CBSE Class 9 Science (Exploration) · Complete Notes · Read only this page → score 100%
🧠 Part 1 Map · How the Atom Story Began
Ancient Ideas (2,000+ yrs ago)
Acharya Kanada → parmanu
Leucippus & Democritus → atomos
Imaginary idea, no experiments
Dalton (1808)
First scientific atomic theory
Atoms = indivisible building blocks
The Crack (late 1800s)
Radioactivity discovered
Atoms NOT indivisible!
Thomson (1897)
Cathode rays → electron
First subatomic particle
Thomson’s Model
Plum pudding / watermelon
+ sphere with − electrons
PART 1 · THE JOURNEY BEGINS
🤔 Think It Over
Are atoms the smallest indivisible particles?
Why do electrons not fall into the nucleus even though they are attracted to protons in it?
Why did scientists keep modifying atomic models?
Everything you see, observe, or feel around is matter.
Matter consists of tiny particles called atoms — both living beings (us) and non-living things (a house) are ultimately composed of atoms.
Atoms are so tiny they cannot be seen with the naked eye.
The big question of this chapter: Is the atom truly the smallest unit of matter, or can it be divided further?
8.1 Rediscovering the Roots of Atomic Theory
More than 2,000 years ago, thinkers in ancient India and ancient Greece asked the same question — What is everything made up of?
Acharya Kanada (India)
If matter (dravya) is divided repeatedly, you reach the smallest particles that can no longer be divided.
He called these particles parmanus.
His ideas are recorded in the Sanskrit text Vaisesika Sutras.
A parmanu is infinitely small and cannot be perceived by the senses.
Parmanus combine to form dyads (groups of two) and triads (groups of three) — the whole material universe, including living bodies, is created from these combinations.
Limitation: this description does not specify the proportions in which parmanus combine to form different substances.
Leucippus and Democritus (Greece)
Proposed a similar idea of indivisible particles, calling them atomos.
In Greek, atomos = indivisible.
EXAM · 1 MARK
The concept of the ‘atom’ originated as an imaginary idea, rather than from experimental observations.
John Dalton’s Atomic Theory (1808)
In 1808, John Dalton proposed his atomic theory — based on scientific experiments of that time.
All matter is composed of indivisible particles called atoms.
Atoms are the fundamental building blocks of matter that cannot be broken down into smaller parts.
It was the first scientific description of how matter is made — the starting point of atomic structure.
💡 Following Dalton’s theory, scientists asked: ① What are atoms made up of? ② What would atoms look like if we could see them? ③ What makes atoms of one element different from another element?
8.2 A Short Historical Journey Through Atomic Models
Over a hundred years ago, scientists proposed simple models of the atom; as new experiments gave new evidence, the models were changed and improved.
Early models were not fully correct, but they show how science moves forward — one step at a time, through curiosity, questioning, and experimentation.
Until the late 19th century, atoms were thought to be indivisible. Then came the crack in the theory ⬇
EXAM · DEFINITION Radioactivity: the phenomenon in which certain elements emit invisible energy and particles called radiation. It proved atoms are composed of smaller particles — they are NOT indivisible.
Discovery of the Electron — J. J. Thomson (1897)
In 1897, J. J. Thomson studied the conduction of electric current through gases at very low pressure.
Set-up: a glass tube with two electrodes + a high voltage applied.
He observed rays moving from the cathode (− electrode) to the anode (+ electrode) — these were called cathode rays.
By studying cathode rays in electric and magnetic fields, he concluded: they are streams of negatively charged particles with a much smaller mass than atoms.
These particles, later called electrons, were emitted from atoms → atoms have smaller subatomic components.
Fig. 8.1: A line diagram of cathode ray tube
👀 LOOK HERE: In Fig. 8.1, the rays travel from cathode (−) to anode (+). Remember the direction — exams love reversing it!
The nature of cathode rays was independent of the cathode material and the gas in the tube.
Conclusion: electrons are a fundamental component of ALL atoms, present in every element.
Charge of an electron = −1.602 × 10⁻¹⁹ C, taken as −1 as a matter of convention and convenience.
📌 Note (NCERT)
Atoms do not show any colour. The colours depicted in the diagrams are for illustrative purposes only.
👨🔬 Meet a Scientist — J. J. Thomson
Most significant discovery: the electron — the first subatomic particle to be identified, part of every atom.
Received the Nobel Prize in Physics in 1906 for his studies of the electrical conductivity of gases.
Head of the famous Cavendish Laboratory, Cambridge — guided many scientists, including Ernest Rutherford.
8.2.1 Thomson’s model of an atom
Thomson’s puzzle: atoms are neutral — so where is the positive charge?
His solution: the atom is a sphere of positive charge with electrons distributed throughout it.
Compared to a pudding with plums embedded in it → the plum pudding model.
Fig. 8.3: Watermelon — a familiar picture of Thomson’s model
EXAM · WATERMELON ANALOGY Red pulp = positively charged matter · Seeds = electrons distributed throughout the atom.
Though later replaced, this was the first genuine attempt to describe how the atom’s positive and negative charges stay balanced.
⏸▶ Pause and Ponder (NCERT)
Q1 Suppose you made up your own ‘atom’, as Thomson described, using clay for the positive charge and small beads for the electrons spread through it. What will happen if: (i) the positive charge on the clay is lesser than the total negative charge of the beads? (ii) by mistake, the clay itself carries a bit of negative charge? Would your model still represent a neutral atom?
Show Answer
(i) Total negative charge > total positive charge → the model has a net negative charge → it does NOT represent a neutral atom.
(ii) If the clay itself is negative, positive charge is missing entirely → total charge is negative → again NOT a neutral atom. Thomson’s model requires positive charge exactly balancing the electrons.
Q2 Could an orange or a lemon, which also contain seeds inside soft pulp, be a good comparison? In what ways does it match Thomson’s idea and where does it fall short?
Falls short: in an orange/lemon the seeds are clustered near the centre in segments, NOT distributed evenly throughout the pulp. Thomson’s model needs electrons spread uniformly through the whole sphere — the watermelon shows this better.
Q3 Why did Thomson conclude that electrons are present in all atoms?
Show Answer
Cathode rays were identical regardless of the cathode material and the gas used in the tube.
Since the same negative particles came from every substance tested, electrons must be a fundamental component of all atoms, in every element.
📝 Check Your Concepts — Part 1
C1 Who proposed the idea of parmanu, and in which text are his ideas recorded?
Show Answer
Acharya Kanada (ancient India); recorded in the Sanskrit text Vaisesika Sutras.
C2 What does the Greek word atomos mean, and who used it?
Show Answer
Atomos = indivisible. Used by the Greek philosophers Leucippus and Democritus for indivisible particles of matter.
C3 How was Dalton’s atomic theory (1808) different from the ancient ideas of the atom?
Show Answer
Ancient ideas were imaginary (not based on experiments). Dalton’s theory was based on scientific experiments of that time — the first scientific description of how matter is made.
C4 What did the discovery of radioactivity prove about atoms?
Show Answer
That atoms must be composed of smaller particles — they are not indivisible as previously believed.
C5 State the direction of cathode rays and the charge they carry.
Show Answer
Cathode rays move from the cathode (−) to the anode (+) and are streams of negatively charged particles (electrons) with much smaller mass than atoms.
C6 Describe Thomson’s model of the atom using the watermelon analogy.
Show Answer
The atom is a sphere of positive charge (red pulp) with electrons distributed throughout it (seeds). Also called the plum pudding model. It was the first attempt to explain how positive and negative charges stay balanced in a neutral atom.
🧠 Part 2 Map · Rutherford’s Discoveries
Gold Foil Experiment (1911)
Geiger & Marsden, under Rutherford
α-particles → thin gold foil
Most pass · some deflect · few bounce back
Rutherford’s Model
Tiny dense + nucleus
Atom mostly empty space
Electrons orbit like planets
Limitation
Orbiting e⁻ should lose energy
Spiral in → atom collapses
Can’t explain stability
Proton Discovered
By Rutherford
+ charge of nucleus
Neutral atom: p = e
PART 2 · RUTHERFORD & THE NUCLEUS
8.2.2 Testing Thomson’s model: The gold foil experiment
In 1911, Geiger and Marsden, working under Ernest Rutherford, tested Thomson’s model through what became famous as the gold foil experiment.
Set-up: a narrow beam of alpha particles aimed at an extremely thin sheet of gold foil.
EXAM · DEFINITION Alpha (α) particles: tiny, positively charged particles emitted from certain radioactive elements. An alpha particle is actually a nucleus of a helium atom containing two protons and two neutrons.
Expectation (as per Thomson’s model): positive charge is spread out evenly → α-particles should pass straight through or be deflected only slightly.
Surprise observations: ① most particles passed through undeflected ② some were sharply deflected ③ a few even bounced back!
Fig. 8.4: Schematic view of the gold foil experiment
👀 LOOK HERE: In Fig. 8.4, count the arrow types — most go straight through, one bends slightly, one bounces back. That “bounce back” is the game-changer for the whole chapter.
EXAM · DEFINITION Scattering: the deflection of α-particles from their straight path. Hence, the gold foil experiment is also called an α-ray scattering experiment.
❌ Thomson’s model FAILED to explain: ① why some α-particles deflected through large angles ② why most α-particles passed undeflected.
A. Rutherford’s model of an atom
From the gold foil experiment, Rutherford concluded that the positive charge of an atom is not spread throughout — it remains concentrated in an extremely small region called the nucleus. He proposed:
Most of an atom is empty space — as most α-particles passed through the gold foil without any deflection.
The nucleus is dense, contains all the positive charge and most of the mass of an atom.
Electrons revolve around the nucleus, somewhat like planets orbiting the Sun → hence called the planetary model of the atom.
Fig. 8.5: Planetary model suggested by Rutherford
Size comparison: nucleus is about 10⁵ (one lakh) times smaller than the atom Diameter of atom ≈ 10⁻¹⁰ m · Diameter of nucleus ≈ 10⁻¹⁵ m
Memory trick: if the atom were a cricket ground (~100 m across), the nucleus would be a tiny black pepper grain (a few mm) at the centre!
✏️ METHOD — STUDY THIS (NCERT: Ready to Go Beyond) Q: How many atoms make a sheet of paper 0.1 mm thick?
Diameter of one atom ≈ 10⁻¹⁰ m; sheet thickness = 0.1 mm = 10⁻⁴ m.
Number of atoms ≈ (10⁻⁴ m) ÷ (10⁻¹⁰ m) = 10⁶
≈ one million atoms stacked together! A simple sheet is a vast assembly of tiny building blocks, all neatly arranged.
🧩 Think as a Scientist (NCERT)
Observe Fig. 8.4 of the gold foil experiment. Predict the observations you would expect if the gold foil were made thicker. Also, draw a simple diagram to show the observations you expect. Hint: Compare thin foil vs thick foil. How does the thickness affect the chances of hitting a nucleus?
Show Expected Answer
Thicker foil = more layers of atoms = higher chance of an α-particle passing close to (or hitting) a nucleus.
So: more α-particles deflected, more bounce back, and fewer pass straight through compared to thin foil.
Diagram idea: same beam, but now several bent arrows and 2–3 bounced-back arrows.
✅ Rutherford’s model was BETTER than Thomson’s at explaining the gold foil results — but it had a limitation: it could not explain the stability of the atom.
B. Limitations of Rutherford’s model
From Chapter 4 (Describing Motion Around Us): a particle moving in a circular path is constantly changing direction → it is accelerating.
An accelerating charged electron should lose energy.
Losing energy → the electron would spiral inward and eventually fall into the positively charged nucleus.
If that really happened → atoms would collapse and would not exist!
But in reality, atoms are stable — matter around us stays intact.
Conclusion: Rutherford’s model was not completely correct; a new explanation was needed for how electrons stay in motion without collapsing into the nucleus.
Fig. 8.6: Spiral path followed by a charged particle on losing energy
👨🔬 Meet a Scientist — Ernest Rutherford
Born in New Zealand; moved to Cambridge to work with J. J. Thomson.
Known as the Father of Nuclear Physics.
Discovered the atomic nucleus; explained how some elements naturally break down → 1908 Nobel Prize in Chemistry.
In 1911, proposed the nuclear model of the atom. His portrait now appears on New Zealand’s $100 banknote.
C. Discovery of the proton
Rutherford showed the nucleus carries positive charge, which comes from particles called protons (discovered and named by Rutherford).
Protons are much heavier than electrons.
Protons have a charge equal and opposite to that of electrons.
EXAM · KEY RULE
For an atom to be electrically neutral: number of protons = number of electrons.
Examples: helium → 2 protons, 2 electrons · sodium → 11 protons, 11 electrons. All atoms are electrically neutral.
⏸▶ Pause and Ponder (NCERT)
Q4 What do you think would happen if α-particles were replaced with negatively charged particles in Rutherford’s gold foil experiment?
Show Answer
Most particles would still pass through (atom is mostly empty space).
But particles passing near a nucleus would be attracted TOWARD it (opposite charges attract) instead of being repelled away.
So deflections would bend toward the nucleus, and no particle would be pushed back by repulsion the way α-particles were.
Q5 Rutherford found that a few α-particles bounced back sharply. How does this single surprising result completely rule out Thomson’s ‘plum pudding model’ of the atom?
Show Answer
In the plum pudding model, positive charge is spread thinly and evenly across the whole atom.
Such spread-out charge can never produce a force strong enough to reverse a fast, heavy α-particle.
A bounce-back needs the positive charge and mass concentrated in one tiny, dense region → the nucleus. So the plum pudding picture cannot be correct.
Q6 If you could ask Rutherford one question about his work, what would it be?
Show Sample Answer
Open-ended — any thoughtful question is correct. Sample: “When the first α-particle bounced back, did you immediately suspect a nucleus, or did you first think the experiment had gone wrong?”
Q7 Assertion (A): Rutherford concluded that most of the mass of an atom is concentrated in a small region at the centre called the nucleus. Reason (R): According to Thomson’s model, electrons are embedded in a uniformly distributed positive charge sphere. Choose the correct option: (i) Both A and R are true, and R is the correct explanation of A. (ii) Both A and R are true, but R is not the correct explanation of A. (iii) A is true, but R is false. (iv) A is false, but R is true.
Show Answer
Option (ii). A is true (gold foil conclusion) and R is true (it correctly describes Thomson’s model) — but R describes Thomson’s model, it does NOT explain Rutherford’s conclusion about the nucleus.
📝 Check Your Concepts — Part 2
C1 Who actually performed the gold foil experiment, and in which year?
Show Answer
Geiger and Marsden, working under Ernest Rutherford, in 1911.
C2 State the three observations of the gold foil experiment.
Show Answer
① Most α-particles passed through undeflected ② some were sharply deflected ③ a few bounced back.
C3 Write the three postulates of Rutherford’s model.
Show Answer
① Most of the atom is empty space ② the nucleus is dense, holding all the positive charge and most of the mass ③ electrons revolve around the nucleus like planets around the Sun (planetary model).
C4 Compare the diameters of the atom and the nucleus.
Show Answer
Atom ≈ 10⁻¹⁰ m; nucleus ≈ 10⁻¹⁵ m — the nucleus is about 10⁵ (one lakh) times smaller than the atom.
C5 Why does Rutherford’s model predict that atoms should collapse?
Show Answer
An electron in a circular path is accelerating (direction keeps changing) → an accelerating charge should lose energy → it would spiral inward and fall into the nucleus → atom collapses. Since real atoms are stable, the model is incomplete.
C6 A magnesium atom has 12 protons. How many electrons does it have, and why?
Show Answer
12 electrons — for an atom to be electrically neutral, the number of protons must equal the number of electrons.
🧠 Part 3 Map · Bohr’s Model & the Neutron
Bohr’s Model (1913)
Fixed shells = energy levels
K, L, M, N (n = 1, 2, 3, 4)
No energy loss in a fixed shell
Shell Energy
K closest → least energy
Farther shell → higher energy
Jump = absorb/release fixed energy
Mass Puzzle
He: 2 protons, but mass = 4 × H
Something else in nucleus?
Neutron (1932)
Chadwick’s discovery
Mass ≈ proton, charge 0
In all nuclei except hydrogen
PART 3 · BOHR’S MODEL & THE NEUTRON
8.2.3 Bohr’s model of the atom
To explain why atoms are stable, Niels Bohr proposed a new model in 1913. According to Bohr:
Electrons do not move randomly — they follow fixed circular paths called stationary states, orbits, or shells.
In each shell, an electron has a definite amount of energy → shells are also called energy levels.
Shells are represented by letters K, L, M, N, … or numbers n = 1, 2, 3, 4, …
Electrons can revolve only in these allowed shells, not in between them.
While moving in a fixed shell, an electron does not lose energy.
The first energy level K (n = 1) is closest to the nucleus and has the least energy.
Energy increases moving away from the nucleus: an electron in the L-shell (n = 2) has more energy than one in the K-shell (n = 1). The farther the shell, the higher its energy.
An electron can move to another shell by absorbing or releasing a fixed amount of energy equal to the difference between the energies of the two levels.
Each shell can hold only a certain number of electrons.
Fig. 8.7: Energy levels in an atom
👀 LOOK HERE: In Fig. 8.7, K is the innermost circle (n=1) and N is the outermost (n=4). Innermost = least energy. Exams often ask “which shell has the least energy?” — answer: K.
EXAM · HOW BOHR EXPLAINS STABILITY
Bohr introduced stationary states as a postulate: in a stationary state, the energy of an electron remains constant, even though it is in motion around the nucleus. So electrons do not lose energy and do not spiral in. Bohr’s model explained many experimental observations — a major step in understanding atomic structure.
🧵 Threads of Curiosity — Why K, L, M, N and not A, B, C, D?
The naming came from early X-ray experiments by physicist Charles Barkla, who called the first observed X-ray line K.
He didn’t start from A — he left room for possible discovery of a series earlier than the K series (none were ever found).
Bohr adopted the same notation for atomic shells.
👨🔬 Meet a Scientist — Niels Bohr
Professor of physics at Copenhagen University, Denmark.
Curious about how atoms exist, because old models could not explain why electrons stay around the nucleus without collapsing.
Received the Nobel Prize in 1922 for his work on the structure of the atom.
⏭ Next Level Up: even Bohr’s model was later found to have limitations — the quantum mechanical model came next. You will learn about it in higher grades.
8.3 What Components Contribute to the Mass of an Atom?
Rutherford’s model showed most of the mass of an atom is concentrated in its nucleus.
Electrons are so light that their mass can be ignored.
The puzzle (early 20th century): hydrogen has 1 proton, helium has 2 protons — yet helium’s mass is about FOUR times that of hydrogen, not double!
Scientists wondered: besides protons, is there something else in the nucleus adding mass without affecting its charge?
8.3.1 Discovery of the Neutron
In 1932, the problem was solved by James Chadwick (a student of Ernest Rutherford).
He discovered a new subatomic particle with mass nearly equal to a proton but no electrical charge.
This neutral particle was named the neutron, represented by the symbol ‘n’.
Neutrons are found in the nucleus of all atoms except hydrogen.
So, the mass of an atom comes mainly from its protons and neutrons packed tightly in the nucleus — this also explains why atoms are heavier than the mass of their total number of protons.
Table 8.1: Symbols and relative charges of subatomic particles
S.No.
Subatomic particle
Symbol
Relative charge
1.
Electron
e⁻
−1
2.
Proton
p⁺
+1
3.
Neutron
n⁰
0
🧵 Threads of Curiosity — Why don’t protons push each other apart?
Lighter atoms often have equal protons and neutrons (carbon: 6 each; oxygen: 8 each).
Heavier atoms have many more neutrons than protons: iron = 26 p, 30 n; uranium = 92 p, 146 n.
Every proton repels every other proton (like charges). Neutrons, being neutral, reduce this repulsion by increasing the distance between protons — and by strengthening the nuclear force that binds all particles together.
So heavier atoms need many more neutrons to hold the nucleus tightly bound.
📦 Ready to Go Beyond — The Atomic Age
The discovery of the neutron opened a new era in atomic physics. Neutrons, being uncharged, can easily penetrate nuclei — leading to breakthroughs like artificial radioactive elements and the splitting of uranium atoms. This gave birth to the ‘atomic age’, allowing the development of both nuclear power and nuclear weapons.
👨🔬 Meet a Scientist — James Chadwick
Worked under Rutherford at the famous Cavendish Laboratory, University of Cambridge.
Discovered the neutron in 1932 — this breakthrough explained atomic mass.
Earned the Nobel Prize in Physics in 1935. The neutron transformed research, enabling scientists to probe nuclear secrets and sparking a chain of discoveries in harnessing atomic energy.
🇮🇳 India’s Scientific Contributions (NCERT)
The Bhabha Atomic Research Centre (BARC), Mumbai, leads advanced neutron-scattering experiments using reactors such as Dhruva.
This has revealed key insights into materials like superconductors, battery electrodes, and drug molecules — helping develop better medicines, energy storage, and industrial alloys right here in India.
Activity: research and explore more about Dhruva!
🧪 By 1869, scientists knew about 69 elements. Today, we know about 118 unique chemical elements — some artificially made, and the search continues.
📝 Check Your Concepts — Part 3
C1 Why did Bohr propose a new model of the atom in 1913?
Show Answer
To explain why atoms are stable — Rutherford’s model predicted collapsing atoms, which contradicts reality.
C2 What are stationary states? How do they solve the stability problem?
Show Answer
Stationary states are fixed shells/orbits where the energy of an electron remains constant even though it is in motion. Since the electron does not lose energy while in a fixed shell, it never spirals into the nucleus — so the atom stays stable.
C3 Which shell has the least energy, and what happens to energy as we move outward?
Show Answer
The K-shell (n = 1), closest to the nucleus, has the least energy. Energy increases as we move away from the nucleus — L (n=2) has more energy than K (n=1), and so on.
C4 How does an electron jump from one shell to another?
Show Answer
By absorbing or releasing a fixed amount of energy equal to the difference between the energies of the two levels.
C5 Why is a helium atom about four times heavier than hydrogen, though it has only two protons?
Show Answer
Because the helium nucleus also contains 2 neutrons. Mass comes from protons + neutrons: 2 p + 2 n = 4 units ≈ four times the mass of hydrogen (1 p, 0 n).
C6 Give the symbol and relative charge of each subatomic particle.
Hydrogen — neutrons are found in the nucleus of all atoms except hydrogen.
🧠 Part 4 Map · Symbols, Z and A
Symbols of Elements
Dalton 1803: pictorial
Berzelius 1813: Latin names
Today: IUPAC approves
Atomic Number (Z)
Z = number of protons
Decides element’s identity
Neutral atom: p = e
Mass Number (A)
A = protons + neutrons
p + n = nucleons
Electron mass ignored
Notation
A on top, Z below
Example: ¹²₆C
PART 4 · SYMBOLS · ATOMIC NUMBER · MASS NUMBER
8.4 Symbols of Elements
John Dalton realised the need for a standard way to represent elements and compounds, to make the study of chemistry easier.
In 1803, he introduced the first pictorial symbols to represent the known elements.
Fig. 8.9: Symbols of some elements given by Dalton
In 1813, Berzelius suggested that symbols should be derived from the Latin names of elements → thus came alphabetic chemical symbols.
Nowadays, the International Union of Pure and Applied Chemistry (IUPAC), an international scientific organisation, approves the names and symbols of elements.
Norms of writing symbols (IUPAC)
Many symbols are the first letter or first two letters of the element’s name.
The first letter is always CAPITAL; the second letter (if any) is always small. Examples: hydrogen, H; aluminium, Al (not AL); cobalt, Co (not CO).
Some symbols use the first letter + a letter other than the second: chlorine, Cl; zinc, Zn.
Some symbols come from Latin, Greek, or German names: iron = Fe (Latin ferrum), mercury = Hg (Greek hydrargyros), tungsten = W (German wolfram).
Table 8.2: Names of some common elements and their symbols
Element
Symbol
Element
Symbol
Element
Symbol
Aluminium
Al
Copper (Cuprum)
Cu
Nitrogen
N
Argon
Ar
Fluorine
F
Oxygen
O
Barium
Ba
Gold (Aurum)
Au
Potassium (Kalium)
K
Boron
B
Hydrogen
H
Silicon
Si
Bromine
Br
Iodine
I
Silver (Argentum)
Ag
Calcium
Ca
Iron (Ferrum)
Fe
Sodium (Natrium)
Na
Carbon
C
Lead (Plumbum)
Pb
Sulfur
S
Chlorine
Cl
Magnesium
Mg
Uranium
U
Cobalt
Co
Neon
Ne
Zinc
Zn
🌍 Why symbols? They are internationally recognised — scientists worldwide communicate clearly, regardless of language barriers.
⏸▶ Pause and Ponder (NCERT)
Q8 Imagine you are a scientist who has discovered a new element. Name this element after yourself and justify that the symbol you have chosen follows the IUPAC rules.
Show Sample Answer
Open-ended. Sample: element “Srikanthium”, symbol Sk — first letter of the name written as a capital (S), second chosen letter written small (k), following the IUPAC norm of first letter + another letter from the name.
Q9 What problems could arise if every scientist used different symbols for the same element?
Show Answer
Confusion — the same symbol could mean different elements to different scientists.
Research could not be shared, compared, or verified across countries and languages.
Chemical formulas and equations would become unreadable — international collaboration would break down.
8.5 Atomic Number
EXAM · DEFINITION
The number of protons in the nucleus of an atom of an element is known as its atomic number, designated by the symbol Z. This number determines the identity of an element and its chemical behaviour.
Atoms of one element are all alike, but different from atoms of other elements — they differ in the number of electrons and protons.
Since the atom as a whole is neutral: number of protons = number of electrons orbiting the nucleus.
Hydrogen: 1 proton, 1 electron → Z = 1. Helium: Z = 2 → 2 protons, 2 electrons.
The atomic number uniquely identifies an element.
Q (NCERT in-text, Fig. 8.10) How many neutrons and protons are present in a lithium atom, and what is its atomic number?
Show Answer
Lithium has 3 protons and 4 neutrons in its nucleus → atomic number Z = 3.
8.6 Mass Number
Helium has 2 protons, but its mass is about four times that of a proton — the extra mass comes from neutrons.
EXAM · DEFINITION
The total number of protons and neutrons in the nucleus of an atom is called its mass number, denoted by A. The protons and neutrons present in the nucleus are called nucleons.
Mass number (A) = Number of protons + Number of neutrons
Since a neutron’s mass ≈ a proton’s mass, helium’s mass = 2 protons + 2 neutrons = 4 units.
Table 8.3: Mass number of different elements
Element
Protons (p⁺)
Neutrons (n⁰)
Mass number (A)
Hydrogen
1
0
1
Helium
2
2
4
Lithium
3
4
7
The electron, in comparison, has almost negligible mass — it can be ignored in calculations.
✏️ METHOD — STUDY THIS (Standard notation)
In the standard notation: Mass Number (A) is written on top, Atomic Number (Z) at the bottom, before the Symbol of the element.
Example: carbon → symbol C, atomic number 6, mass number 12 → written as 126C.
📍 So far: protons + neutrons live in the nucleus; electrons move around the nucleus.
🧮 Numerical Practice (NCERT Pause and Ponder) — try first, then check!
Q10 An atom with an atomic number of 26 has 56 nucleons. Find out its number of electrons, protons and neutrons.
Show Answer
Z = 26 → protons = 26; neutral atom → electrons = 26; neutrons = A − Z = 56 − 26 = 30.
Q11 The nucleus of an atom contains 20 protons. If its mass number is 41, find the number of neutrons in it.
Show Answer
Neutrons = A − p = 41 − 20 = 21 neutrons.
Q12 An atom has 18 neutrons and an atomic number of 17. What is its mass number?
Show Answer
A = p + n = 17 + 18 = 35.
Q13 An atom 23A has 11 electrons. Find the number of neutrons in it.
Show Answer
Neutral atom → protons = electrons = 11. Neutrons = A − p = 23 − 11 = 12 neutrons.
📝 Check Your Concepts — Part 4
C1 Who introduced pictorial symbols for elements, and who suggested Latin-based symbols?
Show Answer
Pictorial symbols: John Dalton (1803). Latin-name-based alphabetic symbols: Berzelius (1813). Today IUPAC approves names and symbols.
C2 Why is cobalt written Co and not CO?
Show Answer
By IUPAC norms the second letter must be small. “CO” would mean a compound of Carbon and Oxygen (carbon monoxide) — “Co” means the single element cobalt.
C3 Give the origins of the symbols Fe, Hg and W.
Show Answer
Fe ← Latin ferrum (iron); Hg ← Greek hydrargyros (mercury); W ← German wolfram (tungsten).
C4 Define atomic number and mass number in one line each.
Show Answer
Atomic number (Z) = number of protons in the nucleus. Mass number (A) = total number of protons + neutrons (nucleons) in the nucleus.
C5 Write the standard notation for sodium (Z = 11, A = 23).
Show Answer
2311Na — mass number 23 on top, atomic number 11 below.
🧠 Part 5 Map · Electron Distribution & Valency
Bohr–Bury Rules
Max per shell = 2n²
Outermost max = 8
Fill stepwise: K → L → M
Electronic Configuration
Distribution of e⁻ in shells
e.g. Na = 2, 8, 1
Valence Shell & Octet
Outermost shell = valence shell
8 e⁻ = octet → stable
Valency
< 4 valence e⁻ → lose
> 4 valence e⁻ → gain
Carbon (4) → shares
PART 5 · ELECTRON DISTRIBUTION & VALENCY
8.7 How Are Electrons Distributed in Different Energy Levels?
Bohr and Bury suggested the following rules:
The maximum number of electrons in a shell is given by the formula 2n², where ‘n’ is the number of the shell.
The maximum number of electrons in the outermost shell is 8 (the first shell can hold a maximum of two).
Electrons fill shells in a stepwise manner, starting closest to the nucleus and moving outward: K, L, M, N, … The L-shell fills only after the K-shell is complete, and so on.
✏️ METHOD — STUDY THIS (2n² in action)
K-shell (n = 1): 2 × 1² = 2 electrons
L-shell (n = 2): 2 × 2² = 8 electrons
M-shell (n = 3): 2 × 3² = 18 electrons
Hydrogen (Z = 1): its only electron has to be in the K-shell.
Q (NCERT in-text) Helium contains two protons in its nucleus and two electrons. In which way will the two electrons be arranged in its atomic shell?
Show Answer
Both electrons go into the K-shell (it can hold a maximum of 2) → configuration: 2. The K-shell is complete, which is why helium is stable and unreactive.
8.7.1 Building up atoms
We build 2-D atomic structures by adding one electron to the appropriate energy level each time the atomic number increases by 1.
EXAM · DEFINITION
The distribution of electrons among various shells is known as the electronic configuration of the atom.
Fig. 8.11: Schematic atomic structure of the first eighteen elements showing how the electrons are filled in the K, L and M shells
👀 LOOK HERE: In Fig. 8.11, watch the pattern — K fills first (max 2), then L (max 8), then M. After Ne (2,8) the next electron starts a NEW shell → Na = 2,8,1.
Table 8.4: Symbols, atomic numbers, number of protons, neutrons, electrons, and the electronic distribution of atoms of the first eighteen elements
Element
Symbol
Atomic number
Protons
Neutrons
Electrons
K
L
M
N
Hydrogen
H
1
1
–
1
1
–
–
–
Helium
He
2
2
2
2
2
–
–
–
Lithium
Li
3
3
4
3
2
1
–
–
Beryllium
Be
4
4
5
4
2
2
–
–
Boron
B
5
5
6
5
2
3
–
–
Carbon
C
6
6
6
6
2
4
–
–
Nitrogen
N
7
7
7
7
2
5
–
–
Oxygen
O
8
8
8
8
2
6
–
–
Fluorine
F
9
9
10
9
2
7
–
–
Neon
Ne
10
10
10
10
2
8
–
–
Sodium
Na
11
11
12
11
2
8
1
–
Magnesium
Mg
12
12
12
12
2
8
2
–
Aluminium
Al
13
13
14
13
2
8
3
–
Silicon
Si
14
14
14
14
2
8
4
–
Phosphorus
P
15
15
16
15
2
8
5
–
Sulfur
S
16
16
16
16
2
8
6
–
Chlorine
Cl
17
17
18
17
2
8
7
–
Argon
Ar
18
18
22
18
2
8
8
–
⏸▶ Pause and Ponder (NCERT)
Q14 Identify the number of electrons in the outermost shell of the following elements: (i) 126C (ii) 199F (iii) 2814Si
Show Answer
(i) Carbon: Z = 6 → config 2, 4 → outermost shell has 4 electrons.
(ii) Fluorine: Z = 9 → config 2, 7 → outermost shell has 7 electrons.
(iii) Silicon: Z = 14 → config 2, 8, 4 → outermost shell has 4 electrons.
Q15 Write the electronic configuration of the elements having atomic numbers 12, 16 and 18.
Show Answer
Z = 12 (Magnesium): 2, 8, 2 · Z = 16 (Sulfur): 2, 8, 6 · Z = 18 (Argon): 2, 8, 8.
Q16 Solve this riddle: I am an atom with a mass number of 23 and 11 protons. I am a soft metal and react vigorously with water. Who am I and how many neutrons do I have? You can also create one such riddle.
Show Answer
The atom is Sodium (Na) — Z = 11, soft metal, reacts vigorously with water. Neutrons = A − Z = 23 − 11 = 12 neutrons.
8.8 Combining Capacity of an Atom: Valency
EXAM · DEFINITION
The number of atoms of hydrogen or chlorine with which one atom of an element can combine to form a compound is called its combining capacity. It is expressed in terms of hydrogen and chlorine because both possess a combining capacity of one.
Example: in H₂O (water), oxygen combines with two hydrogen atoms → combining capacity of oxygen = 2.
Q (NCERT in-text) In NH₃ (ammonia) and MgCl₂ (magnesium chloride), what will be the combining capacities of nitrogen and magnesium respectively?
Show Answer
In NH₃, nitrogen combines with 3 hydrogen atoms → combining capacity of nitrogen = 3. In MgCl₂, magnesium combines with 2 chlorine atoms → combining capacity of magnesium = 2.
Valence shell, valence electrons and the octet
The outermost shell containing electrons is the valence shell; electrons in it are valence electrons.
If the outermost shell has 8 electrons, it is called an octet.
Elements with a complete octet (8 electrons) — or 2 electrons in the case of helium — are largely unreactive and more stable.
Atoms with incomplete valence shells are usually more reactive — they lose, gain, or share electrons to complete their octet.
EXAM · DEFINITION
The number of electrons gained, lost, or shared to complete the octet is called the valency of the element.
Fewer than 4 valence electrons → tends to LOSE electrons to complete its octet. Example: sodium (2, 8, 1) → loses 1 electron → valency 1.
More than 4 valence electrons → tends to GAIN electrons. Example: oxygen (2, 6) → gains 2 electrons → valency 2.
Some compounds appear to violate the usual valency rule — you will learn about them in higher grades.
Q (NCERT in-text) Can you predict what happens to atoms that already have eight electrons in their outermost shell (except one-shell elements, where only two electrons are possible)? Will they still try to lose or gain electrons?
Show Answer
No. Their octet is already complete → they are stable and unreactive → they do not lose or gain electrons → their valency is 0 (e.g., neon 2,8 and argon 2,8,8).
Q (NCERT activity) Examine Table 8.4. Add one more column to it, and write down the common valency of each element.
🔎 Coming up: all atoms of an element have the same protons and electrons (= Z). But the same regarding neutrons? Scientists observed atoms of the SAME element can have DIFFERENT numbers of neutrons → Part 6!
📝 Check Your Concepts — Part 5
C1 State the three Bohr–Bury rules for filling electrons.
Show Answer
① Max electrons per shell = 2n² ② max in the outermost shell = 8 (first shell max 2) ③ shells fill stepwise from the innermost outward (K, then L, then M, …).
C2 Why can’t the M-shell hold 18 electrons in argon (Z = 18)?
Show Answer
Because the outermost shell can hold a maximum of 8 electrons. In argon, M is the outermost shell → config is 2, 8, 8 (not 2, 8, 8+ beyond 8).
C3 Why are neon and argon unreactive?
Show Answer
Their valence shells hold a complete octet (Ne: 2,8 · Ar: 2,8,8) → stable → no need to lose, gain or share electrons → valency 0.
C5 Why is combining capacity expressed in terms of hydrogen and chlorine?
Show Answer
Because both hydrogen and chlorine possess a combining capacity of one — they act as the reference standard.
🧠 Part 6 Map · Isotopes & Isobars
Isotopes
Same Z, different A
H: protium, deuterium, tritium
C: C-12, C-13, C-14
Isotope Properties
Chemical properties SAME
Physical properties differ
Average Atomic Mass
Weighted by abundance
Cl = 35.5 u
Isobars
Same A, different Z
Ca-40, K-40, Ar-40
PART 6 · ISOTOPES & ISOBARS
8.9 A Deeper Look into Atomic Structure
8.9.1 Isotopes
Dalton proposed all atoms of an element are identical with the same mass — but scientists later discovered atoms of the same element with the same protons (Z) yet different neutrons → different mass numbers (A = p⁺ + n⁰).
EXAM · DEFINITION Isotopes: ‘twin atoms’ with the same atomic number but different mass numbers.
Example 1: Isotopes of hydrogen
Naturally occurring hydrogen is a mixture of three isotopes: 11H (protium, ~99.98%), 21H (deuterium, ~0.015%), and 31H (tritium, in traces).
All contain one proton each; deuterium has one neutron, tritium has two neutrons.
Fig. 8.12: Schematic representation of isotopes of hydrogen
Q (NCERT in-text) Can you guess how many electrons each of these isotopes have?
Show Answer
All three have 1 electron each — the atomic number (protons = 1) is the same, and a neutral atom has electrons = protons.
EXAM · WHY CHEMICAL PROPERTIES ARE SAME
Isotopes have the same number of electrons and the same electronic configuration. Chemical properties depend mainly on valence electrons → all isotopes show the same chemical properties. They differ only in physical properties, e.g. boiling and melting points.
Example 2: Isotopes of carbon
Carbon has three isotopes: 126C, 136C, and 146C. Each has six protons and six electrons.
126C is the most abundant isotope in nature. The three differ only in the number of neutrons (6, 7, 8).
Fig. 8.13: Schematic representation of isotopes of carbon
📦 Ready to Go Beyond — the unified atomic mass unit
Atoms are too tiny to weigh in kilograms or grams. Just as it is easier to weigh a grain of wheat in milligrams rather than kilograms, scientists use a special unit called the unified atomic mass unit (u) to measure the mass of atoms — a kilogram is simply too large for such small particles. Earlier, atomic mass was expressed in atomic mass unit (abbreviated ‘amu’).
🌏 Bridging Science and Society — Applications of Isotopes (NCERT)
23592U (uranium isotope) → used as fuel in a nuclear reactor to generate electricity in a nuclear power plant.
6027Co (radioactive cobalt isotope) → used in radiation treatment for cancer.
13153I (iodine isotope) → used to treat goitre and thyroid cancer.
146C (carbon isotope) → used in archaeology and geology to determine the age of ancient fossils and artefacts.
A. Average atomic mass
Chlorine occurs in nature in two isotopic forms: mass 35 u and mass 37 u, in the ratio 3 : 1.
The question: should we call the mass of a chlorine atom 35 u or 37 u?
✏️ METHOD 1 — STUDY THIS (Simple average — the WRONG way for nature)
Simple arithmetic mean of isotope masses, without accounting for relative abundances:
Average atomic mass = (35 + 37) ÷ 2 = 36 u
❌ But this does NOT accurately reflect nature — isotopes do not occur in equal ratios!
✏️ METHOD 2 — STUDY THIS (Weighted average — the CORRECT way) ³⁵Cl ≈ 75% of natural chlorine; ³⁷Cl ≈ 25%. Multiply each isotope’s mass by its percent relative abundance, then add:
EXAM · WHAT 35.5 u MEANS
No single chlorine atom has a fractional mass of 35.5 u! It means: in 1 million chlorine atoms, there are 7.5 lakh 3517Cl and 2.5 lakh 3717Cl atoms, with a weighted average atomic mass of 35.5 u. The simple average ignores abundance; the weighted average atomic mass accurately reflects the mass of the element as it occurs in nature.
⏸▶ Pause and Ponder (NCERT)
Q17 Two different atoms have 11 protons each, but one has 12 neutrons, and the other has 13 neutrons. How do their atomic numbers and mass numbers compare? Are they the same element or different elements?
Show Answer
Atomic numbers: both Z = 11 (same).
Mass numbers: 11+12 = 23 and 11+13 = 24 (different).
Same Z → they are the SAME element (sodium) — they are isotopes of each other.
Q18 If a bromine atom is available in the form of, say two isotopes, 7935Br (49.7%) and 8135Br (50.3%), calculate the average atomic mass of the bromine atom.
Show Answer
Average atomic mass = (79 × 49.7/100) + (81 × 50.3/100)
= 39.263 + 40.743
= 80.006 u ≈ 80.01 u
8.9.2 Isobars
Consider three elements: calcium (Z = 20), potassium (Z = 19), and argon (Z = 18).
Different numbers of protons — yet each has a mass number of 40 → same total number of nucleons, though they are different elements.
EXAM · DEFINITION Isobars: atoms of different elements with the same mass number but different atomic numbers.
🧠 Memory trick: isoTOPES = same Type (same element, same Z) · isoBARS = same Bulk (same mass number A).
The story does not end here…
Later, scientists discovered that even Bohr’s model was not entirely correct.
Electrons do not follow well-defined paths like fixed Bohr orbits — today we understand they exist as ‘electron clouds’ around the nucleus.
We can predict regions where electrons are most likely to be, not exactly where they are. You will learn these details in higher grades — the journey of exploring the atom is far from over!
Fig. 8.16: Journey of the development of atomic models
👀 LOOK HERE: Fig. 8.16 is the whole chapter in one line — Dalton → Thomson → Rutherford → Bohr → Modern (quantum) → still being discovered. Perfect last-minute revision!
At a Glance (NCERT Summary)
Atoms are the building blocks of matter.
J. J. Thomson proposed that in an atom, electrons are embedded in a positively charged sphere.
Rutherford’s model described the atom as mostly empty space, with a dense, positively charged nucleus at its centre and electrons orbiting it.
Niels Bohr’s model proposed that electrons move in fixed energy levels (shells) around the nucleus.
The shells of an atom are named as K, L, M, N, and so on.
James Chadwick discovered the presence of neutrons in the atom.
The three subatomic particles of an atom are electrons, protons, and neutrons.
If the outermost shell of an atom has an octet of electrons (or two electrons in case of helium), the atom is stable and largely unreactive.
Valency is the combining capacity of an atom. It is equal to the number of electrons which can be gained, lost or shared by an atom to achieve a stable configuration.
The atomic number of an element is equal to the number of protons in its nucleus.
The mass number of an atom is equal to the total number of nucleons (protons and neutrons) in its nucleus.
Isotopes are atoms of same element that have the same atomic number but different mass numbers.
The average atomic mass of an element is calculated based on the relative abundance of its isotopes in nature.
Isobars are atoms of different elements with the same mass number but different atomic numbers.
PART 7 · NCERT EXERCISES + CHAPTER QUIZ
Revise, Reflect, Refine (NCERT Exercises)
Q1 Choose the correct options and explain the reason for the correct and incorrect options in the context of Ernest Rutherford’s gold foil experiment: (i) The experiment clearly showed the existence of neutrons in the nucleus. (ii) The results disproved the plum pudding model and led to the idea of a nucleus at the centre of the atom. (iii) The large deflection of a few alpha particles indicated that most of the mass of the atom and positive charge are packed into a tiny centre. (iv) The way alpha particles were deflected showed that electrons move around the nucleus.
Show Answer
Correct: (ii) and (iii).
(i) ❌ Incorrect — neutrons were discovered by Chadwick in 1932, much later; the gold foil experiment revealed nothing about neutrons.
(ii) ✅ Correct — sharp deflections and bounce-backs could not happen if positive charge were spread out evenly → plum pudding disproved, nucleus proposed.
(iii) ✅ Correct — only a tiny, dense centre holding most of the mass and all positive charge can bounce back a fast α-particle.
(iv) ❌ Incorrect — the deflections were caused by the nucleus repelling α-particles; the experiment gave no direct information about how electrons move.
Q2 Which of the following statements are correct or incorrect according to the Bohr’s atomic model? Give a reason for each statement. (i) Electrons lose energy while moving in fixed orbits and slowly fall into the nucleus. (ii) Electrons can exist anywhere around the nucleus with no fixed energy. (iii) Electrons revolve around the nucleus in orbits of fixed energy without losing energy. (iv) Electrons can be found between energy levels as they move around the nucleus.
Show Answer
(i) ❌ Incorrect — in a fixed shell (stationary state), an electron does not lose energy, so it never spirals into the nucleus.
(ii) ❌ Incorrect — electrons exist only in allowed shells, each with a definite energy.
(iii) ✅ Correct — this is Bohr’s central postulate: fixed-energy orbits with no energy loss.
(iv) ❌ Incorrect — electrons revolve only in allowed shells, not in between them; they jump between levels by absorbing/releasing fixed energy.
Q3 The composition of the nuclei of three atomic species X, Y, and Z: X → 18 protons, 19 neutrons · Y → 17 protons, 18 neutrons · Z → 17 protons, 20 neutrons. Explain the relation between (i) Y and Z (ii) Z and X.
Show Answer
First calculate: X → Z = 18, A = 37 · Y → Z = 17, A = 35 · Z → Z = 17, A = 37.
(i) Y and Z: same atomic number (17), different mass numbers (35, 37) → they are ISOTOPES (both are chlorine).
(ii) Z and X: same mass number (37), different atomic numbers (17, 18) → they are ISOBARS.
Q4 What conclusion did Rutherford draw about the position and characteristics of the atom’s positively charged part based on the few alpha particles that bounced back or were deflected at large angles in the gold foil experiment?
Show Answer
The positive charge is not spread throughout the atom — it is concentrated in an extremely small region at the centre, called the nucleus.
The nucleus is dense and contains all the positive charge and most of the mass of the atom — only such a centre could repel fast α-particles straight back.
Q5 Explain and arrange the following statements in the correct chronological order to show how atomic models have evolved over time. (i) Bohr’s model — fixed orbits with definite energy (ii) Thomson’s model — ‘plum pudding’ (iii) Rutherford’s model — dense central nucleus (iv) Dalton’s model — indivisible particles.
Show Answer
Correct order: (iv) → (ii) → (iii) → (i)
(iv) Dalton (1808): atoms are indivisible particles — the first scientific theory.
(ii) Thomson (after discovering the electron, 1897): atom = positive sphere with electrons embedded (plum pudding).
(iii) Rutherford (1911): gold foil experiment → atoms have a dense central nucleus, mostly empty space.
Q6 Electrons move around the nucleus in orbits. Why do they not fly away from the atom? Explain what keeps them attracted to the nucleus.
Show Answer
The nucleus contains positively charged protons, and electrons are negatively charged. The electrostatic force of attraction between the opposite charges keeps the electrons bound to the nucleus — this attraction prevents them from flying away, just as the Sun’s gravity keeps planets in orbit.
Q7 Assertion (A): The discovery of subatomic particles helped in understanding the atomic structure. Reason (R): The number of electrons is equal to the number of protons in an atom. Choose: (i) Both A and R are true, and R is the correct explanation of A. (ii) Both A and R are true, but R is not the correct explanation of A. (iii) A is true, but R is false. (iv) A is false, but R is true.
Show Answer
Option (ii). A is true (electrons, protons, neutrons revealed atomic structure) and R is true (neutral atoms have p = e) — but R states a property of neutral atoms; it does not explain WHY discovering subatomic particles helped understand structure.
Q8 Magnesium is essential for many biological processes, including muscle contraction. For an atom of magnesium with a mass number of 24 and atomic number 12, determine the number of (i) protons, (ii) neutrons, (iii) electrons, and also illustrate the arrangement of electrons in a magnesium atom.
Q9 Find the following information for the elements shown in Fig. 8.17: (i) Name of the element (ii) Symbol (iii) Total number of electrons (iv) Number of valence electrons (v) Valency of the element (vi) Number of protons (vii) Atomic number.
Q10 Both Rutherford’s and Bohr’s models have electrons orbiting the nucleus. Why did Rutherford’s model fail to explain atomic stability, while Bohr’s model succeeded?
Show Answer
Rutherford: an electron in a circular path is accelerating → it should continuously lose energy, spiral inward, and fall into the nucleus → atom collapses. He had no answer for why this doesn’t happen.
Bohr: introduced stationary states as a postulate — electrons revolve only in fixed allowed orbits where their energy remains constant. No energy loss in a fixed shell → no spiralling → atom is stable.
Q11 An atom 70X has 31 electrons. How many neutrons are there in its nucleus?
Show Answer
Protons = electrons = 31 (neutral atom). Neutrons = A − p = 70 − 31 = 39 neutrons.
Q12 An atom has 79 protons and a mass number of 197. Calculate (i) the number of neutrons, and (ii) the number of electrons.
Show Answer
(i) Neutrons = 197 − 79 = 118. (ii) Electrons = protons = 79 (neutral atom). [This is gold, Au!]
Q13 Complete the Table 8.5 (Atomic number · Mass number · Neutrons · Protons · Electrons · Name).
Show Answer
Atomic number
Mass number
Neutrons
Protons
Electrons
Element
5
11
6
5
5
Boron
7
14
7
7
7
Nitrogen
12
24
12
12
12
Magnesium
15
31
16
15
15
Phosphorus
1
1
0
1
1
Hydrogen
(Bold values = the blanks filled in. Working: A = p + n; Z = p = e.)
Q14 Aman learnt that an element X has a mass number of 35 and contains 18 neutrons. Answer: (i) How many electrons and protons does X have? (ii) What is its atomic number? (iii) Identify the element X. (iv) Write its electronic configuration. (v) How many valence electrons does it have? (vi) What will be the mass number if two neutrons are added to its nucleus? (vii) What will be the relation of X with the new atom?
Show Answer
(i) Protons = A − n = 35 − 18 = 17; electrons = 17.
(ii) Atomic number Z = 17.
(iii) X is Chlorine (Cl).
(iv) Electronic configuration: 2, 8, 7.
(v) Valence electrons = 7.
(vi) New mass number = 35 + 2 = 37.
(vii) Same Z (17), different A (35 vs 37) → they are ISOTOPES.
Q15 In an atom, there are 12 protons and 12 neutrons in the nucleus. Now, imagine that all the electrons are replaced with some hypothetical particles that have the same charge as electrons but are 500 times heavier. What effect will this replacement have on the atom’s: (i) Atomic number (ii) Atomic mass (iii) Mass number (iv) Overall charge.
Show Answer
(i) Atomic number: unchanged (12) — Z depends only on protons.
(ii) Atomic mass: increases — the heavy particles’ mass (500 × electron mass each) is no longer negligible, so the total mass of the atom goes up.
(iii) Mass number: unchanged (24) — A counts only nucleons (protons + neutrons), not the orbiting particles.
(iv) Overall charge: unchanged (neutral) — the particles carry the same charge as electrons, so 12 positive protons still balance 12 negative particles.
The Journey Beyond (NCERT Activities)
Create an ‘Atomic Prediction Board’ game based on atomic number, mass number, number of electrons, protons, neutrons and valency. Students may predict elements using atomic clues.
Prepare a report on how the properties of atoms impact us in everyday life across fields, such as healthcare, energy, agriculture and technology.
Create a role-play, stage play or story about the ‘Journey Inside the Atom’, and the scientists who discovered and contributed to the identification of atomic structure.
Use selected software or digital tools and try to create animations or simulations of various atomic models, and share them in the class.
Watch a film or documentary about the structure of the atom and write a report answering: which film and its main idea; what it taught about atomic structure and model(s); which scientists were mentioned and their contributions; the most interesting part and a question you still have.
Draw a bar graph showing the number of electrons in each energy level for any three elements.
To learn more about atoms, explore: phet.colorado.edu/en/simulations/rutherford-scattering and phet.colorado.edu/en/simulations/isotopes-and-atomic-mass
🗝 The Quest Continues …
Is it possible to completely understand everything that happens inside an atom?
