A complete visual guide — from the discovery of electrons, protons and neutrons to Thomson, Rutherford and Bohr models, atomic & mass number, electronic configuration, valency, and isotopes & isobars.
📌 Chapter 2
Sub-atomic Particles
Atomic Models
Electronic Configuration
Valency
Isotopes & Isobars
150 Practice Qs
Section 2.1 – 2.2
The Atom and Its Size
The idea of the smallest particle of matter goes back to the Indian philosopher Maharishi Kanad ("parmanu") and the Greek Democritus ("atomos" = indivisible). We now know the atom is divisible — it has an inner structure of sub-atomic particles.
⚛ What is an atom?
An atom is the smallest unit of an element which may or may not exist independently, but always takes part in a chemical reaction. Atoms are so small that over a million of them stacked together would only be as thick as a sheet of paper.
Atomic size
~0.1 nm
Measured as a radius in nanometres; 1 nm = 10⁻⁹ m. Hydrogen \u2248 0.037 nm.
Seen by
STM
A Scanning Tunnelling Microscope shows computer-generated images of atoms indirectly.
1 u
1/12 of C-12
One unified atomic mass unit = 1/12 the mass of one carbon-12 atom.
❖ Atomic mass
The atomic mass of an element is the number of times one atom is heavier than 1/12 of a carbon-12 atom. Carbon-12 is the modern standard because hydrogen's natural isotopes give an awkward average of 1.008 u.
💡 Did You Know?
The gram-atomic mass of an element holds one mole of atoms = 6.022 × 10²³ atoms. So 12 g of carbon and 14 g of nitrogen each contain 6.022 × 10²³ atoms.
Section 2.3 – 2.5
The Three Sub-atomic Particles
An atom is built from electrons (negative), protons (positive) and neutrons (neutral). Protons and neutrons sit in the central nucleus; electrons revolve around it in shells.
Thomson's discharge-tube experiment that discovered the electron
Particle
Symbol
Charge
Relative mass
Location
Discovered by
Electron
e⁻
−1
1/1836 u (negligible)
Revolves around nucleus
J. J. Thomson (1897)
Proton
p⁺
+1
1.007 u
In the nucleus
E. Goldstein (canal rays)
Neutron
n⁰
0
1.008 u
In the nucleus
James Chadwick (1932)
Cathode rays → electron
At low pressure and high voltage, negative particles stream from the cathode. Their nature is the same for every gas, so all atoms contain electrons.
Canal (anode) rays → proton
Goldstein's perforated cathode produced positive rays moving the opposite way. With hydrogen these are bare protons — the positive particle in every atom.
❖ Why a neutron was needed
Nitrogen has 7 protons but a mass of 14 u — the protons alone can't account for the mass. Chadwick proposed a neutral particle of nearly proton mass: the neutron. (Ordinary hydrogen, ¹H, is the only atom with no neutron.)
Section 2.6
Thomson's Model
The first model of how the particles are arranged — often called the "plum pudding" (or watermelon) model.
Positive sphere with electrons embedded like seeds in a watermelon
The idea
The atom is a sphere of positive charge with negatively charged electrons embedded in it, so the total positive and negative charges balance and the atom is electrically neutral.
⚠️
Drawback: it could not explain the results of Rutherford's scattering experiment — it had no dense central nucleus.
Section 2.7
Rutherford's Model & the Nucleus
Rutherford fired fast, positive alpha particles (helium nuclei, mass 4 u, charge +2) at a very thin gold foil and watched where they went on a zinc-sulphide screen.
Alpha-particle scattering: most pass straight (atom is mostly empty), a few deflect (small dense positive nucleus), very few rebound
1
Most pass straight → the atom is mostly empty space.
2
A few deflect → there is a concentrated positive charge in the atom.
3
Very few rebound → that positive mass is in a tiny, dense nucleus.
Rutherford's atom
A small, dense, positively charged nucleus at the centre (holding the protons) with electrons revolving around it in orbits; the atom is mostly empty and electrically neutral.
⚠️
Drawback: a revolving charged electron should continuously radiate energy and spiral into the nucleus — so this model could not explain the atom's stability.
💡 Did You Know?
If an atom were blown up to the size of a cricket stadium (~100 m), the nucleus would be just a pea at the centre — the atom is almost entirely empty space, with nearly all its mass packed into the tiny nucleus.
Section 2.8
Bohr's Model
Niels Bohr (1913) fixed the stability problem: electrons revolve only in certain fixed orbits (shells / energy levels) and do not lose energy while doing so.
Bohr's atom: electrons occupy discrete shells (energy levels); they jump between shells only by absorbing or emitting a fixed amount of energy
Electrons revolve in fixed orbits called energy levels or shells: K, L, M, N (or 1, 2, 3, 4).
While in a shell, an electron does not radiate energy — this explains the atom's stability.
The shell nearest the nucleus (K) has the lowest energy; energy rises outward.
An electron jumps to a higher shell by absorbing energy and falls to a lower shell by emitting it.
Section 2.9
Atomic Number & Mass Number
Two numbers fix the identity of any atom: how many protons it has, and how many protons plus neutrons.
An atom is written ᵃX with mass number A on top-left and atomic number Z on bottom-left (e.g. ²³Na with Z = 11)
Key relations
Z (atomic number) = number of protons = number of electrons (neutral atom) A (mass number) = protons + neutrons Number of neutrons = A − Z
Worked ExampleSodium, ²³Na (Z = 11)
Protons = 11, electrons = 11, neutrons = A − Z = 23 − 11 = 12. The atomic number stays 11 even when sodium loses an electron to form Na⁺ (protons don't change in a chemical reaction).
Section 2.10 – 2.12
Electronic Configuration (Bohr–Bury Scheme)
Rules that decide how many electrons go into each shell, and in what order.
The 2n² rule
Maximum electrons in a shell = 2n² K (n=1)=2 L (n=2)=8 M (n=3)=18 N (n=4)=32
But the outermost shell can hold at most 8 electrons, and the penultimate shell at most 18.
Fill inner shells first — electrons enter a new shell only after the inner ones are appropriately filled.
A new shell starts forming once the outermost shell reaches 8 (which is why calcium is 2,8,8,2 not 2,8,10).
Na → K=2, L=8, M=1
Cl → K=2, L=8, M=7
❖ Valence electrons
The electrons in the outermost (valence) shell are the valence electrons — they decide an atom's chemical behaviour and valency. Oxygen (2,6) has 6; chlorine (2,8,7) has 7; sodium (2,8,1) has 1.
Section 2.13
Cause of Combination & Valency
Noble gases are unreactive because their outermost shell is complete — a duplet (2, like helium) or an octet (8). Other atoms react to reach this stable arrangement by losing, gaining or sharing electrons.
Sodium donates its 1 valence electron (forming Na⁺) and chlorine accepts it (forming Cl⁻) — both reach a noble-gas octet
Working out valency
If valence electrons = 1, 2 or 3 → valency = number of valence electrons (metals, lose e⁻) If valence electrons = 5, 6 or 7 → valency = 8 − valence electrons (non-metals, gain e⁻) Complete duplet/octet → valency = 0 (noble gases)
Element
Config.
Valence e⁻
Valency
Sodium (Na)
2,8,1
1
1
Magnesium (Mg)
2,8,2
2
2
Aluminium (Al)
2,8,3
3
3
Carbon (C)
2,4
4
4 (shares)
Oxygen (O)
2,6
6
8−6 = 2
Chlorine (Cl)
2,8,7
7
8−7 = 1
Neon (Ne)
2,8
8
0
Electrovalency
Electrons are transferred (lost or gained), forming ions. Losing gives a positive ion (cation, e.g. Na⁺, Mg²⁺); gaining gives a negative ion (anion, e.g. Cl⁻, O²⁻).
Covalency
Electrons are shared. Carbon shares its 4 valence electrons (covalency 4, as in CH₄); in Cl₂ each chlorine shares 1; in O₂ each oxygen shares 2.
💡 Did You Know?
Some metals show variable valency. Iron is 2+ (ferrous, FeO) or 3+ (ferric, Fe₂O₃) — the lower valency takes the suffix -ous and the higher -ic, or Roman numerals Fe(II) and Fe(III).
Section 2.14 – 2.15
Isotopes & Isobars
Atoms can share an atomic number but differ in mass (isotopes), or share a mass number but differ in atomic number (isobars).
The three isotopes of hydrogen — same 1 proton, different numbers of neutrons
Isotopes
Isobars
Same
Atomic number (protons)
Mass number
Different
Mass number (neutrons)
Atomic number (protons)
Element
Same element
Different elements
Chemical properties
Identical
Different
Example
³₅Cl & ³⁷Cl
⁴⁰Ar & ⁴⁰Ca
Average atomic massChlorine: Cl-35 and Cl-37 in ratio 3 : 1
Average = (35×3 + 37×1) ÷ 4 = (105 + 37) ÷ 4 = 142 ÷ 4 = 35.5 u. This is why chlorine's atomic mass is the fraction 35.5 u — it's a weighted average of its isotopes, not the mass of any single atom.
☢ Uses of radioactive isotopes
U-235 — fuel in nuclear reactors for electricity.
Co-60 — treatment of cancer.
C-14 — dating fossils (carbon dating).
I-131 — treatment of goitre and thyroid disorders.
Solved Examples
Worked Sample Problems
Model answers in the CBSE style.
Example 1Helium has mass number 4 and 2 protons. How many neutrons?
Neutrons = mass number − atomic number = 4 − 2 = 2 neutrons.
Example 2Why does Na⁺ have completely filled K and L shells?
Sodium (Z=11) is 2,8,1. Losing its single outermost electron leaves 10 electrons → 2,8, i.e. completely filled K and L shells (the stable neon configuration).
Example 3Bromine occurs as ⁷⁹Br (49.7%) and ⁸¹Br (50.3%). Find its average atomic mass.
Average = (79×49.7 + 81×50.3) ÷ 100 = (3926.3 + 4074.3) ÷ 100 = 8000.6 ÷ 100 ≈ 80 u.
Glossary
Key Terms
Quick definitions for fast revision.
Atom
The smallest particle of an element that takes part in a chemical reaction.
Electron / Proton / Neutron
Negative particle outside the nucleus / positive particle in the nucleus / neutral particle in the nucleus.
Cathode rays / Canal (anode) rays
Streams of electrons / streams of positive particles in a discharge tube.
Nucleus
The small, dense, positively charged centre of an atom containing protons and neutrons (nucleons).
Atomic number (Z)
Number of protons in the nucleus (= electrons in a neutral atom).
Mass number (A)
Total number of protons and neutrons in the nucleus.
Electronic configuration
The distribution of electrons in the shells of an atom.
Valence shell / Valence electrons
The outermost shell / the electrons in it, which decide chemical behaviour.
Valency
The number of electrons an atom loses, gains or shares to attain a stable octet/duplet.
Electrovalency / Covalency
Valency by transfer of electrons (ionic) / by sharing of electrons (covalent).
Isotopes
Atoms of the same element with the same atomic number but different mass numbers.
Isobars
Atoms of different elements with the same mass number but different atomic numbers.
Duplet / Octet
A stable valence shell of 2 electrons (like helium) / of 8 electrons (other noble gases).
Exam Strategy
Top Exam Tips
High-yield reminders that catch most students out.
1
Neutrons = A − Z
Mass number minus atomic number. Don't confuse mass number (A) with atomic mass.
2
Z never changes in a reaction
Electrons can be lost/gained (forming ions), but the proton count — the element's identity — stays fixed.
3
2n² with a cap of 8 outside
Shells hold 2n², but the outermost can never exceed 8 — that's why Ca is 2,8,8,2.
4
Metal vs non-metal valency
1–3 valence e⁻ (metal): valency = that number. 5–7 (non-metal): valency = 8 minus that number.
5
Isotopes vs isobars
IsoTOPES = same element, different mass; IsoBARS = same mass, different elements. Match the "same" carefully.
6
Rutherford → nucleus; Bohr → stability
Scattering found the nucleus; Bohr's fixed shells explained why the atom doesn't collapse.
7
Fractional atomic mass = isotopes
Cl is 35.5 u because it's a weighted average of Cl-35 and Cl-37, not any single atom.
8
Noble gases → valency 0
Complete duplet (He) or octet means no tendency to lose, gain or share — hence unreactive.
Self-Evaluation
Practice Question Bank — 150 Questions
CBSE-pattern questions across all five sections with toggle answers. Section A: 50 MCQ & Assertion-Reason (1 mark) · Section B: 30 Very Short Answer (2 marks) · Section C: 30 Short Answer (3 marks) · Section D: 20 Long Answer (5 marks) · Section E: 20 Case-Based (4 marks).