Structure of Atom

NCERT Class 11 Chemistry Chapter 2

Structure of Atom for Class 11 Chemistry, JEE and NEET: Bohr’s model, spectra, dual behaviour, uncertainty principle, quantum numbers and configurations.

6 topics Updated 8 Nov 2026

For: Class 11, JEE and NEET, unless a topic says otherwise

In the syllabus: JEE Class 11 syllabus, NEET Class 11 syllabus

With your textbook

NCERT Chemistry Part I (Class 11), Chapter 2 “Structure of Atom”, reprint 2026-27.

Open the official NCERT chapter (PDF, 5.5 MB, ncert.nic.in) (opens in a new tab)

Checked on 19 Oct 2026.

Learn in this order

Discovery of sub-atomic particles
Cathode rays, Thomson’s e/m experiment, Millikan’s oil drop, canal rays and Chadwick’s neutron.

Class 11 only NCERT Section 2.1 13 min read 4 solved examples 6 practice questions
Atomic models
Thomson’s plum pudding model, Rutherford’s gold foil experiment and nuclear model, mass number, isotopes and isobars.

Class 11 only NCERT Section 2.2 15 min read 5 solved examples 7 practice questions
Developments leading to Bohr’s model
Work function, dual nature of light, hydrogen spectrum and solved examples.

NCERT Section 2.3 20 min read 6 solved examples 10 practice questions
Bohr’s model for the hydrogen atom
Postulates, derivation of orbit radius and energy, hydrogen-like ions, the hydrogen spectrum and limitations.

NCERT Section 2.4 19 min read 6 solved examples 8 practice questions
Towards the quantum mechanical model
Dual nature of matter, NCERT problems and why Bohr’s model fails.

NCERT Section 2.5 19 min read 6 solved examples 10 practice questions
Quantum mechanical model of the atom
Quantum numbers, shapes and nodes of s, p and d orbitals, the n + l rule, aufbau, Pauli and Hund, and electronic configurations for Class 11, JEE and NEET.

NCERT Section 2.6 28 min read 6 solved examples 14 practice questions

Key formulas in this chapter

Sub-atomic particles

e/m_ₑ = 1.758820 × 10^¹¹ C kg^⁻¹
e = 1.602176634 × 10^⁻¹⁹ C (exact); |q| = ne (n = 0, 1, 2, …)
m_ₑ = 9.1094 × 10^⁻³¹ kg
Proton 1.6726 × 10^⁻²⁷ kg; neutron 1.6749 × 10^⁻²⁷ kg
Proton mass ≈ 1836 × electron mass

Atomic models

Z = number of protons = number of electrons (neutral atom)
A = Z + number of neutrons; neutrons = A − Z
Electrons in an ion = Z − charge (cation), Z + size of charge (anion)
Nucleus about 10^⁻¹⁵ m; atom about 10^⁻¹⁰ m

Developments leading to Bohr’s model

c = νλ; ν̄ = 1/λ
E = hν = hc/λ; h = 6.626 × 10^⁻³⁴ J s
hν = hν_₀ + ½m_ₑv^²; W_₀ = hν_₀
KEmax = eV_₀ (stopping potential)
E (eV) ≈ 1240/λ (nm)
ν̄ = 109 677 (1/n_₁^² − 1/n_₂^²) cm^⁻¹

Bohr’s model

ν = |E_f − E_i|/h (photon frequency)
m_ₑvr = nh/2π
r_ₙ = n^²a_₀, a_₀ = 52.9 pm; r_ₙ = 52.9 n^²/Z pm
E_ₙ = −2.18 × 10^⁻¹⁸ Z^²/n^² J (−13.6 Z^²/n^² eV)
v_ₙ = 2.19 × 10^⁶ Z/n m s^⁻¹; KE: PE: E = 1: −2: −1
ΔE = 2.18 × 10^⁻¹⁸ Z^² × (1/n_₁^² − 1/n_₂^²) J, n_₁ lower, n_₂ upper (absorbed going up, emitted coming down)
ν̄ = 1.09677 × 10^⁷ Z^² × (1/n_₁^² − 1/n_₂^²) m^⁻¹ (n_₁ lower, n_₂ upper)
Lines from n_₂ down to n_₁ (many atoms) = (n_₂ − n_₁)(n_₂ − n_₁ + 1)/2; to the ground state, n(n − 1)/2

Towards the quantum mechanical model

λ = h/mv = h/p
λ = h/√(2mK); electron through V volts: λ ≈ 1.23/√V nm
2πr = nλ (gives m_ₑvr = nh/2π)
Δx × Δp ≥ h/4π
Δx × Δv ≥ h/(4πm)

Quantum mechanical model

Ĥψ = Eψ
Shell n: n subshells, n^² orbitals, 2n^² electrons
l = 0, 1, …, n − 1; m_ₗ = −l, …, +l (2l + 1 values); m_ₛ = ±½
Nodes: radial n − l − 1, angular l, total n − 1
(n + l) rule: lower n + l, lower energy; equal n + l, lower n first
Exchanges = k(k − 1)/2 for each set of k parallel electrons; add the two spin sets