Class 11 Chemistry – Structure of Atom

CBSE 2026–27 | NCERT-aligned free study resource

Chapter Overview

This chapter traces the discovery of sub-atomic particles and the development from early atomic models to the quantum mechanical model.

Sub-atomic Particles

Electrons, protons and neutrons form the basic structure of atoms. Thomson established the existence of electrons; Rutherford’s scattering experiment provided evidence for a small, dense nucleus; Chadwick discovered the neutron.

Atomic Models

Thomson proposed a positive sphere containing electrons. Rutherford proposed a nuclear model with most of the atom empty space. Bohr introduced quantised energy levels for hydrogen-like atoms.

Bohr Model

Electrons occupy permitted stationary orbits. Angular momentum is quantised. Radiation is emitted or absorbed when an electron moves between allowed energy levels. The model successfully explains the hydrogen spectrum but has limitations for multi-electron atoms.

Quantum Mechanical Model

Modern theory describes electrons using wave functions and probability. Heisenberg’s uncertainty principle states that position and momentum cannot both be known with unlimited precision. Orbitals are regions of high probability of finding an electron.

Quantum Numbers

Principal quantum number n describes the shell; azimuthal quantum number l describes subshell shape; magnetic quantum number m_l describes orbital orientation; spin quantum number m_s describes electron spin.

Orbitals and Filling

s orbitals are spherical; p orbitals have directional lobes. Electron configurations follow the Aufbau principle, Pauli exclusion principle and Hund’s rule.

MCQs

1. The nucleus was established by: (A) Rutherford scattering (B) cathode ray experiment (C) oil drop experiment (D) photoelectric effect. Answer: A.
2. For a p subshell, l equals: (A) 0 (B) 1 (C) 2 (D) 3. Answer: B.
3. Maximum electrons in an orbital are: (A) 1 (B) 2 (C) 6 (D) 10. Answer: B.
4. Hund’s rule concerns: (A) equal-energy orbitals (B) nuclear size (C) molar mass (D) isotopes. Answer: A.

Practice

1. Compare Rutherford and Bohr models.
2. Explain all four quantum numbers.
3. Write configurations for selected atoms and ions.
4. Explain why the quantum mechanical model replaced the Bohr model.

HOTS

Explain why two electrons in the same orbital must have opposite spins using the Pauli exclusion principle.

Revision

Remember: nucleus → Bohr levels → wave nature → uncertainty → orbitals → four quantum numbers → electron configuration.