Summary
NCERT Class 12 Physics Chapter 12 covers atomic models — from Thomson's plum pudding model and Rutherford's nuclear model (derived from the gold foil alpha-particle scattering experiment) to Bohr's quantised orbit model, which successfully explains the hydrogen atom's discrete line spectrum and ionisation energy of 13.6 eV.
Chapter 12 of NCERT Class 12 Physics (Part II) traces the development of atomic models. J. J. Thomson proposed the plum pudding model in 1898. Rutherford's 1911 gold-foil alpha-particle scattering experiment revealed a tiny, dense, positively charged nucleus at the atom's centre, with electrons revolving around it. To resolve the classical instability of this model, Niels Bohr (1913) introduced three postulates: electrons occupy stable stationary orbits, angular momentum is quantised as L = nh/2π, and photons are emitted when electrons transition between orbits with energy hν = Ei − Ef. The Bohr model gives hydrogen energy levels En = −13.6/n² eV and correctly predicts the ionisation energy and line spectrum. De Broglie later explained Bohr's quantisation condition through electron wave-particle duality.
Key points & formulas
- 01Rutherford's alpha-particle scattering experiment (Geiger-Marsden, 1911) used 5.5 MeV alpha-particles on a thin gold foil; only ~1 in 8000 deflected by more than 90°, proving the nucleus is tiny (~10⁻¹⁵ m) and dense
- 02Bohr's first postulate: electrons revolve in stable stationary orbits without radiating energy, contradicting classical electromagnetic theory
- 03Bohr's second postulate: angular momentum of the orbiting electron is quantised — L = nh/2π, where n is the principal quantum number
- 04Bohr's third postulate: a photon of frequency ν is emitted when an electron transitions from a higher to a lower energy state, with hν = Ei − Ef
- 05Energy of the nth orbit in hydrogen is En = −13.6/n² eV; ground state (n = 1) energy is −13.6 eV, giving an ionisation energy of 13.6 eV
- 06De Broglie explained Bohr's quantisation by treating the orbiting electron as a standing wave: the orbit circumference must equal a whole number of de Broglie wavelengths (2πrn = nλ)
Frequently asked questions
01What did Rutherford's alpha-particle scattering experiment prove about atomic structure?
The Geiger-Marsden experiment showed that most alpha-particles passed straight through a thin gold foil, but about 1 in 8000 deflected by more than 90°. Rutherford concluded that almost all of an atom's mass and its entire positive charge are concentrated in a tiny nucleus (about 10⁻¹⁵ m), while the rest of the atom is largely empty space, with electrons revolving far from the nucleus.
02What are Bohr's three postulates for the hydrogen atom?
(1) Electrons revolve in certain stable stationary orbits without emitting radiation. (2) Only orbits where angular momentum L = nh/2π (n = 1, 2, 3…) are allowed — angular momentum is quantised. (3) When an electron transitions from a higher energy state Ei to a lower energy state Ef, a photon is emitted with frequency ν given by hν = Ei − Ef.
03What is the ground state energy and ionisation energy of the hydrogen atom according to Bohr's model?
According to Bohr's model, the energy of the nth orbit is En = −13.6/n² eV. For the ground state (n = 1), E₁ = −13.6 eV. The ionisation energy — the minimum energy needed to free the electron from the ground state — is therefore 13.6 eV, which matches the experimentally observed value.
04Is the NCERT Class 12 Physics Chapter 12 PDF free to download?
Yes, the NCERT Class 12 Physics Part II Chapter 12 (Atoms) PDF is completely free to download on cbseprepmaster.com.
More chapters in Physics Part II
This is the complete Physics Part II Chapter 12 as published by NCERT — every diagram, solved example, and exercise included, free. Browse all CBSE Class 12 textbooks.
Read offline with notes, solutions & mock tests
CBSE Prepmaster — free on iOS & Android