Atoms | Class 12 Physics Notes
By ConceptScroll Team · Published on 17 July 2026 · 2 min read
Atoms – this guide gives you a concise, exam-ready overview of Atoms from Class 12 Physics, written by ConceptScroll editors and reviewed against the latest NCERT textbook.
Bohr’s model of the hydrogen atom
Niels Bohr proposed a revolutionary model of the hydrogen atom to explain the discrete emission spectra observed experimentally. Classical physics predicted that an electron orbiting the nucleus should continuously emit electromagnetic radiation due to acceleration, causing it to spiral into the nucleus and collapse the atom. This was contrary to observations that atoms are stable and emit light at discrete frequencies. Bohr introduced the concept of quantized orbits where electrons can revolve around the nucleus without radiating energy. These orbits correspond to specific energy levels. The electron can jump between these energy levels by absorbing or emitting a photon of energy equal to the difference between the levels. Bohr’s postulates explained the stability of atoms and the origin of spectral lines. The model successfully explained the hydrogen emission spectrum and introduced the idea of quantization in atomic physics.
📊 Diagram: See figure_6: FIGURE 12.5 Emission lines in the spectrum of hydrogen.; See figure_7: Niels Henrik David Bohr (1885 - 1962) Danish physicist who explained the spectrum of hydrogen atom based on quantum ideas. He gave a theory of nuclear fission based on the liquid-drop model of nucleus
🧪 Activity: Calculation of wavelengths of spectral lines using Bohr’s formula.
🔗 Connection: Leads to discussion on limitations of classical physics and the problem of atomic stability.
Frequently asked questions
The angular momentum of an electron in the third orbit of an atom is
3.15 x 10 -34 Js
As an electron makes a transition from an excited state to the ground state of a hydrogen-like atom/ion
its kinetic energy increases but potential energy and total energy decreases
An alpha particle of energy 5 MeV is scattered through 180 0 by a fixed uranium nucleus. The distance of closest approach is of the order of
10 -12 cm
De Broglie’s hypothesis gave an explanation for Bohr’s quantised orbits by bringing in the wave particle duality. According to this, the orbits correspond to circular standing waves in which the circumference of the orbit is
an integral multiple of wavelength
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