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.
Energy levels and spectral lines of hydrogen
This section elaborates on the discrete energy levels of the hydrogen atom as proposed by Bohr and their relation to the observed emission spectrum. The electron in a hydrogen atom can occupy only certain allowed energy states, each corresponding to a quantized orbit. The energy of the nth level is given by E_n = -13.6 eV / n², where n is the principal quantum number. The negative sign indicates that the electron is bound to the nucleus. Transitions of the electron from a higher energy level (n_i) to a lower energy level (n_f) result in the emission of a photon with energy equal to the difference between the two levels: E_photon = E_ni - E_nf. This energy corresponds to a specific wavelength λ = hc / E_photon, producing discrete spectral lines. The energy level diagram shows the ground state (n=1) as the most stable state. Ionization occurs when the electron gains enough energy to reach n = ∞, i.e., to escape the atom. The section also discusses the Balmer series and other spectral series observed in hydrogen.
📊 Diagram: See figure_9: FIGURE 12.7 The energy level diagram for the hydrogen atom. The electron in a hydrogen atom at room temperature spends most of its time in the ground state. To ionise a hydrogen atom an electron from
🧪 Activity: Calculation of spectral line wavelengths using energy level differences.
🔗 Connection: Leads to the wave nature of electrons and the de Broglie hypothesis.
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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