# Atoms Notes, Class 12 Physics | SwaVid Atoms: NCERT Class 12 Physics notes with key ideas, formulas, common mistakes and 3 quick checks with worked answers. Written by SwaVid from the textbook. Canonical: https://www.swavid.com/cbse/class-12/physics/chapter/atoms Source: https://www.swavid.com/cbse/class-12/physics/chapter/atoms # Atoms ## Formulas and definitions ## Key ideas ## Common mistakes ## Quick checks ## Keep going ## Questions parents ask ## Class 12 Physics on SwaVid ### Alpha-particle scattering experiment ### Impact parameter and trajectory ### Rutherford atomic model and its limits ### Bohr model postulates ### Hydrogen energy levels and radii ### Hydrogen spectral series ### De Broglie explanation of Bohr postulate ### Next chapter ### Related chapter ### All chapters SwaVid › CBSE › Class 12 › Physics › Atoms Notes written by SwaVid from the NCERT textbook. Class 12 Physics, Chapter 12. Key ideas, formulas and common mistakes. CBSE prints 12 marks for Units VII-VIII together, not for this chapter alone. Short answer. This chapter describes the structure of the atom through alpha-particle scattering experiments and Bohr's model of the hydrogen atom. It explains how quantized energy levels account for discrete spectral series and atomic stability. Source: NCERT Physics Part II (Reprint 2026-27) , Chapter 12, Atoms Geiger and Marsden directed alpha-particles at a thin gold foil under Rutherford's guidance. Most particles passed straight through, but a tiny fraction deflected by more than 90°, showing that the positive charge and most atomic mass are packed into a tiny, dense nucleus. The trajectory of an alpha-particle depends on its impact parameter, which is the perpendicular distance of its initial velocity vector from the nucleus center. A smaller impact parameter produces a larger scattering angle, while a head-on collision causes a rebound. Rutherford pictured the atom as a central nucleus orbited by electrons, held by Coulomb attraction. According to classical electromagnetic theory, accelerating orbiting electrons must continuously radiate energy and spiral into the nucleus, which fails to explain atomic stability and discrete line spectra. Bohr proposed that electrons occupy stable, non-radiating orbits where orbital angular momentum is an integral multiple of h / (2π). Radiation is emitted or absorbed only when an electron jumps between two stable states, with photon energy equal to the energy difference between these states. For hydrogen, the orbit radius scales as n² and electron orbital speed scales as 1 / n. Total energy is negative and scales inversely with n², meaning the electron is bound to the nucleus, with -13.6 eV in the ground state. Transitions between quantized energy levels produce distinct spectral series: Lyman in the ultraviolet, Balmer in the visible, and Paschen, Brackett, and Pfund in the infrared region. The wavenumber of each spectral line is calculated using the Rydberg formula. De Broglie explained the angular momentum quantization by treating the orbiting electron as a standing matter wave. A stationary orbit forms only when the circumference of the circular orbit contains an integral number of de Broglie wavelengths. Try each question first. Then tap it to see the answer. Kinetic energy K = -E = -(-13.6 eV) = 13.6 eV Potential energy U = 2E = 2 × (-13.6 eV) = -27.2 eV K = 13.6 eV, U = -27.2 eV Use rₙ = n² a₀ with n = 3 r₃ = 3² × (5.3 × 10⁻¹¹ m) = 9 × 5.3 × 10⁻¹¹ m = 4.77 × 10⁻¹⁰ m 4.77 × 10⁻¹⁰ m 1 / λ = R ((1 / 1²) - (1 / ∞²)) = R × 1 λ = 1 / R = 1 / (1.097 × 10⁷ m⁻¹) ≈ 9.116 × 10⁻⁸ m 91.2 nm Nuclei . Dual Nature of Radiation and Matter . The 14 chapters of Class 12 Physics . CBSE does not give marks to one chapter. It prints 12 marks for Units VII-VIII together (Dual Nature of Radiation and Matter; Atoms and Nuclei). Atoms is part of that group. No. They are written by SwaVid from the NCERT textbook. The CBSE curriculum is the reference for what is assessed. Nuclei. SwaVid is an AI tutor for Indian students in Classes 6 to 12. It finds the earlier ideas a child missed, then teaches today’s chapter. Start free to see how it works. - Ch 12 NCERT book - 12 marks, Units VII-VIII together - 3 quick checks - Using total energy instead of kinetic energy when calculating orbit velocity . Remember that total energy is negative (E = -K = U / 2). Kinetic energy is always positive, so set K = (1/2) m v² = |E| to find velocity. - Identifying the Balmer series as ending on n = 1 . The Lyman series terminates at n = 1 in the ultraviolet region. The Balmer series terminates at n = 2 and is the only series in the visible spectrum. - Confusing the shortest and longest wavelengths in a spectral series . The shortest wavelength (series limit) corresponds to the maximum energy transition where n₂ = ∞. 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