$word_{count}$: 200

Atomic structure contributes ~4% of JEE Main marks. High-priority topics: (1) Bohr model calculations — E, r, v for hydrogen-like species. Master E = -13.6$Z^2$/$n^2$ eV and the Rydberg formula. (2) Quantum number validity — quickly check l < n, |$m_l$| <= l. Count orbitals ($n^2$) vs electrons (2$n^2$). (3) Electronic configuration — know exceptions (Cr, Cu), cation configurations (remove 4s first), and Hund's rule for counting unpaired electrons. (4) Spectral lines — n$\frac{n-1}{2}$ formula and identifying the series. (5) De Broglie wavelength — lambda = 12.27/sqrt(V) angstroms for electrons. (6) Node counting — radial = n-l-1, angular = l. Common mistakes: (a) confusing orbits (Bohr) with orbitals (QM), (b) using stoichiometric coefficients as quantum numbers, (c) forgetting l < n (not <=), (d) removing 3d electrons before 4s in cation formation, (e) forgetting half-filled/fully-filled stability for Cr and Cu. Quick checks: E is always negative for bound electrons. Node total = n-1 (always). s orbital probability at nucleus is nonzero.