JEE Main · 2026 · 05 Apr 2026 · Shift 1 · Atomic Structure

The uncertainty principle governs every moving microscopic particle, so it applies to the electron and statement A is correct. Orbital size grows with the principal quantum number, hence a $2p_x$ orbital is smaller than a $3p_x$ orbital and statement B is correct. Hydrogen is a one-electron atom whose orbital energy depends only on $n$, whereas in lithium the same $2s$ electron experiences a larger, screened nuclear charge, so the two $2s$ energies are unequal and statement C is wrong. Chromium takes the configuration $[\mathrm{Ar}]3d^{5}4s^1$ because the half-filled $3d$ set is extra stable, so statement D is correct. The correct statements are A, B and D.