Atomic Structure & Electron Configuration

Orbitals, energy levels, and periodic trends

Atoms consist of a nucleus (protons + neutrons) surrounded by electrons in quantum orbitals. Principal quantum number n defines energy shells. Subshells: s (1 orbital, 2e), p (3 orbitals, 6e), d (5 orbitals, 10e), f (7 orbitals, 14e). Electron configuration follows the Aufbau principle (fill lowest energy first), Pauli exclusion (max 2e per orbital, opposite spins), and Hund's rule (fill each orbital singly before pairing). Orbital shapes: s = sphere, p = dumbbell (3 orientations), d = cloverleaf (5 orientations). Periodic trends: atomic radius decreases across a period (increasing nuclear charge pulls electrons in) and increases down a group (more shells). Ionization energy (IE) and electronegativity (EN) show opposite trends to radius.