$word_{count}$: 190

Nodes = regions where psi = 0 (zero electron probability). Radial nodes: spherical surfaces at certain distances r. Count = n - l - 1. Angular nodes: planes or cones through the nucleus. Count = l. Total = n - 1. Examples: 1s (0 nodes), 2s (1 radial), 2p (1 angular), 3s (2 radial), 3p (1 radial + 1 angular), 3d (2 angular), 4f (3 angular, 0 radial). The probability density |psi|^2 at the nucleus: nonzero for s orbitals (l=0), zero for all others (angular node at nucleus). Radial distribution function P(r) = 4pi$r^2$$R^2$(r): always zero at r=0. Most probable distance: where P(r) is maximum. For 1s: $r_{mp}$ = $a_0$. For 2s: $r_{mp}$ = ~5.2 $a_0$. Average distance: = integral(rP(r)*dr). for 1s = 1.5 $a_0$ (larger than most probable distance). Node questions are among the most commonly tested topics in JEE atomic structure.