Zeff = Z - sigma, where Z = atomic number and sigma = shielding constant. Slater's rules give sigma: (1) Electrons in same group (n,l): each contributes 0.35 (0.30 for 1s). (2) Electrons in (n-1) shell: 0.85 each for s/p electrons being shielded; 1.00 each for d/f electrons being shielded. (3) Electrons in (n-2) or lower: 1.00 each. Penetration order: s > p > d > f (s-electrons get closest to nucleus, experience least shielding). This is why 4s fills before 3d but IE removes 4s first (4s has lower Zeff in a multi-electron atom due to greater shielding from 3d). In JEE, Slater's rules are used for calculating Zeff to explain trends.