Aufbau principle: electrons fill the lowest available energy orbital first. Order determined by (n+l) rule: lower (n+l) fills first; if (n+l) is equal, lower n fills first. Filling order: 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p < 5s < 4d < 5p < 6s < 4f < 5d < 6p < 7s < 5f < 6d < 7p. Note: 4s fills before 3d (both have n+l=4, but 4s has lower n... actually 3d has lower n but n+l for 4s = 4 and 3d = 5, so 4s fills first). Wait: 4s: n+l = 4+0 = 4. 3d: n+l = 3+2 = 5. So 4s < 3d. But 4s ionises before 3d — in cations, 4s electrons are lost first. This is a common JEE trap: filling order is NOT the same as ionisation order.