Work out electron configuration instantly with clear inputs, formula shown and shareable results.
Electrons fill subshells in order of increasing n + l, the Madelung rule, which is why 4s fills before 3d. Each subshell holds 2(2l+1) electrons, so s holds 2, p holds 6, d holds 10 and f holds 14. Iron (Z = 26) therefore reads 1s2 2s2 2p6 3s2 3p6 4s2 3d6, with two electrons in the fourth shell. Ionic charge is applied by removing that many electrons before filling.
Filling order
Madelung rule: increasing n + l, then increasing n
Subshell capacity
capacity = 2(2l + 1)
Because 4s has the lower n + l sum (4 versus 5), so it lies lower in energy in the neutral atom despite the larger principal number.
No. A handful of elements promote an s electron to half-fill or fill the d subshell; those need to be corrected by hand.