Following the order below to fill the orbitals of the first 36 elements in the Period Table:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, …
A maximum of two electrons are put into orbitals in the order of increasing orbital energy: the lowest-energy orbitals are filled before electrons are placed in higher-energy orbitals.
Exp. 11Na: 1s2, 2s2, 2p6, 3s1 or [Ne] 3s1
