The Pauli exclusion principle states that two electrons can occupy the same orbital as long as they have opposite spin.
Hund's Rule states that the electrons fill the
orbitals singly before pairing as this minimises repulsion and requires less energy. For neutral atoms the 4s orbital is of a slightly lower energy than the 3d orbital. This can change when
ions are formed.
The lowest energy orbitals are filled first. Each orbital can hold a maximum of two electrons and the order of filling for the first thirty elements is: 1s 2s 2p 3s 3p 4s 3d.
There is one s orbital, three p orbitals (p
x, p
y, p
z), five d orbitals and seven f orbitals. The maximum number of electrons that can be held in the s,p,d and f orbitals is 2,6,10 and 14.