the number of electrons that can fill the first four shells

atomic orbitals, including:

(i) as a region around the nucleus that can hold up to two electrons, with opposite spins
(ii) the shapes of s- and p-orbitals
(iii) the number of orbitals making up s-, p- and d-sub-shells, and the number of electrons that can fill s-, p- and d-sub-shells

filling of orbitals:

(i) for the first three shells and the 4s and 4p orbitals in order of increasing energy
(ii) for orbitals with the same energy, occupation singly before pairing

Learners are expected to be familiar with the 'electrons in box' representations.

deduction of the electron configurations of:

(i) atoms, given the atomic number, up to Z = 36
(ii) ions, given the atomic number and ionic charge, limited to s- and p-blocks up to Z = 36.

Learners should use sub-shell notation, i.e. for oxygen: 1s²2s²2p⁴.