Application only for single electron species. For 2S ⇒ n=2 Order of energy E2s(H)>E2s(Li)>E2s(Na)>E2s(K) (ii) Maximum number of electron which can accommodate in a principal energy shell is equal to 2n2. (iii) Extra stability of half -field subshell is due to to higher exchange energy. (iv) Only two electron with opposite spin can exists in same orbital. So correct statement – (i), (ii).