The increasing order of the first ionization enthalpies of the elements B, P, S and F (lowest first) is:

- F is the smallest and has the highest nuclear charge among these, so it has the highest IE.B is in period 2, but its IE is lower than period 3 elements like P and S because IE decreases significantly down a group but increases across a period (B is far to the left).Between P and S: P ($3s^{2}3p^3$) has a half-filled $p$ -subshell which is stable. S ($3s^{2}3p^4$) has one electron more than half-filled, which is easier to remove due to electron-electron repulsion. Thus, ${\text{IE}}_P>{\text{IE}}_S$.Order: B < S < P < F.