Answer:
Because the highest s-orbital or p-orbital is partially filled.
Explanation:
All representative elements are part of the columns of the periodic table which contain the letter A, namely IA, IIA, IIIA, IVA, VA, VIA, VIIA, VIIIA.
These columns have as common feature to all, the fact that the valence layer has the highest energy sub-level S or P, having the same number representing the column as the valence number.
That is, the representative elements are all those that do not have their complete sub-levels. The family IA and IIA having the highest energy sublevel S, and the rest having the highest energy sublevel P.
