Amongst the following, the total number of species not having eight electrons around the central atom in its outermost shell is:${\text{NH}}_3,{\text{AlCl}}_3,{\text{BeCl}}_2,{\text{CCl}}_4,{\text{PCl}}_5$

Let's check the valence electrons for each:${\text{NH}}_3$: $3$ bonds $+1$ lone pair $=8$ electrons. (Follows octet)${\text{AlCl}}_3$: Aluminum has $3$ valence electrons, forms $3$ bonds $=6$ electrons. (Incomplete octet)${\text{BeCl}}_2$: Beryllium has $2$ valence electrons, forms $2$ bonds $=4$ electrons. (Incomplete octet)${\text{CCl}}_4$: Carbon has $4$ valence electrons, forms $4$ bonds $=8$ electrons. (Follows octet)${\text{PCl}}_5$: Phosphorus has $5$ valence electrons, forms $5$ bonds $=10$ electrons. (Expanded octet)The species that do not have exactly 8 electrons are ${\text{AlCl}}_3,{\text{BeCl}}_2,$ and ${\text{PCl}}_5$. Total $=3$.