Consider the following five chemical species:

NH₃, AlCl₃, BeCl₂, CCl₄ and PCl₅

How many of these species violate the octet rule by having either fewer or more than
eight electrons in the valence shell of their central atom?

Which pair of ions among the following list do not constitute an iso-electronic pair?

1. Mn²⁺, Fe³⁺

2. Fe²⁺, Mn²⁺

3. O^2–, F^–

4. Na⁺, Mg²⁺