Question

The atomic number of elements A, B and C are (Z - 1), Z and (Z + 1) respectively. B is an inert gas (not helium). Which element has the lowest ionization energy? Why?

Answer 1

Ionisation energy is the amount of energy required to remove the valence electron from an atom.

As C is an alkali metal with atomic number Z+ 1 and has one electron in the valence shell. Therefore, it will readily loose the valence electron to achieve stable noble gas configuration.

Thus $C$ has the minimum ionisation energy.