Question

Why do elements in the same group have a similar reactivity?

Answer 1

Because to a first approximation, all elements of the same group have the same valence electronic shell.

Explanation:

The valence shell of the carbon atom (`Z=6`) is described as `2s^(2)2p^2`; that of silicon (`Z=14`), `3s^(2)3p^2`. Given this electronic configuration, where the silicon atoms have orbitals of precisely the same symmetry as does carbon, we might expect that the chemistry of silicon and carbon would be similar, and we would be right.

Of course, the Periodic Table necessarily groups atoms with the valence shell into the same column, the same Group. The common chemistry of the Group is not an accident.