Answer: All the elements in a group have the same number of valence electrons, so they show the same valency.
The group number in the modern periodic table is decided by the number of electrons in the outermost shell. Since every element of a group has the same number of valence electrons, its combining capacity — the number of electrons it loses, gains or shares — is the same. For example, all group 1 elements (Li, Na, K) have one valence electron and show valency 1.