What is the valence of chemical elements, explain this with specific examples?

Valence (from Latin “having force”) – the ability of atoms of chemical elements to form a certain number of chemical bonds.
There are elements with constant valence (there are relatively few of them) and elements with variable valence (of which the majority).
Constant valence:
I-H, Na, Li, K, Rb, Cs
II-O, Be, Mg, Ca, Sr, Ba, Ra, Zn, Cd
III-B, Al, Ga, In
For example, manganese is in group VII (a side subgroup), the highest valency Mn is seven. Silicon is located in group IV (main subgroup), its highest valency is four.