How do valence electrons relate to the chemical reactions of an element?

Valance electrons are loosely held electrons of an atom. They are involve in chemical reaction. Consider the example of metals such as group two metals. All these have two valance electrons. They needed six electrons to complete the octet or loses two valance electrons to get complete octet. Thus its easier to remove two electrons than getting six electrons. These metals remove two electrons and form cations.

Now consider the example of nonmetals such group sixteen. They needed two electrons to get complete octet or remove six electrons to get complete octet. Thus its easier for them to get two electrons and they form anion. When group two metals cation and group sixteen anions combine they form compound and chemical reaction occur.

Group two metals also combine with halogens. Two halogens atoms combine with one alkaline earth metal atom to cancel the charge and make compound neutral.

They react with oxygen and form oxide.

2Ba + O₂ → 2BaO

2Mg + O₂ → 2MgO

2Ca + O₂ → 2CaO

Oxygen carry - 2 charge while Ca, Mg and Ba + 2 and make the compound neutral because charges are equal in magnitude.

With sulfur,

Mg + S → MgS

Ca + S → CaS

Ba + S → BaS

Sulfer carry - 2 charge while Ca, Mg and Ba + 2 and make the compound neutral because charges are equal in magnitude.