The bonding of two atoms will most likely occur if: a more stable state can result from the union the outer shell can be completely filled all the valance electrons are removed the electronegativity of both atoms are similar

A more stable state can result from the union.

Explanation:

Consider the following reaction:

Mg + Cl₂ → MgCl₂

when magnesium combine with chlorine it form magnesium chloride.

Magnesium is alkaline earth metal and present in group two. Magnesium has two valance electrons. Magnesium atom loses its two electron to complete the octet.  Chlorine is halogen element and have seven valance electrons. Chlorine needed one electron to complete the octet. When it combine with magnesium atom it accept the two electrons from magnesium atom that's why two chlorine atoms are combine with one magnesium atom to make the compound neutral.