Valence electrons predict the formation of bonds based on how MANY electrons are placed on the elements outer shell.
Initially, all elements want an outer valence electron shell of 8 electrons. This is called the Octet Rule.
Here’s the key part: In order to gain those 8 valence electrons there must either be a LOSS or a GAIN in electrons. The elements that give give up their electrons are metals, while nonmetals gain the electrons that metals offer to them. This way, they’re able to form a bond together that makes them “wholesome.” (FULL 8 electron outer shell)
It’s important to keep in mind the families of the periodic table; alkali metals, alkaline earth metals, transition metals, halogens, etc,...
For example:
Oxygen is a nonmetal and has a valence electron shell of six, therefore it will want to gain two of its valence electrons to make a full outer shell.
In order to make a wholesome bond, oxygen MUST bond with an element in the Alkaline Earth Metal family in order to be whole since Alkaline Earth Metals have a valence electron outer shell of two. Being a metal, they will want to give away their two valence electrons.
Let’s just say that oxygen decided to pair with beryllium.
Therefore the bond would be;
BeO