Octet rule: A rule stating that atoms lose, gain, or share electrons in order to have a full valence shell of 8 electrons. (Hydrogen is excluded because it can hold a maximum of 2 electrons in its valence shell. )

What is it called when the valence shell is full?

In general, atoms are most stable, least reactive, when their outermost electron shell is full. Most of the elements important in biology need eight electrons in their outermost shell in order to be stable, and this rule of thumb is known as the octet rule.

Why do valence shells need to be full?

Atoms want a full outer shell because it completes all the gaps in the outside. With no gaps, other electrons don't want to fit into those spaces. For example, a noble gas like Neon has a full outer shell. It doesn't react with other chemicals because the electrons from other chemicals can't fall in anywhere.