The electrons on the outermost energy level of an atom are called valence electrons.
Valence electrons play a crucial role in determining how an atom will interact with others, particularly in the formation of chemical bonds. Atoms tend to seek a stable configuration, often adhering to the octet rule, whereby they prefer to have eight electrons in their outermost shell. This is the reason why valence electrons are so important in chemistry; they are the electrons that participate in chemical reactions.
For instance, if an atom has four valence electrons, it may tend to either gain four more, giving it a full outer shell, or lose its outer shell entirely to achieve stability. Understanding valence electrons helps us predict how different elements will bond and react with one another.