The electronic configuration of an element gives an idea about its reactivity.
The elements having a completely filled outermost orbit (or shell) will be chemically inert (non-reactive). For example, the outermost shell in the case of helium (He), neon (Ne) and argon (Ar) are completely filled as shown below. As a result, helium, neon, and argon are chemically inactive (or inert). These elements do not form compounds with other elements.
Because of this chemical inactivity, these gases are called as noble gases (earlier these were called inert gases).
The elements containing only one or seven electrons in their outermost shell show greater chemical reactivity, i.e., such elements react very fast with other elements. For examples, sodium and chlorine having the follows electronic configurations are highly reactive.
Sodium 2, 8, 1 —- Here, the outermost shell has been one one electron: one more than the completely filled shell.
Chlorine 2, 8, 7 —– Here, the outermost shell has seven electrons: one less than that required to fill the shell completely.