Octet Rule

The tendency of main group elements atoms to form chemical bonds in a way that each atom obtains eight valence electrons is described by the "octet rule". Having eight electrons in the valence shell is a particularly stable state with the same electron configuration as a noble gas.

Some exceptions to the rule are hydrogen, lithium and helium. Hydrogen only needs one additional electron and lithium needs to lose one electron to attain a stable configuration with two electrons. Helium has a total of two valence electrons and, with this, in its most stable form.