Isoelectronic elements are elements that have the same electron configuration in the outermost valence shells. These elements can either be atoms or ions, and in the case of isoelectronic compounds, molecules. This similar arrangement of electrons leads to similar chemical properties.
Nitrogen and oxygen with an extra electron are isoelectronic, because their valence electron shells each contain five electrons. The positively-charged lithium atom is an isoelectronic partner with the helium atom, because they both have two electrons in their outermost shells. Some isoelectronic pairs have identical pairs of electrons extending deep into the inner electron shells, such as ionized sulfur and argon.