Lithium belong to group 1 metals. Hence, it can loose one electron to form lithium ion i.e. Li⁺¹ or Li⁺
While, Nitrogen is non-metal and hence has the ability to gain the electron lost by lithium metal. Furthermore, Nitrogen can gain maximum 3 electrons to acheive noble gas configuration. Hence, three Li atoms will loose their electrons and Nitrogen will gain those three electrons to form nitride ion i.e. N³⁻.
