Let us start with #NH_3#; there are 5 valence electrons from nitrogen, and 3 valence electrons from hydrogen.....8 electrons in total, and 6 of these form the #3xxN-H#, and the remaining 2 electrons are localized on nitrogen, to give a lone pair of electrons on nitrogen.
In liquid ammonia, the following equilibrium is presumed to operate....
#2NH_3(l) rightleftharpoonsNH_4^(+) + NH_2^(-)#
And this equilibrium is PRECISELY AKIN to the acid-base equilibrium that operates in water:
#2H_2O(l) rightleftharpoonsH_3O^+ + HO^-#
Going back to the amide ion...we gots #""^(-):ddotNH_2#...there are SIX electrons associated with nitrogen rather than five, and with 2 inner shell electrons, this gives 8 electrons in total associated with nitrogen, and thus a FORMAL nitrogen-centred, negative charge.
Certainly I could use #NaNH_2# as an ionic species in a non-aqueous medium.....
