Why is Na^- not possible??

Valency is defined as the number of electrons in the outermost shell of an element . As the last shell(third shell) has one electron in it the valency of sodium(Na) is 1 or +1 as it looses that electron to form a cation and achieve noble gas configuration.But if valency is -1, it will gain one electron ,so that last shell contain 2 electron ,will not achieve noble gas configuration.So that is not possible.