In binary compounds the more electropositive element is placed first in the formula. The formal list is used. The name of the most electronegative element is modified to end in -ide and the more electropositive elements name is left unchanged.
Taking the binary compound of sodium and chlorine: chlorine is found first in the list so therefore comes last in the name. Other examples are
* PCl phosphorus pentachloride
* CaP dicalcium triphosphide
* NiSn nickel stannide
* CrC tricosachromium hexacarbide