Group I elements (M) have electronic configuration of outermost shell as ns1 and the group 16 elements (X) have electronic configuration of outermost shell as ns2np4, where ' n ' is the principal quantum number of outermost shell. M is strong reducing agent and can easily lose its . 1 electron to acquire the stable electronic configuration of noble gas. The group 16 elements require 2 electrons to complete their octet, so each X atom will bind with two M atoms. Therefore, M2X compound is formed.