Explanation:
Atomic number \(20\) will be distributed in electron shell as \(2,8,8,2\).
Its valency is \(2\). It will loose this \(2\) electron to gain stability and noble gas configuration. If atomic number \(11\) is distributed in electron shell it will be \(2,8,1\).
Atom with valence electron \(1\) will loose this atom to gain stability. Hence, both \({ }_{20} X\) and \({ }_{11} X\) will gain nothing i.e. will not gain stable electronic configuration after combining with each other.
An atom with atomic number \(16\) has electronic configuration as \(2,8,6\). It will gain \(2\) electron from \({ }_{20} X\) and complete its octet. Thus atom having atomic number \(20\) will combine most likely with atom having atomic number \(16\) as both will gain stable electronic configuration.
