The lowest oxidation number of an atom in a compound \(A _2 B\) is -2 . The number of electrons in its valence shell is …

\({A}_{2}B\to 2{A}^{+}+{B}^{−2},{B}^{−2}\) has complete octet in its dianionic form, thus in its atomic state it has 6 electrons in its valence shell. As it has negative charge, it has acquired two electrons to complete its octet.