The correct order of the atomic radii of C, Cs, Al, and S is :

The electronegativity of aluminium is similar to :

When the first electron gain enthalpy $$\left( {{\Delta _{eg}}H} \right)$$ of oxygen is $$-$$ 141 kJ/mol, its second electron gain enthalpy is :

Aluminium is usually found in +3 oxidation state. In contrast, thallium exists in + 1 and + 3 oxidation states. This is due to :