Question

Which one of the following pairs of the element has the second element with greater first ionization energy?

• A. P, As
• B. Si, Ge
• C. Al, Ga
• D. S, Se

Answer 1

The correct option is C Al, Ga

Gallium-$Ga$-outermost valence shell electronic configuration-${4s}^2{4p}^1$

Aluminium-$Al$-outermost valence shell electronic configuration-${3s}^2{3p}^1$

Before valence shell configuration, $Ga\left(10\right)$ electrons are filled in $d-$orbitals which has poor shielding effect. So effective nuclear charge will increase and hence $Ga$ will be smaller in size than $Al$.

As nuclear charge is more, it is difficult to remove an electron from outermost shell of $Ga$, hence ionization energy is more than $Al.$