Question

The increasing order of atomic radii of the following Group 13 elements is :

• A. $Al<Ga<In<Tl$
• B. $Ga<Al<In<Tl$
• C. $Al<In<Ga<Tl$
• D. $Al<Ga<Tl<In$

Answer 1

The correct option is B $Ga<Al<In<Tl$

The increasing order of atomic radii of group 13 elements is :

$Ga\left(135pm\right)<Al\left(143pm\right)<In\left(167pm\right)<Tl\left(170pm\right)$
On moving down the group, the atomic radii increases. However due to poor shielding of nuclear charge by $3d$ electrons, $\underset{\left(135pm\right)}{r_{Ga}}<\underset{\left(143pm\right)}{r_{Al}}$

$Ga$ has 10 d electrons which do not screen the nuclear charge in an effective manner.

The electrons in the outermost shell of $Ga$ experience more force of attraction towards nucleus than $Al$ which decreases atomic radius.

Hence, the correct option is B.