Question

The element with the smallest size in group $13$ is:
A. Beryllium
B. Gallium
C Aluminium
D. Boron

Answer 1

The $1st$ element of every group in the periodic table has the smallest size as compared to all other corresponding metals in the same group. This is the main reason for the anomalous behaviour of the $1st$ element of every group.

Complete answer:
In the periodic table the $1st$ member of every group has the smallest size because the number of shells in that particular element will be less than other elements in the group.
Size of an element is related to many properties of elements.
The ionization enthalpy of an element is inversely proportional to its size. Thus as the size increases on moving down the group the ionization enthalpy decreases. Also as we move left to right the ionization enthalpy increases due to decrease in size by addition of electrons in the same shell which results in enhanced nuclear attraction.
It is mentioned in the question that the group number is $13$ .
In this group we have $B;Al;Ga;In;TI$ respectively.
As boron is the $1st$member of group$13$ , hence it will have the smallest size in its respective group. The electronic configuration of boron is $1{s^2}2{s^2}2{p^1}$.
The most common oxidation state for this group is $+ 3$ .
Hence option (a) cannot be the correct answer as $beryllium$ belongs to alkaline earth metals i.e. group number$2$. It is also the $1st$ member of group$2$.
Other elements mentioned in the options are not the $1st$ element hence their size is big as compared to boron.

So, Correct option is (d).
Note:
Boron hydride exists as ${B_2}{H_6}$ which has a different kind of bond known as banana bond with$3c - 2{e^ - }$. Also the most common oxidation state of the Boron family is $+ 3$. For e.g. $A{l^{3 + }},{B^{3 + }},etc$.Boron has very less electronegativity due to this boron hydride acts as a Lewis acid and accepts electron pairs from bases.