Question

The electronegativity of B, Al, and Ga are 2.0, 1.5, and 1.6 respectively. The trend is not regular. Explain?

Answer 1

The transition of B to Al is as expected. But, the transition in electronegativity from Al to Ga is unexpected, i.e. Ga is less electronegative than Al. This is due to the presence of d-orbital in the case of Ga, the shielding effect of the d-orbital electrons is very poor which makes it able to attract electron pairs easily, hence making it more electronegative.

Complete step by step answer:
Let’s start by discussing electronegativity.
We know that when atoms bond, they share electrons between each other and the electronegativity of an element is described as the tendency of an element to attract the shared pair of electrons towards itself.
So when two atoms are bonded with each other the electron pair is shifted more towards the more electronegative atom.
In the problem, we are told that the electronegativity of B (boron), Al (Aluminium) and Ga (Gallium) are 2.0, 1.5 and 1.6 respectively.
We know that it is a general trend in a group that the electronegativity decreases as we move down in the group because as the atoms get larger the ability to attract the shared pair of electrons towards it decreases. But, the given data of electronegativity deviates from the ideal condition.
The transition of B to Al is as expected, i.e. Al is less electronegative than B. But, the transition in electronegativity from Al to Ga is unexpected, i.e. Ga is less electronegative than Al. This is due to the presence of d-orbital in the case of Ga, the shielding effect of the d-orbital electrons is very poor so the size of the Ga atom is smaller than expected and so it can attract the shared pairs of electrons more effectively. Hence, the electronegativity of Ga is more than Al.

Note: In the solution, we discussed how the electronegativity of elements decreases as we move down the group, but when moving from left to right in the periodic table the electronegativity of atoms generally increases as the number of electrons in the atom is increasing the size of the elements is decrease.