Question

In group 13, electronegativity first decreases from B to Al and then increases marginally down the group. This is because of:
A) Non-metallic nature of B
B) Discrepancies in the atomic size of the element
C) The ability of B and Al to form $\text{ p }\\!\\!\pi\\!\\!\text{ -p }\\!\\!\pi\\!\\!\text{ }$ multiple bonds
D) Irregular trend in electronegativity throughout the periodic table

Answer 1

Electronegativity is the ability of an atom to attract the shared pair of an electron from the bond. In a group, the electronegativity decreases as we move top to bottom. However, in the period, it increases as we move from the left to right horizontally. The inner electrons shield the nucleus from interacting with the valence shell electrons.

Complete Solution :
- Electronegativity is defined as the ability of an atom to attract the shared pair of electrons. It is a special feature of the periodic table. The electronegativity increases as we move from the left to right along the period. In a group, the electronegativity decreases as we move from top to bottom.
The electronegativity of an atom depends on the following factors:

1. Size of an atom
2. Magnitude of the nuclear charge.

- In the periodic table, the group 13 elements are collectively known as the boron family. It has the elements B, Al, Ga, In, Tl.
- As we move down from the boron family, the atomic size increases from B to Tl . Each element has an additional shell compared to the previous elements. The increase in the shell number increases the size of an atom.
- The atomic size of an atom is inversely related to the electronegativity. $\text{ Electronegativity }\propto \text{ }\dfrac{1}{\text{Atomic size}}\text{ }$

- Thus, the electronegativity of aluminium is less than boron. It is expected that electronegativity should decrease further.
- As we move from the aluminium to the Thallium, the electrons start to fill in the d-orbitals. The inner electrons tend to prevent the interaction between the nucleus and the valence shell electrons. The shielding or the screening effect is greater in the s orbitals and goes on decreasing for p, d, and f orbitals.
- Here, d-orbitals have a poor shielding effect. Thus, they are incapable of reducing the interaction between the nucleus and the electrons. Thus, as we move down in the groups the attraction reduces the size of the atom.
- Since the size of the atom is inversely related to the electronegativity, the electronegativity is found to be decreasing as we move from B to Al and then increases further due to poor shielding effect. This is also known as discrepancies in the atomic size of the element.
So, the correct answer is “Option B”.

Note: Note that the d orbitals have the five electrons. This orbital does not have a regular shape. This is a double dumbbell-shaped orbitals. Because of their shape they cannot cover the nucleus to the highest extent. The shielding effect reduces further for the f-orbitals. The shielding effect is observed less as the number of shells increases in the atom. In group 14, the electronegativity trend decreases from carbon to silicon and remains steady afterwards.