Question

In which of the following molecules is the covalent bond most polar?
A. HI
B. HBr
C. HCl
D. \[{{H}_{2}}\]

Answer 1

Hint: A covalent bond is formed by the sharing of one electron by two atoms with each other. There are mainly two types of covalent bonds, one is polar and the other is non polar. In this question, we need to find the molecule formed with a bond between its atoms having more covalent character than the covalent bond in other molecules. The covalent bond will be most polar when there is a great electronegativity difference between the atoms of the molecule.

Complete Step by Step Solution:
Electronegativity is defined as the tendency of an atom to attract the shared electron from the covalent or chemical bond towards itself. The electronegativity difference between the atoms of a molecule lies between 0.4 to 1.7 termed as polar in nature.

In \[{{H}_{2}}\]molecules, both the hydrogen atoms are bounded with a covalent bond. As both the atoms forming covalent bonds are same thus, electronegativity difference between the two is 0. Thus, the covalent bond is nonpolar.

In HCl, the electronegativity difference can be calculated just by subtracting the lower electronegativity from the higher electronegativity. The electronegativity of hydrogen is 2.1 and the electronegativity of chlorine is 3. Thus subtraction of both electronegativity of hydrogen and chlorine we get electronegativity difference such as
\[3\text{ }\text{ }2.1\text{ }=\text{ }0.9\] \[3\text{ }\text{ }2.1\text{ }=\text{ }0.9\]
Thus, it is polar in nature
The electronegativity difference between the atoms of HBr is
\[2.8\text{ }\text{ }2.1\text{ }=\text{ }0.7\] \[2.8\text{ }\text{ }2.1\text{ }=\text{ }0.7\]
So, HBr is also a polar molecule.
The electronegativity difference between the atoms of HI is
\[2.5\text{ }\text{ }2.1\text{ }=\text{ }0.4\] \[\]
So, HI is also a polar molecule.
Thus, the correct option is C as the electronegativity difference between H and Cl is greater than the electronegativity difference between the atoms of the other two molecules.

Note: It is not a fixed rule that the covalent bond of a molecule will be polar if the electronegativity difference lies between 0.4 to 1.7. Somewhere it is also considered that polar character starts at 0.5, electronegativity difference. Also, greater the electronegativity difference between the atoms of the molecule, greater the polar character and less will be covalent character.