Question

The electronegativity of $C$, $H$, $O$, $N$ and $S$ are $2.5$, $2.1$, $3.5$, $3.0$ and $2.5$ respectively. Which of the following bonds is most polar?
A. $O - H$
B. $S - H$
C. $N - H$
D. $C - H$

Answer 1

Electronegativity of an element refers to its ability to attract the shared electron pairs in a covalent bond. Electronegativity of elements varies in the modern periodic table in a predictable manner. On moving top to bottom in a group, the electronegativity of the elements decreases whereas it increases on moving left to right across a period.

Complete answer: Bond polarity in a chemical compound is the separation of electric charge along a chemical bond which leads the compound to have an electric dipole or dipole moment. Bond polarity in a chemical compound occurs due to the electronegativity difference between the bonded atoms. The electronegativity of elements is represented by a symbol $\chi$.
Greater the electronegativity difference of atoms, the higher will be the polarity of a bond. Hence among given options, electronegativity difference of bonds will be as follows:
Electronegativity difference in $O - H$ bond:
$\chi$of $O = 3.5$
$\chi$ of $H = 2.1$
Therefore, difference in electronegativity $= 3.5 - 2.1 \Rightarrow 1.4$
Electronegativity difference in $S - H$ bond:
$\chi$of $S = 2.5$
$\chi$ of $H = 2.1$
Therefore, difference in electronegativity $= 2.5 - 2.1 \Rightarrow 0.4$
Electronegativity difference in $N - H$ bond:
$\chi$of $N = 3.0$
$\chi$ of $H = 2.1$
Therefore, difference in electronegativity $= 3.0 - 2.1 \Rightarrow 0.9$
Electronegativity difference in $C - H$ bond:
$\chi$of $C = 2.5$
$\chi$ of $H = 2.1$
Therefore, difference in electronegativity $= 2.5 - 2.1 \Rightarrow 0.4$
As the highest value of electronegativity difference is for $O - H$, so it will be the most polar bond among given options.
Hence, option (A) is the correct answer.

Note:
It is important to note that on increasing the polarity of a bond, the difference in electronegativity of atoms increases which lead to increase in the ionic character of the compound. If the difference in electronegativity of atoms is less than $0.4$ then the compound is pure covalent, if the value lies between $0.4$ and $1.7$ then the compound is polar covalent and when the value of electronegativity difference is greater than $1.7$ then the compound is ionic.