CBSE Class 10 Answered

The tendency of an atom in a molecule to attract the shared pair of electrons towards itself is called its electronegativity.

Electronegativity values depend on:

(i) Atomic size: The greater the size of the atom, the lesser the electronegativity, since the electrons being farther away from the nucleus, experience a lesser force of attraction.

(ii) Nuclear charge: The greater the nuclear charge, the greater the electronegativity, because an increase in nuclear charge causes electron attraction with a greater force.

Trends in electronegativity

Across a Period

Since the nuclear charge increases due to an increase in atomic number, electronegativity increases from left to right in a period.

For example, in the second period, electronegativity increases from lithium to fluorine.

Down a group

There is an increase in atomic number down a group, i.e., nuclear charge increases, but due to the addition of extra shells, the atomic size increases. The effect of an increase in the atomic size overcomes the effect of an increase in the nuclear charge; hence electronegativity decreases down a group.

For example, in the first group, it significantly decreases from lithium downwards to francium.

In case of molecule: Example- Hydrogen chloride

Hydrogen chloride is a polar covalent compound. Chlorine is more electronegative than hydrogen.

So after the covalent bond formation, chlorine pulls the shared pair of electrons towards itself.

In doing so chlorine develops a slight negative charge on it and hydrogen develops a slight positive charge resulting in the formation of polar covalent bond.