Periodicity of electronegativity

Electronegativity is the tendency of an atom to attract electrons towards itself in a molecule of a compound. The value of electronegativity of an element describes the ability of its atom to compete for electrons with the other atom to which it is bonded. Electronegativity is however not the property of an isolated atom.

Electronegativity increases from left to right in each period ending at group VII.

In the 3^rd period, electronegativity increases from sodium to chlorine i.e., chlorine can accept electrons most easily in that period followed backwards by sulphur, phosphorus, silicon, aluminium, magnesium and sodium. All the atoms of the above mentioned elements have three shells but chlorine has the smallest atomic radii. Hence chlorine experiences more positive charge from the nucleus than all other atoms in that period. So, if one electron is available, chlorine can attract it most easily.

Types of Electronegativity

• When the molecule is formed by transfer of electrons (ionic bonding) the transfer takes place from electropositive atom to electronegative atom (Fig. 2.4). In the example below, Na is electropositive and Cl is electronegative.

• If the molecule is formed by sharing of electrons (covalent bond) the bonded pair of electrons shift towards more electronegative atom resulting in the formation of polar molecule. In the example below, chlorine atom is more electronegative as compared to hydrogen atom, resulting in a covalent bond where the shared pair of electron shifts towards the more electronegative atom. This results in polar molecules (Fig. 2.5).

The electron pair is more closer to the chlorine atom and so the molecule gets polarized i.e., the chlorine atom gets a negative charge while the hydrogen atom gets a positive charge.

Remember :

• Fluorine is the most electronegative element.

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A summary of periodic properties and their variation in groups and periods is given below:

Variation along the period

Due to the increased nuclear charge across the period, the electronegative or non-metallic character increases in going from left to right in a period.

Variation down the group

Due to the increased atomic size the non-metallic or electronegative character decreases as we go down the group.

Electronegativity is however not the property of an isolated atom. Electronegativity is measured on a number of scale levels, the most commonly used are of Pauling or Mulliken.

Factors Affecting Electronegativity

Atomic size

As the size of the atom decreases it has greater tendency to attract the bonding electrons towards itself. Therefore smaller atoms have higher electronegativity values than the larger ones.

Ionisation energy and electron affinity

Higher Ionisation energy and electron affinity lead to higher electronegativity.

Number and nature of atoms

The electronegativity depends on the number and nature of atoms bonded to it.

Type of hybridization

The electronegativity increases with the increase in 's' character in the hybrid orbital. This is because the 's' orbitals being more near to the nucleus have greater tendency to attract the shared pair of electron.

Charge on the ion

A cation has high electronegativity while an anion has less electronegativity than its parent atom. A cation with a higher positive charge is more electronegative.