Electronegativity

What is Electronegativity?

It may be defined as the tendency of an atom to attract a shared pair of electrons towards itself in a covalently bonded molecule.

The numerical value of the electronegativity of an element depends upon its ionisation potential and electron affinity values. Higher ionisation potential and higher electron affinity values imply a higher electronegativity value.

Electronegativity Scale

Some arbitrary scales for the quantitative measurement of electronegativities are as follows:

Pauling's scale: 

Pauling related the resonance energy(Δ_AB) of a molecule AB with the electronegativities of the atoms A and B. If x_A and x_B are the electronegativities of atoms A and B, respectively, then

0.208 = √Δ_ABA – x_B if x_A>x_B or Δ_AB = 23.06 (x_A – x_B)²

= E_{A-B(experimental)} – E_{A-B(theoretical)} where E_A-B is the energy of A-B bond. In a purely covalent molecule, AB, the experimental and theoretical values of the bond energy A-B are equal.

So Δ_AB = 0

or 0=23.06 (x_A – x_B)² or x_A = x_B

In an ionic molecule AB, E_{A-B(experimental)} is more than E_{A-B(Theoretical)}.

Pauling assumed the electronegativity value of fluorine 4 and calculated the electronegativity values of other elements from this value.

Mulliken's electronegativity: 

According to Mulliken, the electronegativity of an element is the average value of its ionisation potential and electron affinity.

or Electro-negativity = Electron affinity + Ionisation potential/2

= Electron affinity + Ionisation potential/5.6 (on Pauling scale)

When both are expressed in electron volts

Factors affecting the magnitude of electronegativity

Atomic radius: As the atomic radius of the element increases, the electronegativity value decreases. Electronegativity Α 1/Atomic radius

iii) Effective nuclear charge: The electronegativity value increases as the effective nuclear charge on the atomic nucleus increases.

Electronegativity Α Effective nuclear charge (Z_eff)

iii) Oxidation state of the atom: The electronegativity value increases as the oxidation state (i.e. the number of positive charge) of the atom increases.

iv) Hybridisation state of an atom in a molecule: (Reference: Chemical Bonding) If the s- character in the hybridisation state of the atom increases, the electronegativity increases because s-electrons are comparatively nearer to the nucleus. For example, the electronegativity values of the C-atom in various hybridisation states are as follows:

SP> SP2 > SP3

s-character is increasing

So the electronegativity value is increasing

Periodicity in Electronegativity

i. In a period moving from left to right, the electronegativity increases due to the increase in effective nuclear charge.

ii. In a period, the electronegativity value of IA alkali metal is minimum, and that of VIIA halogen is maximum.

iii. In a group moving from top to bottom, the electronegativity decreases because the atomic radius increases.

iv. The electronegativity value of F is maximum, and that of Cs is minimum in the periodic table.

v. The electronegativity of Cs(55) should be more than Fr(87) but it is less. This is due to the increase of +32 units in the nuclear charge of Fr, which makes the effective nuclear charge comparatively high.

vi. On moving from the second to third transition series in a group [except Y(39) → La (57)], electronegativity increases due to the increase of +18 units in nuclear charge.

vii. The variation of electronegativity along any period or row of the periodic table may be understood with reference to the following table.

Electronegativity increases form left to right of the periodic table, while it decreases down any group. These observations are consistent with other periodic properties of the atom. The alkali metals possess very feeble attraction for electrons, as it is to be expected from their electronic configurations. The halogens, on the other hand, have the highest electronegativity in each row owing to their ns² np⁵ configuration. The decrease of the electronegativity down any vertical group in the periodic table is consistent with the variation of the effective nuclear charge.

Applications of electronegativity

i) Partial ionic character in a covalent Bond: The ionic character of a covalent bond increases as the electronegativity difference of bonded atoms increases. According to Haney and Smith if the electronegativity difference of bonded atoms is Δx then percentage ionic character of the bond = 16Δx+3.5Δx²

If the value of Δx is 2.1 then ionic character percentage is about 50. For example the order of ionic character in H–X bond is as follows–

H–F>H–Cl>H–Br>H–I

Because the electronegativity difference of bonded atoms (Δx) decreases.

ii) Bond strength: If the electronegativity difference of covalently bonded atoms (Δx) increases, the bond energy of the covalent bond also increases. For example – the order of the H–X bond strength is –

H – F > H – Cl> H – Br > H – I

As the bond strength is decreasing the acid strength is increasing. So order of increasing acid strength is

HF <HCl<HBr< HI

iii) Acidic and basic nature of oxides of normal elements in a period: The acidic nature of the oxides of normal elements increases as we move from left to right in a period. In a period from left to right the electronegativity of the elements increases. So the difference of the electronegativities of oxygen and the elements (x_O –x_E) decreases. If the (x_O – x_E) values is about 2.3 or more, then oxide will be basic. If (x_O – x_E) values is less than 2.3 the oxide will be acidic. The oxides of the IIIA elements are amphoteric.

The value of x_O – x_E is decreasing

Basic nature is decreasing

The acidic nature is increasing

(iv) Metallic and non-metallic properties of elements:

i) The metallic character decreases as the electronegativity of the element increases.

ii) On moving from left to right in a period, the electronegativity of the elements increases. So the metallic character decreases.

iii) On moving down a group, the electronegativity of the elements decreases. So the metallic character increases.

v) Basic nature of the hydroxides of elements: A hydroxide MOH of an element M may ionise in two ways in water.