Some arbitrary scales for the quantitative measurement of electronegativities are as under

i) `text(Pauling's Scale)` : Pauling related the resonance energy(`DeltaAB`) 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 sqrt(delta_(AB)) = X_A -X_B` if `X_A > X_B` or `Delta_(AB) = 23.06(X_A-X_B)^2`

`Delta_(AB) = E_(A-B text(experimental)) - E_(text(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 bond energy `A-B` are equal.

So `Delta_(AB) = 0`

or `0=23.06 (X_A - X_B)^2` or `X_A = X_B`

In an ionic molecule `AB`, `E_(text(A-B experimental))` is more than `E_(text(A-B Theotrical))`

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

ii) `text(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 = `(text(Electron affinity) + text(Ionisation potential))/2`

`= (text(Electron affinity) + text(Ionisation potential))/(5.6)` (on pauling scale)

When both are expressed in electron volt per atom

i) `text(Atomic Radius)` : As the atomic radius of the element increases the electronegativity value decreases.

`text(Electronegativity ) prop 1/(text(atomic radius))`

ii) `text(Effective Nuclear Charge)` : The electronegativity value increases as the effective nuclear charge on the atomic nucleus increases.

Electronegativity and Effective nuclear charge `(Z_(eff))`

iii) `text(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) `text(Hybridisation State of an Atom in a Molecule)` : If the `s`-character in the hybridisation state of the atom increases electronegativity increases because `s`-electrons are comparatively nearer to the nucleus. For example the electronegativity values of `C`-atom in various hybridisation states are as under:

Hybridisation states `sp^3 quad sp^2 quad sp`

`s`-Character `25% quad 33.33% quad 50%`

Electronegativity `2.48 quad 2.75 quad 3.25`

`s`-character is increasing.

So, the electronegativity value is increasing.

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 `lA` alkali metal is minimum and that of `VIlA` halogen is maximum.

iii) In a group moving from top to bottom, the electronegativity decreases because 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 nuclear charge of `Fr` which makes the effective nuclear charge comparatively high.

vi) On moving from 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 :

`text(Electronegativity values of some elements in Pauling scale)` : See fig.

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 highest electronegativity in each row owing to their `ns^2 quad np^5` configuration. The decrease of the electronegativity down any vertical group in the periodic table is consistent with the variation of effective nuclear charge.

i) `text(Partial Ionic Character in 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 `Delta_x` then percentage ionic character of the bond = `16 Delta_x + 3.5 Delta_x^2`

If the value of `Delta_x` 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 (`Delta_x`) decreases.

ii) `text(Bond Strength)` : If the electronegativity difference of covalently bonded atoms (`Delta_x`) 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) `text(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 period. In a period from left to right the electronegativity of the element increases. So the diffrence 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 order of acidic or basic nature of the oxides of third period elements may be given as under :

`Na_2O quad MgO quad SiO_2 quad P_2O_5 quad SO_3 quad Cl_2O_7`

The value of `X_O - X_E` is decreasing

Basic nature is decreasing

Acidic nature i s increasing

iv) `text(Metallic and non metallic properties of elements)` :

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

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

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

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

`M-O-H + H_2O ⇋ MO^(-) + H_3O^(+) ... (1)`

`M- O -H +H_2O ⇋ MOH_2^(+) +OH^(-) ... (2)`

If the ionisation is according to eqn(1) then it is acidic. It is possible when ionic characctor of `O-H` bond is more then the ionic character of `M-O` bond i.e., `(X_O-X_H) > (X_O + X_M)` where `X_O`, `X_H` and `X_M` are the electronegativities of oxygen, hydrogen and element respectively.

If the ionisation is according to eqn.(2) then it is basic. This is only possible when ionic character of `O-H` bond is less than `M-O` bond i.e., `(X_O-X_H) < (X_O - X_M)`