Electronegativity

Electronegativity (χ)

It is the ability of an atom to attract the shared electron pair towards itself. Generally it is measured on following scales:

Mulliken scale: According to Mulliken, electronegativity ‘M’ of any atom ‘M’ can be taken as the average of its ionization enthalpy and electron affinity (Ae) [or electron gain enthalpy ]

i.e.,

Above relation applies when ΔI.E. H and Ae are taken in eV.

, when : Δ_I.E. H and A_e are taken in kcal/mol.

when : Δ_I.E. H and A_e are taken in kJ/mol.

Pauling scale: On this scale electronegativity ‘’of any atom ‘M’ can be determined w.r.t Hydrogen (which is given an arbitrary value of 2.1) with the help of following formula :

where : (called extra bond energy) and EH–M, EH–H and EM–M represent bond energies (in kcal/mol) of H–M, H – H and M – M bonds respectively.

If bond energies are given in kJ/mol, then .

• In a period from left to right, the value of electronegativity increases while in a group from top to bottom, the value of electronegativity decreases.
• Nature of pond between two atom A and B can be determined from their electronegativities values, if , (predominantly covalent).
• Percentage of ionic characters
• When electronegativities of atoms A and B are different then, bond length of A – B bond is given by, Bond length (A – B) = rA + rB – 0.09

Where : rA, rB are covalent radii of A and B respectively.

• Electronegativity of an atom also depends upon state of hybridization of atom. For Carbon, electronegativity in three states of hybridization varies as: sp > sp2 > sp3.
• Electronegativity of an element increases with increase in Oxidation number or formal charge on an atom, e.g. E.N. of Pb2+ = 1.87 and E.N. of Pb4+ = 2.33.
• The Pauling and Mulliken scales are related to each other by relation :

Difference between electron gain enthalpy and electronegativity