Electronegativity ( χ )
Ability of an atom in a chemical compound to attract shared electrons towards itself is called electronegativity. Unlike ionization enthalpy and electron gain enthalpy, it is not a measurable quantity so it is unitless.
Mulliken define the elecrtonegativity of an atom as arithmetic mean of ionization potential and electron affinity.
χA = 1/2(ionization potential + electron affinity)
If the values are given in ev-
χA = (ionization potential + electron affinity)/5.6
Factors affecting the Electronegativity
The value of electronegativity depends upon number of factors, some of which are discussed below:
- Atomic size
- Effective Nuclear charge
- Hybridization State of an Atom
- Oxidation State
- Effect of Substituent
- Charge on Ion
- Number of Inner Shells
- Number and Nature of Bonded Atoms
Atomic size
Electronegativity is inversely proportional to the atomic size of the element. Smaller the size of an atom, greater the electronegativity and vice-versa.
Effective Nuclear charge
Electronegativety is directly proportional to effective nuclear charge. Greater the effective nuclear charge, greater would be the electronegativity. For example, electronegativity of oxygen (3.44) is more than that of nitrogen (3.04) as the effective nuclear charge of oxygen is more than that of nitrogen.
Hybridization State of an Atom
Electronegativity is directly proportional to the per centage of s-character in hybridized atom. Greater the per centage of s-character, greater the electronegativity. For example, in ethane, ethene and ethyne the hybridization is sp3, sp2 and sp respectively. The per centage of s-character in ethane is 25, in ethene is 33.3 and in ethyne is 50. Therefore, The order of electronegativity of carbon in ethane, ethene, and ethyne is ethane < ethene < ethyne.
Oxidation State
Electronegativity is directly proportional to the oxidation state. Greater the oxidation state, greater would be the electronegativity.
Effect of Substituent
The electronegativity of an atom depends upon the nature of substituent attached to that atom. For example, the carbon atom in CF3I acquires a greater positive charge than CH3I. Therefore, carbon atom in CF3I is more electronegative than in CH3I.
Charge on Ion
Electronegativity is a variable quantity. It varies with the change in the oxidation state of the element. The element in higher oxidation state is more electronegative than that in the lower oxidation state. Thus the value of elctronegativity of Fe3+ is higher than that of Fe2+.
Number of Inner Shells
The atom with greater number of inner shells is less electronegative than the atom with smaller number of inner shells. For example, electronegativity values of halogens decrease from Flourine to Iodine as the number of inner shells increases from Flourine to iodine.
Number and Nature of Bonded Atoms
As we know that the electronegativity of an atom is not the property of isolated atom, it depends on the number and nature of the atoms to which the atom is bonded. Therefore, the electronegativity of an atom is not constant. For example, electronegativity of Phoshorous atom in PCl3 molecule is different from that in PF5 molecule as the number and nature of the bonded atoms (Cl and F) in both molecules are different.
Variation of Electronegativity in Periodic Table
In period from left to right electronegativity increases while down the group it is decreases.Importance of Electronegativity
The electronegativities of elements are widely used throughout the study of Chemistry. The important applications of electronegativities are listed below-
- In predicting the polarity of a particular bond.
- In predicting the degree of ionic character of a covalent bond.
- In predicting of inductive effects in organic chemistry.
- In understanding the shapes of molecules.
