Electronegativity may be defined as the tendency of an atom in a molecule to attract towards itself the shared pair of electrons. The main factors, which the electronegativity depends, are effective nuclear charge and atomic radius. Greater the effective nuclear charge greater is the electronegativity. Smaller the atomic radius greater is the electronegativity.
In a period electronegativity increases in moving from left to right. This is due to the reason that the nuclear charge increases whereas atomic radius decreases as we move from left to right in a period. Halogens have the highest value of electronegativity in their respective periods.
In a group electronegativity decreases on moving down the group. This is due to the effect of the increased atomic radius. Among halogens fluorine has the highest electronegativity. In fact fluorine is the most electronegative element and is given a value of 4.0 (Pauling's scale) whereas cesium is the least electronegative element (E.N. = 0.7) because of its largest size and maximum screening effect. In other words, cesium is the most electropositive element and hence is the most metallic element in the periodic table.
The main differences between Electron gain enthalpy (electro affinity) and electronegativity are given below :
Electron gain Enthalpy
1. It is the tendency of an isolated gaseous atom attract an electron.
2. It is measured in electron volts/atom or kcal/mole kj/mole.
3. It is the property of an isolated atom.
4. An atom has an absolute value of electron gain enthalpy.
5. It does not change regularly
6. in a period or group.
1. It is the tendency of an atom in a molecule to attract the shared pair of electrons.
2. It is a number and has no units.
3. It is property of a bonded atom.
4. An atom has a relative value of electronegativity depending upon its bonding state.
5. For example, sp-hybridized carbon is more electronegative than sp2-hybridized carbon which, in turn, is more electronegative than sp3- hybridized carbon.
6. It changes regularly in a period or a group.
Electronegativity of an element is important in determining the bond character. If two atoms have the same electronegativities the bond between the two will be covalent, while a large difference in electronegativity leads to ionic bond. Between the extremes the purely covalent bond and purely ionic, the bonds will have different degrees of ionic character. As a rough estimate it is seen that a difference of 1.7 in electronegativities, the bond has 50% ionic character. If the difference is less than 1.7, the bond is considered covalent, and greater than 1.7 it is considered ionic.