(solved)Question 3.22 of NCERT Class XI Chemistry Chapter 3

Difference 1: Purpose

Electronegativity is a qualitative measure of the ability of an atom to attract the shared electrons of a chemical bond.

But electron gain enthalpy is a quantitative measurement of the enthalpy change that takes place when an electron is added to an isolated atom in gaseous state.

Difference 2: Constant or not

Electronegativity is not constant for an element. It depends on the other participating elements. It is a property of an atom "in a molecule".

NOTE: But we find that the data shows a single value for electronegativity. Then how can we say that it is NOT constant? Actually, the calculated and tabulated value is meant as an indicator of the property in a wide variety of molecules. Pauling scale has a relative error of an order of 10% but can be used to get a rough qualitative idea for of any molecule.

on the other hand, electron gain enthalpy, being a property of an isolated atom in gaseous state, is a fixed constant.

Difference 3: Measurability

Electronegativity is not a measurable quantity. Mulliken calculated on the basis of absolute values of ionization enthalpy and electron gain enthalpy. Pauling calculated it on the basis of ionic resonance energies [0.79 caesium to 3.98 florine].

Electron gain enthalpy, however, is measurable.

Difference 4: Sign

Electronegativity is a positive number less than 4.0 on Pauling scale.

Electron gain enthalpy is positive for an endothermic process and negative for an exothermic process.

Difference 5: Units

Electronegativity is a dimensionless quantity with no units. It is a number.

Electron gain enthalpy has the units of energy and expressed in kJ/mol.

Examples

Florine F is the most electronegative of all the elements.

Chlorine Cl has the most negative electron gain enthalpy.