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What do you mean by electronegativity?

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Hint: Electronegativity relates to the ease with which an atom can attract a shared pair of electrons in a covalent compound.

Complete answer:
> Electronegativity is defined as the tendency of the atom of an element in a chemical compound to attract a shared pair of electrons towards it in a covalent bond. Since electronegativity is just a tendency, it is a dimensionless quantity.
> A number of electronegativity scales are present. These include Pauling scale, Milliken-Jaffe scale, Allred Kochow scale etc. Out of these scales, Pauling scale is usually used in which fluorine (the most electronegative element) was arbitrarily assigned a value of 4.0. Least electronegative element cesium is assigned a value of 0.7.
> Electron gain enthalpy is closely related to electron affinity but significantly differs from electronegativity.
> Electron gain enthalpy is defined as the enthalpy change when a neutral gaseous atom takes up an extra electron to form an anion.
> Electron gain enthalpy is the tendency of an isolated gaseous atom to accept an additional electron to form a negative ion. Electronegativity is the tendency of the atom of an element in a chemical compound to attract a shared pair of electrons towards it in a covalent bond.
> Electronegativity is a qualitative measure. It is not a measurable quantity. However, electron gain enthalpy is a quantitative measure. It is a measurable quantity.

Note: In a period, on moving from left to right, with increase in the atomic number, the nuclear charge and the electronegativity increases. In a group, on moving down, with increase in the atomic number, the effective nuclear charge decreases due to an additional shell of electrons. Hence, electronegativity decreases.