A. The repetition of similar chemical properties of elements at regular intervals with increasing atomic number is called periodic property or periodicity.
a) Ionization enthalpy : A quantitative measure of the "tendency of an element to lose electron is given by its Ionization Enthalpy. It- represents- the energy required to remove an electron from an isolated gaseous atom (X) in its ground state. In other words, the first ionization,enthalpy for an element X is the enthalpy change (DjH) for the reaction depicted,in the following equation. `X(g) to X^(+) (g) + e^(-)`
The ionization, enthalpy is expressed in units of kJ `"mol"^(-1)`. We can define the second ionization enthalpy as the energy required to remove the second most loosely-bound electron, it is the energy required to carry out the reaction shown in the following equation.
`X^(+)(g) to X^(+)(g) + e^(-)`
b) Electronegativity: As we go from left to right in.a period, the electronegativity gradually increases. Reason : In a period, as we go from- left to. fight, atomic radius gradually decreases: As a result of which the tendency of attraction of nucleus on the bonded pair increase.s Hence the values of electronegativity increases in a period.
As we go from top to bottom in a group,-the electronegativity values gradually decrease.
Reason : As we go from top to bottom in a.group, the-atorhic radius gradually increases: As a result of which the tendency .of attraction of nucleus oh the bonded pair decreases. Hence, the values of electronegativity decreases in a group
c). Electron gain enthalpy or Electron affinity: In a period, as we go from left to right. Electron affinity gradually, increases. Reason.: As we go from left to right in a period, the atomic radius gradually decreases. Asa.result of which, the attraction between the added electron and the nucleus increases. Hence, electron affinity increases in a period.
In a groupas we.go from top to bottom, the electron affinity values gradually decrease.
Reason: As we go down a group, the atomic radius gradually increases. As a result of which, the attraction between the added electron and the nucleus decreases. Hence, electron affinity decreases in a group.
The repetition of similar physical and chemical.properties of elements at regular intervals with increasing.atomic number is called periodic property pr periodicity.
