What is periodic property ? How are the following properties vary in a group and in a period ? Explain.
a) Ionization Enthalpy (I. E)
b) Electronegativity (E. N)
c) Atomic radius
d) Electron gain enthalpy
e) Nature of oxides

Periodicity :
The properties of elements repeats itself at regular intervals is known as periodicity (or) periodic properties.
a) Ionization Enthalpy :
The amount of energy required to remove an electron from outermost orbit of an isolated neutral gaseous atom is known as ionization enthalpy (or) Ionization potential.
i) In a group from top to bottom atomic size increase, Ionization potential value decrease.
ii) In a period from left to right atomic size decrease, ionization potential value increase.
$\text{ I.P }\mathrm{\alpha }\frac{1}{\text{ Atomic size }}$
b) Electronegativity :
The ability of an atom to attract the bond pair electrons towards itself is known as electronegativity.
i) In a group from top to bottom atomic size increases, electronegativity decreases.
ii) In period from left to right atomic size decreases, electronegativity increase.
c) Atomic radius :
The distance between the nucleus and outer most electron of an atom is known as atomic radius.
i) In group from top to bottom atomic radius increases because new shells are added.
ii) In period from left to right atomic radius decreases because nuclear attraction increases on outer most shell.
d) Electron gain enthalpy :
The amount of energy released when an electron is added to the valency shell of neutral gaseous atom is called Electron gain enthalpy In group from top to bottom atomic size increases, Electron gain enthalpy decreases. In period from left to right atomic size decreases, Electron gain enthalpy increases.
e) Nature of oxides :
Generally, metal oxides have basic nature and non-metal oxides have acidic nature.
i) In group from top to bottom, metallic nature of elements increases so basic character of oxides increases.
ii) In a period from left to right non-metallic nature of elements increases. So acidic nature of oxides increases
f) Metallic nature:
The tendency of an element to lose electrons is called metallic nature.
Atomic radius ∝ metallic nature
Variation in a group :
In a group from top to bottom, metallic nature gradually increases due to increase in atomic
radius.
Variation in a period :
In a period from left to right metallic nature gradually decreases due to decrease in atomic radius.