Periodicity in Properties

Repetition of properties after intervals of atomic number values 2, 8, 18 and 32 is called periodicity. The properties are called periodic properties.

1. Atomic Radius

The distance between the centre of the nucleus and electron cloud of the outermost energy level is called the atomic radius.

The atomic radius increases down the group due to the addition of new shells and increase in screening effect. The atomic radius decreases from left to right in a period due to the increase in nuclear positive charge. In a given period alkali metal is the largest atom and halogen is the smallest atom.

2. Ionisation Potential

Ionization potential decreases down the group due to increase in size and increase in screening effect. lonisation potential increase in a period form left to right due to decrease in size and increase in nuclear charge.

3. Electron Affinity

The amount of energy released when an electron is added to a neutral gaseous atom is called electron affinity. Energy is released when the first electron is added to the neutral atom.

Electron Affinity decreases from top to bottom in a group due to increase in size and increase in nuclear charge. Electron Affinity increases from left to right in a period due to decrease in atomic size and increase in nuclear charge.

4. Electronegativity

The tendency of an atom to attract the shared electron pair towards itself in a molecule is called its electronegativity.

Electronegativity decreases with increase in size and increase in screening effect in a group from top to bottom. Electronegativity increases with decrease in size and increase in nuclear charge in a period from left to right.

5. Valency

Valency is the combining capacity i.e. number of bonds formed by an atom. Valency is a whole number without positive or negative sign.

Valency of a given group element will be same in general. Highest valency of an element will never exceed its group number. Valency of an element may be equal to its group number or 8 - group number.

Valency of a given period elements will be different and it changes by 1 unit from one element to next element.

6. Oxidation States

The charge which an atom appears to be possessed in a molecule or ion is called its oxidation state. Oxidation state may be positive or negative or a fraction or zero.

For s-block elements, oxidation number is equal to the group number. p-block elements show different oxidation states which often differ by 2 units.