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When we move from left to right in the periodic tble atomic radii

decrease due to increase in effective nuclear charge.

 Variation of atomic radii in a group :

When we move from top to bottom in a group in the periodic table

atomic radii increase due to increase in number of shells.

 Ionic radius :

It is defined as the effective distance from the centre of the nucleus

of the ion up to which it exerts its influence on its electronic cloud.

The radius of the cation is always smaller than that of its parent atom

because after the removal of valence electron the number of shells in
the cation decreases and effective nuclear charge increases,there fore

size becomes smaller.

The radius of the anion is always larger than that of parent atom

because on addition of electron in the valence shell,the inter
electronic repulsion is more. So ionic size is more.

 Isoelectronic ions or species :

Ions of the different elements which have same number of electrons
but different magnitude of the nuclear charge are called iso electronic

ions.

Iso electronic ions have the same number of electrons . Their radii

would be different because of their different nuclear charges. The
cation with the greater positive charge will have a smaller radius

because of the greater attraction of the electrons to the nucleus. Anion

with the greater negative charge will have the larger radius. In this

case, the net repulsion of the electrons will outweigh the nuclear
charge and the ion will expand in size.