Ionic radius is the distance between the nucleus and outermost electrons of an ion (cation and anion). The trend in ionic radii depends on two factors:
- Nuclear charge
- Repulsion among valence shell electrons
Periodic Trends Going Left to Right
Periodic trends in ionic radii are difficult compared to atomic radii. The reason is that, along a period from left to right, the elements from group I to IV form cations while those in group V to VII, elements form anions. Ionic radii of cations and anions need to be studied with different parameters: nuclear charge and repulsions among valence shell electrons.
Ionis radii of cations decrease from left to right going through group I to IV because nuclear charge increases. The number of valence shell electrons remain the same, so inter-electronic repulsions do not matter.
When we switch from group IV to group V along a period, the ionic radii increase because three electrons are added to the valence shell of group V elements which increases repulsion between newly added three electrons and already lying five outermost shell electrons. This repulsion increases the size of the anion. Moving across the period, the ionic radii of anions decrease through group V to VII because nuclear charge increases.