Rank these atoms and ions according to radius.

K-, K+, K

The answer is K⁺ < K < K⁻.

- The radius of cation is smaller than its atomic radius.

Cation is formed by losing electrons. When electrons are less, the attraction towards the remaining electrons by the nucleus is larger than the attraction in neutral atom. As well as, the repulsions between electrons are less. Hence, the radius of cation is smaller than its atomic radius.

- The radius of anion is larger than its atomic radius.

Anion is formed by gaining electrons. When more electrons are present, the attraction towards the electrons by the nucleus is smaller than the attraction in neutral atom. As well as, the repulsions between electrons are high due to the higher number of electrons. Hence, the radius of anion is larger than its atomic radius.

Largest radius to smallest radius: K -, K, K+

Ionic radius is the radius of cation/anion that affects physical and chemical properties of substance of which its part increases as you go down a group.

A trend for atomic radius is that it decreases as you move across the period.