Question

why size of cation is always smaller than parent atom?

Answer 1

Parent atom has more electrons so the effective nuclear charge on each electron is less. Now when a cation is formed electron(s) is/are lost. So the effective nuclear charge or simply put, the attraction of the nucleus towards the electrons increases. So due to greater pull, the nucleus pulls the shells towards it.. there by reducing the size.

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Answer 2

Cations are always smaller than their parent atoms this is because they have lesser electrons, while their nuclear charge remains the same. The remaining electrons are, therefore held more tightly by the protons in the nucleus and thus their radii.are smaller than the parent atoms.