Question

Why cations are smaller and anions larger in radii than their parental atoms?

Answer 1

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Cations are always smaller than their parent atoms this is because theyhave lesser electrons, while theirnuclear charge remains the same. Theremaining electrons are, therefore held more tightly by the protons in thenucleus and thus their radii aresmaller than the parent atoms. This is opposite in case of anions.
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Answer 2

Answer:

Cations are formed by expelling an electron from outermost orbit of an atom, thus cation has less electrons compared to parent atom which results in increased effective nuclear charge but the total nuclear charge remains same which results in increased attraction of electrons towards nucleus than that of parent atom. Thus, cations are having smaller radii then that of their parent atom.

Anions are formed by gaining an electron in the outermost orbit of an atom, thus anion has more electrons compared to parent atom which results in decreased effective nuclear charge but the total nuclear charge remains same which results in increased distance the nucleus and the valence electrons as the attraction of electrons towards nucleus decreases than that of parent atom. Thus, anions are having larger radii then that of their parent atom.