Question

Explain why cations are smaller and anions larger in radii than their parent atoms?

Answer 1

1. Cations (Smaller Than Parent Atoms)

• A cation is formed when an atom loses one or more electrons.
• After loss of electrons:
  • The number of electrons decreases, but the number of protons (nuclear charge) remains the same.
  • So, the same positive charge is now attracting fewer electrons.
  • Effective nuclear charge per electron increases; the nucleus pulls the remaining electrons more strongly.
• This stronger attraction draws the electron cloud closer to the nucleus, so the ionic radius decreases.
• In many cases, one whole outer shell may be lost (e.g. Na → Na⁺), causing a large drop in size.

2. Anions (Larger Than Parent Atoms)

• An anion is formed when an atom gains one or more electrons.
• After gain of electrons:
  • The number of electrons increases, but the number of protons remains the same.
  • Now more electrons share the same nuclear attraction.
  • Electron–electron repulsion in the valence shell increases.
  • Effective nuclear charge per electron decreases; the hold of the nucleus on each electron becomes weaker.
• The electron cloud spreads out due to increased repulsion and weaker pull, so the ionic radius increases.

3. Key Idea (Compact Statement)

Losing electrons → fewer electrons, stronger effective nuclear attraction → cations smaller than parent atoms.
Gaining electrons → more electrons, more repulsion, weaker effective attraction → anions larger than parent atoms.