Question

How does atomic radius vary in a period and in a group? How do you explain the variation?

Answer 1

1. Variation Across a Period (Left → Right)

• Atomic radius generally decreases from left to right in a period.
• Reason:
  • Electrons are added to the same principal shell (same n), so the distance of the valence shell from the nucleus does not increase significantly.
  • However, the atomic number (number of protons) increases, so nuclear charge increases.
  • Screening (shielding) by inner electrons remains almost the same.
  • Effective nuclear charge on valence electrons therefore increases, pulling them closer to the nucleus.
• Result: Stronger attraction between nucleus and outer electrons → smaller atomic radius across a period.

2. Variation Down a Group (Top → Bottom)

• Atomic radius generally increases from top to bottom in a group.
• Reason:
  • Each successive element down a group has an additional electron shell (n increases: 2, 3, 4, ...).
  • Valence electrons are in orbitals farther from the nucleus, increasing the size of the electron cloud.
  • Inner shells of electrons provide shielding, reducing the effective nuclear attraction on the outermost electrons.
• Result: Larger distance between nucleus and valence electrons → larger atomic radius down a group.

3. Compact Statement

Across a period: ↑ nuclear charge with same shell → stronger pull → radius decreases.
Down a group: ↑ number of shells and shielding → weaker pull at outer shell → radius increases.