Question

Why does the atomic radius decrease as you go to the right?


The atoms on the right have more protons (p+) in the nucleus


The atoms on the right have fewer energy levels

Answer 1

The correct reason is: the atoms on the right have more protons in the nucleus, which increases the effective nuclear charge and pulls the electron cloud closer, so the atomic radius decreases.

Why that works:
- Across a period electrons are being added to the same principal energy level, so the number of energy levels does not decrease — it stays the same.
- Each added proton increases the nuclear positive charge (Z). Because the added electrons enter the same shell, they do not fully shield the extra positive charge, so the effective nuclear charge (Zeff ≈ Z − shielding) felt by the valence electrons increases.
- Greater Zeff pulls the electrons closer to the nucleus → smaller atomic radius.

(By contrast, going down a group the radius increases because additional energy levels are added, placing electrons farther from the nucleus.)