Question

Why do atoms get bigger as you go down the group and smaller as you go across the periodic table?

a) Due to an increase in the number of protons in the nucleus
b) Because of an increase in the number of electron shells
c) Because of a decrease in the number of neutrons
d) It's a random phenomenon with no specific explanation

Answer 1

Atoms increase in size down a group due to additional electron shells and larger quantum numbers. They decrease in size across a period because of the stronger pull of an increasing number of protons on the valence electrons in the same shell.

Step-by-step explanation:

Atoms get bigger as you go down a group due to an increase in the number of electron shells and larger principal quantum numbers, which means valence electron shells are farther from the nucleus. Conversely, atoms get smaller across a period due to the addition of more protons in the nucleus, resulting in a greater pull on the valence electrons, despite them being in the same shell. The size of the atomic radius also depends on the balance between the electron shielding effect and the increase in nuclear charge.