Final answer:
Atomic size increases when moving down the periodic table due to the addition of electron shells which are farther from the nucleus and larger, as well as electron shielding, which decreases the effective nuclear attraction for the valence electrons.
Step-by-step explanation:
When considering atomic size and how it changes with increasing energy levels as one goes down the periodic table, it's important to understand the periodic trend of atomic sizes. As the energy levels increase, which occurs when moving from top to bottom in a group, there is an overall increase in atomic size.
This is because higher principal energy levels consist of larger orbitals. As a result, with every added shell, the valence electrons are further from the nucleus, and the atomic radius increases despite the increase in nuclear charge. This effect is also partly due to electron shielding, whereby the inner electrons repel the outermost electrons, making them even less tightly bound to the atom's nucleus.
Additionally, while moving down a group, the number of filled principal energy levels increases, leading to a larger atomic radius. The increase in nuclear charge does not fully counteract this increase in electron shell size due to the electron shielding effect and the greater distance between the outermost electrons and the nucleus in higher principal energy levels.
The trend is summarized as atoms generally getting larger as you go down any one column of the periodic table.