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How do electron shells and orbitals affect the size of an atom?

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subatomic particle The Large Hadron Collider (LHC), the world's most powerful particle accelerator. At the LHC, located underground in Switzerland, physicists study subatomic particles.

Electron shells and orbitals are fundamental in determining the size of an atom.

The size of an atom is primarily influenced by the principal quantum number (n), which defines the electron shell. As n increases, electrons occupy shells that are, on average, farther from the nucleus. Consequently, adding electrons to a new, higher-energy shell increases the atomic radius, which is why atoms generally grow larger as you move down a group in the periodic table.123

Conversely, moving across a period, the atomic size typically decreases. Although more electrons are added, they are placed into the same shell and do not effectively shield each other from the increasing positive nuclear charge. This stronger attraction pulls the valence electrons closer to the nucleus, resulting in a more compact atom.1

Additionally, the specific shapes of orbitals—such as spherical s orbitals or double-lobed p orbitals—describe the probability distribution of electrons, influencing how they occupy space around the nucleus.4