Periodic Trends: Atomic Radius

Periodic Trends in Atomic Radius

Electrons occupy orbitals rather than fixed paths. Orbitals describe regions where electrons are likely to be found, not sharply defined boundaries. Although atomic orbitals can have many shapes, a simplified model can help explain atomic radius.

Atomic radius is the typical distance from the nucleus to the outer boundary of the electron cloud. It depends on the number of protons and electrons in an atom.

Trends Down a Group

Atomic radius increases as you move down a group. Each step down adds an electron shell, placing the outer electrons farther from the nucleus.

For example, hydrogen, lithium, and sodium have progressively more electron shells, so their atomic radii increase.

Trends Across a Period

Atomic radius decreases as you move from left to right across a period. This trend is explained by effective nuclear charge and electron shielding:

1. Protons in the nucleus attract negatively charged electrons.
2. Inner-shell electrons repel outer-shell electrons, reducing the nucleus’s pull on them. This effect is called electron shielding.
3. Moving from lithium to beryllium adds a proton and an electron, while the number of inner-shell electrons providing shielding stays the same.
4. The added proton increases the effective nuclear charge, pulling the electrons closer to the nucleus.
5. As a result, the atomic radius becomes smaller. The same general decrease continues across the period.

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