How does atomic radius vary in a period and in a group?

The atomic radius changes systematically across the periodic table, both within periods and groups.

Atomic Radius Trends:

The atomic radius, defined as the distance between the nucleus and the outermost electron, follows specific patterns:

Decrease in Atomic Size: As you move from left to right across a period, the atomic radius generally decreases.
Increased Nuclear Charge: This decrease occurs because the number of protons in the nucleus (nuclear charge) increases across a period. The increased nuclear charge attracts the electrons more strongly, pulling them closer to the nucleus.
Same Shell: Electrons are added to the same energy level or shell within a period. This means there is no increase in shielding effect, and the increased nuclear charge dominates, resulting in a smaller atomic size.
- Example: In the third period, as we move from sodium (Na) to chlorine (Cl), the atomic radius progressively decreases.

Increase in Atomic Size: As you move down a group, the atomic radius generally increases.
Addition of Shells: This increase happens because each successive element in a group adds a new energy level or electron shell.
According to the reference: "Atomic size increases down the group because of the addition of an extra shell."
Shielding Effect: The inner electrons shield the outermost electrons from the full positive charge of the nucleus, causing a decrease in the effective nuclear charge experienced by the outer electrons. Thus, with an increased number of shells, the electrons are farther from the nucleus.
Dominant Effect: The addition of a new shell is more dominant than the effect of an increasing nuclear charge, resulting in a larger atomic radius down a group.
- Example: In Group 1 (alkali metals), lithium (Li) has a smaller radius than sodium (Na), which has a smaller radius than potassium (K) and so on.

Trend	Description	Reason
Across a period	Decreases from left to right	Increasing nuclear charge attracts electrons more strongly; no new shell is added.
Down a group	Increases from top to bottom	New electron shells are added, increasing the distance of outermost electrons from the nucleus and increasing shielding.