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If you add 8 protons, 8 neutrons, and 8 electrons to Lithium to make Fluorine, you might expect the atom to balloon in size. Instead, Fluorine is almost three times smaller than Lithium.
Why does adding more matter shrink an atom? The answer comes down to an atomic tug-of-war between nuclear pull and electron push.

The Pull and The Shield

The atomic radius is half the distance between the nuclei of two identical atoms bonded together. It defines the practical boundary of an atom's electron cloud.
Two forces determine this boundary: effective nuclear charge (ZeffZ_{\text{eff}}), the net positive pull felt by valence electrons, and electron shielding, where inner-shell electrons repel outer electrons and block the full pull of the nucleus.
In 1930, physicist John C. Slater formulated rules showing that core electrons shield valence electrons almost completely, while electrons in the same valence shell barely shield each other at all.
We approximate this with the formula Zeff=Z−SZ_{\text{eff}} = Z - S, where ZZ is the total number of protons (atomic number) and SS is the number of inner core shielding electrons.
How do these competing forces shift when we travel across a row or down a column of the periodic table?

Trends in Atomic Radii

Moving left to right across a period, protons are added to the nucleus while electrons enter the same outer energy level. Because shielding remains constant, ZeffZ_{\text{eff}} climbs steadily, pulling the valence cloud tighter and decreasing atomic radius.
Moving down a group, each step adds an entire new principal energy level (nn). Even though nuclear charge increases, the inner shells stack up and place outer electrons much farther from the nucleus, dramatically increasing atomic radius.