What periodic trends of Atomic size occur with the halogens?

As we move from left to right across the Period, nuclear charge increases sequentially and the additional electrons do not effectively shield one another, causing a contraction across the Period. However, when moving down the Group, the additional electrons have completed a shell and offer some shielding of the nuclear charge, meaning that the valence electrons build on an already-existing shell of electrons. Shielding is the dominant effect, and atomic radii should logically increase.

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The atomic size of halogens decreases as you move from top to bottom within the group. This is because, as you move down the group, the number of electron shells increases, leading to a greater distance between the nucleus and the outermost electrons, resulting in larger atomic sizes.