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The reason why Sodium's Atomic Radius is greater than Chlorine's Atomic Radius.
Chlorine has 17 protons and Sodium has 11 protons. The more protons in the atoms' nuclei the stronger inward pull the protons have on the valence electrons, thus decreasing atomic radii. Therefore Sodium's 11 protons do not have as strong of a pull as Chlorines 17 protons, thus giving Sodium a greater atomic radius.