So valence electrons shield other valence electrons from the nucleus. Shielding happens not only from core electrons, but also from electrons in the same shell. So, while a chloride ion has the same electron configuration as a neutral argon atom, they have different radii because of the different number of protons in the nuclei. The chlorine ion example is keeping the same number of protons but adding an electron. If the effective nuclear charge for elements increases as you move to the right, the electrons feel a greater force of attraction for the nucleus and the valence electrons orbit closer resulting in a smaller atomic radius. While you are also adding an extra electron, the extra proton results in a net increase in the effective nuclear charge because the attractive pull of a proton is greater than the shielding of an extra electron in the same shell. As you move left to right, you’re changing the type of element the atom is which means you’re adding an extra proton each step to the right. This is different from the trend of decreasing atomic radii as you move left to right along a period. You’ve decreased the effective nuclear charge felt by the electrons towards the nucleus and so they feel less attractive force towards the nucleus and the valence electrons orbit farther from the nucleus resulting in a larger atomic radius. If you have a neutral chlorine atom and all you do is add an electron, then you’ve added to the repulsive force felt by the electrons.
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