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It actually does, which is why a negative ion is always larger than a neutral atom of the same element. The trend you are talking about is atomic radii getting smaller as you move right in the periodic table. That happens because you're adding protons to the nucleus, which increases the pull on the electrons, bringing them closer to the nucleus and making the atom smaller. So even though adding electrons makes the atom larger, the effect of adding protons completely outweighs it. Basically, if you add an electron, the atom gets larger. If you add a proton, the atom gets smaller. If you add a proton and an electron, which is what you do when you move right in the periodic table, then the added proton has a greater impact than the added electron, so the atom gets smaller.
Increasing the number of electrons would lead to the orbital expanding in ions. However, the periodic trend refers to neutral atoms, so protons are also added along with the electrons leading to a greater attraction between the nucleus and shell. This causes the shell to contract and the atom gets smaller.
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