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Cation size decreases up and decreases to the right. They get smaller as you go up because number of orbitals decrease, and get smaller as you go right as you add more positive charge (holds in electrons tighter). Same trend as atomic radius. The smaller the cation leads to more polarizing power.
I'm assuming if you're asked to find the smallest cation, it is given that all the cations have the same amount of valence electrons. In this case, the smallest cation would have the highest amount of protons because this would mean the nucleus has a stronger pull on the electrons.
Cation radii follows the normal atomic radii trend. It increases across a period and decreases down a group. This is because moving across, the number of protons increases, thus increasing effective nuclear charge and decreases the size. If we have 3 cations that are isoelectronic, for instance: Na+, Mg2+, and Al3+, the smallest cation would be the one with the greatest number of protons, since it has the strongest effective nuclear charge and is able to pull in the remaining electrons closer. In this case, Al3+ would be the smallest.
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