When electrons are removed from the outermost shell of a calcium atom, the atom becomes

Removing electrons gives the calcium atom more protons than electrons, so it becomes a positively charged ion, or cation. Calcium loses its two outermost electrons, and the resulting Ca²⁺ ion is smaller than the neutral atom because it has less electron–electron repulsion and its outer shell is gone. Therefore, the correct choice is a cation that has a smaller radius than the atom.

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