Chlorine is the only nonmetal listed which would gain electrons to ionize. For a nonmetal, becoming an ion usually means gaining electron(s), and therefore means an increase in radius. Therefore, since is the only element in this list that will gain electrons to ionize, it is the correct answer.

Electronegativity increases from the bottom to the top, and from the left to the right of the periodic table. Of the choices given, oxygen is closest to the top right corner.

The answer is . While chloride ion is not farthest downward and right on the periodic table of the answers, it is the largest radius in this case. Potassium ion, although isoelectronic with chloride, would have a smaller radius because it would have more protons pulling at the same number of electrons.

The most chemically similar elements can are in the same family on the periodic table. Calcium and barium are both in group II of the periodic table. In all other answer choices, the two elements listed are in different families and are therefore chemically dissimilar.