Correct Response: D. Chlorine is element 17, and sulfur is element 16. Chlorine has a slightly smaller radius than sulfur because of the extra proton in the nucleus; this extra proton causes the positively charged nucleus to hold the electrons more tightly, and the resulting atomic radius is smaller. A is incorrect because selenium is element 34 and so has an atomic radius considerably larger than that of sulfur. Selenium is larger and has 18 more protons and 18 more electrons than sulfur does. B is incorrect because phosphorus is element 16 and therefore has 1 less proton in the nucleus than sulfur does; because there is 1 less proton, the electrons are held less tightly to the positively charged nucleus and the resulting atomic radius is larger. C is incorrect because oxygen is element 8 and so is considerably smaller; oxygen has 8 fewer protons and 8 fewer electrons than sulfur does.
