MCAT General Chemistry Question 18: Answer and Explanation

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Question: 18

3. What determines the length of an element's atomic radius?

I. The number of valence electrons

II. The number of electron shells

III. The number of neutrons in the nucleus

  • A. I only
  • B. II only
  • C. I and II only
  • D. I, II, and III

Correct Answer: C

Explanation:

Atomic radius is determined by multiple factors. Of the choices given, the number of valence electrons does have an impact on the atomic radius. As one moves across a period (row), protons and valence electrons are added, and the electrons are more strongly attracted to the central protons. This attraction tightens the atom, shrinking the atomic radius. The number of electron shells is also significant, as demonstrated by the trend when moving down a group (column). As more electron shells are added that separate the positively charged nucleus from the outermost electrons, the electrostatic forces are weakened, and the atomic radius increases. The number of neutrons is irrelevant because it does not impact these attractive forces.

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