MCAT General Chemistry Question 28: Answer and Explanation
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Question: 28
13. Which of the following atoms or ions has the largest effective nuclear charge?
- A. Cl
- B. Cl–
- C. K
- D. K+
Correct Answer: D
Explanation:
The effective nuclear charge refers to the strength with which the protons in the nucleus can pull on electrons. This phenomenon helps to explain electron affinity, electronegativity, and ionization energy. In choice (A), the nonionized chlorine atom, the nuclear charge is balanced by the surrounding electrons: 17 p+/17 e-. The chloride ion, choice (B), has a lower effective nuclear charge because there are more electrons than protons: 17 p+/18 e-. Next, elemental potassium, choice (C), has the lowest effective nuclear charge because it contains additional inner shells that shield its valence electron from the nucleus. Choice (D), ionic potassium, has a higher effective nuclear charge than any of the other options do because it has the same electron configuration as Cl- (and the same amount of shielding from inner shell electrons as neutral Cl) but contains two extra protons in its nucleus: 19 p+/18 e-.