Answer Key and Explanations

The correct answer is (E). For the same reasons that it has the lowest electronegativity, potassium has the lowest ionization energy. The outermost electron has the most shielding between it and the nucleus with the smallest effective nuclear charge. 

The correct answer is (C). Lithium has the smallest ion. When a lithium atom loses its outermost electron, it loses the entire second energy level. Its only remaining electrons are in the first energy level and have one more proton than helium with which to attract them. 

The correct answer is (A). Fluorine has the smallest atomic radius. The fluorine atom has the highest effective nuclear charge of the elements in the list. Because there are no elements on the list with a greater effective nuclear charge or a smaller amount of shielding, fluorine will have the smallest atomic radius. 

The correct answer is (D). Isotopes of an element have the same atomic number (numbers of protons), but different mass numbers (numbers of neutrons). The only choice that represents the same element is (D), where each isotope has 9 protons. One isotope has 9 neutrons, while the other has 10 neutrons. 

The correct answer is (B). Although choices (B) and (D) each have 24 electrons, (B) is the better answer because of chromium s exceptional electron configuration. The chromium atom gains increased stability by promoting one of the 4s electrons to the vacant 3d orbital. This promotion creates half-filled orbitals, which increase stability. 

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