Kaplan MCAT General Chemistry Review

C. Within the same row, the halogens have smaller atomic radii than the alkaline earth metals.

D. The halogens have much higher electron affinities than the alkaline earth metals.

16. What is the outermost orbital of elements in the third period?

A. s-orbital

B. p-orbital

C. d-orbital

D. f-orbital

Small Group Questions

1. Mercury (Hg) exists as a liquid at room temperature. Why, then, is it classified as a metal? What metallic properties might it possess?

2. The transition metals in group VIIIB could theoretically have eight different oxidation states. In reality, that does not hold true. Why not?

Explanations to Practice Questions

1. B

To answer this question, one must first recall that the periodic table is organized with periods (rows) and groups (columns). This method of organization allows elements to be organized such that some chemical properties can be predicted based on an element’s position in the table. Groups (columns) are particularly significant because they represent sets of elements with the same outer electron configuration. In other words, all elements within the same group will have the same configuration of valence electrons, which in turn will dictate many of the chemical properties of those similar elements. Although (A) is true, the fact that both ions are positively charged does not explain the similarity in chemical properties as effectively as does answer (B); most other metals, whether or not they are similar to lithium and sodium, produce positively charged ions. (C) is not true, because periods are rows and lithium and sodium are in the same column. Finally, although lithium and sodium do have relatively low atomic weights, so do several other elements that do not share those same properties.

2. A

This question assesses understanding of a key periodic trend: atomic radii. As one moves from left to right across a period (row), atomic radii decrease. This occurs because as more protons are added to the nucleus and more electrons are added within the same shell, there is no increased shielding between the protons and electrons, but there is increased attractive electrostatic force. This effect decreases the atomic radius. In contrast, as one moves from top to bottom down a group (column), extra electron shells accumulate, despite the fact that the valence configurations remain identical. These extra electron shells provide shielding between the positive nucleus and the outermost electrons, decreasing the electrostatic forces and increasing the atomic radius. Because carbon and silicon are in the same group, and silicon is farther down the periodic table, it will have a larger atomic radius because of its extra electron shell. (C) and (D) are incorrect because all elements in the same group have the same number of valence electrons.

3. C

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, III, is irrelevant, because it does not impact these attractive forces. Statements I and II are correct, and III is incorrect.

4. C