Kaplan MCAT General Chemistry Review

Ionization energy is related to the same set of forces that explains atomic radius, as well as the rules governing maintenance of a full valence shell octet. The first set of rules dictates that the stronger the attractive forces between the outer electron (the electron to be ionized) and the positively charged nucleus, the more energy will be required to ionize. As a result, strong attractive forces, which make the atomic radius smaller toward the right of a period or the top of a group, will also increase the first ionization energy. With this information alone, one could guess that ionization energies for beryllium (Be) should be higher than those for Li (lithium), eliminating (A) and (B). How should we choose between the two ionization energies for beryllium? The first ionization energy is usually dramatically lower than the second. This property holds true for the same reasons previously discussed. For example, upon removing one electron from beryllium, the ion is Be¹⁺, which has one more proton in its nucleus than it has electrons surrounding it. Thus, there is a heightened electrostatic force between the positive nucleus and the now less-negative electron cloud, meaning that all remaining electrons will be held more tightly than before. Removing a second electron will be even more difficult than the initial electron removal, making the second ionization energy higher than the first. Furthermore, the first ionization energy for Li is 520.2 kJ/ mol, the first ionization energy for Be is 899.5 kJ/mol, and the second ionization energy for Be is 1,757.1 kJ/mol.

5. D

Selenium is on the right side of the periodic table (atomic number 34), too far right to be a metal or metalloid. However, it does not lie far enough to the right to fall under column 7A, which would classify it as a halogen. It is a nonmetal.

6. C

The trend in the periodic table demonstrated by the figure is correct for increasing electronegativity and first ionization energy. Electronegativity describes how strong an attraction an element will have for electrons in a bond. A nucleus with a stronger electrostatic pull due to its positive charge will have a higher electronegativity. An arrow pointing toward the right represents this because effective nuclear charge increases toward the right side of a period. This mirrors the trend for ionization energies, because a stronger nuclear pull will also lead to increased first ionization energy, as the forces make it more difficult to remove an electron. The vertical arrow can be explained by the size of the atoms. As size decreases, the proximity of the outermost electrons to the positive inner nucleus increases, making the positive charge more effective at attracting new electrons in a chemical bond (which leads to higher electronegativity). Similarly, the more effective the positive nuclear charge, the higher the first ionization energy. Because I and III are both correct, we can choose answer choice (C).

7. C

All four descriptions of metals are true, but the most significant property that contributes to their ability to conduct electricity is the fact that they have valence electrons that can move freely (C). Large atomic radii, low ionization energies, and low electronegativities all contribute to the ability of metals’ outermost electrons to be easily removed, but it’s the free movement of electrons that actually conducts the electricity.

8. B

This block represents the alkaline earth metals, which form divalent cations, or ions with a +2 charge. All of the elements in Group IIA have two electrons in their outermost s-orbital. Because loss of these two electrons would then leave a full octet as the outermost shell, becoming a divalent cation is a stable configuration for all of the alkaline earth metals. Although some of these elements might be great conductors, it’s an exaggeration to say that they are the best ones on the periodic table, so (A) is incorrect. (C) is also incorrect because although forming a divalent cation is a stable configuration for the alkaline earths, the second ionization energy is still always higher than the first due to the increased positive nuclear charge when compared with the outer negative charge from the electrons. Finally, (D) is incorrect because atomic radii increase when moving down a group of elements because the number of electron shells increases.

9. B

Iron, Fe²⁺, is a transition metal. Transition metals can often form more than one ion. Iron, for example, can be Fe²⁺ or Fe³⁺. The transition metals, in these various oxidation states, can often form hydration complexes (complexes with water). Part of the significance of these complexes is that when a transition metal can form a complex, its solubility within the complex solvent will increase. The other ions given might dissolve readily in water, but because none of them are transition metals, they won’t form complexes.

10. D