Free-Response Questions
Both authors have been AP free-response graders for years. Here are some free-response questions
for practice. You have 10 minutes. You may not use a calculator. You may use the tables at the
back of the book.
Use the periodic table and other information concerning bonding and electronic
structure to explain the following observations.a. The radii of the iron cations are less than that of an iron atom, and Fe^3 +is smaller
than Fe^2 +.
b. When moving across the periodic table from Li to Be to B, the first ionization energy
increases from Li to Be, then drops for B. The first ionization energy of B is greater than
that of Li.
c. The electron affinity of F is higher than the electron affinity of O.
d. The following observations have been made about the lattice energy and ionic radii of
the compounds listed below. Compare NaF to CaO, and then compare CaO to BaO.
All of the solids adopt the same crystal structure.Ionic radius Ionic radius Lattice energy
Compound of cation (pm) of anion (pm) (kJ/mole)
NaF 116 119 911
CaO 114 126 3566
BaO 149 126 3202Answers and Explanations
Notice that all the answers are very short. Do not try to fill all the space provided on the
exam. You score points by saying specific things, not by the bulk of material. The graders
look for certain keywords or phrases. The answers should not contain statements that con-
tradict each other.a. The observed trend of radii is: Fe >Fe^2 +>Fe^3 +. There is an increase in the effective
nuclearcharge in this series. As electrons are removed, the repulsion between the electrons
decreases. The larger the effective nuclear charge, the more the electrons are pulled
towards the nucleus and the smaller the atom or ion becomes. Give yourself 1 point for
“effective nuclear charge,” and 1 point for explaining the effective nuclear charge discussion.b. When moving across a period on the periodic table, the value of the effective nuclear
charge increases with atomic number. This causes a general increase from Li to Be to B.
This effective nuclear charge argument is worth 1 point.
The even higher value of Be (greater than B) is due to the increased stability of the
electron configuration of Be. Beryllium has a filled s-subshell. Filled subshells have an
increased stability, and additional energy is required to pull an electron away. Give
yourself 1 point for the filled subshell discussion.c. The effective nuclear charge in F is greater than the effective nuclear charge in O. This
causes a greater attraction of the electrons. You get 1 point for this answer.d. Because all the solids adopt the same structure, the structure is irrelevant. The sizes of
the anions are similar; thus, anion size arguments are not important.
Two factors, other than structure, are important here. The two compounds
with the highest lattice energies contain divalent ions (+2 or −2) while NaF contains64 STEP 4. Review the Knowledge You Need to Score High