A. An electron moves from n = 6 to n = 2.
B. An electron moves from n = 2 to n = 6.
C. An electron moves from n = 3 to n = 4.
D. An electron moves from n = 4 to n = 3.
11. Suppose that a chemical species fills its orbitals as shown.
This compound could be said to obey which of the following laws of atomic physics?
A. Hund’s rule
B. Heisenberg uncertainty principle
C. Bohr’s model
D. Pauli exclusion principle
12. How many total electrons are in a 133Cs cation?
A. 54
B. 55
C. 78
D. 133
13. The atomic mass of hydrogen is 1.008 amu. What is the percent composition of hydrogen by isotope, assuming that hydrogen’s only isotopes are 1H and 2D?
A. 92% H, 8% D
B. 99.2% H, 0.8% D
C. 99.92% H, 0.08% D
D. 99.992% H, 0.008% D
14. Consider the two sets of quantum numbers shown in the table, which describe two different electrons in the same atom.
Which of the following terms best describes these two electrons?
A. Parallel
B. Opposite
C. Antiparallel
D. Paired
15. Which of the following species is represented by the electron configuration 1s22s22p63s23p64s13d5 ?
A. Cr
B. Mn+
C. Fe2+
D. both (A) and (B)
16. Which of the following statements is NOT true of an electron’s ground state?
A. The electron is at its lowest possible energy level.
B. The electron is in a quantized energy level.
C. The electron is traveling along its smallest possible orbital radius.
D. The electron is static.
17. Which of the following experimental conditions would NOT excite an electron out of the ground state?
A. Radiation
B. High temperature
C. High pressure
D. None of the above
Small Group Questions
1. Which orbital fills first: 4s or 3d ? Which is first to give up electrons?
2. Why do atoms have neutrons? What purpose do they serve?
Explanations to Practice Questions
1. A
Because an electron was pulled off the neutral parent atom, consider how that parent atom was formed and which electron it would be willing to give up. Zn° has 30 electrons, so it would have an electron configuration of 1s22s22p63s23p64s23d10. Subshells strongly prefer to be completely empty, exactly half full, or completely full. Therefore, the best way to lose two electrons and form Zn2+ is to pull them both out of the 4s-orbital. (B) implies that electrons are pulled out of the d-orbital, (C) presents the configuration of the uncharged zinc atom, and (D) shows the configuration that would exist if four electrons were removed.
2. B
The azimuthal quantum number l cannot be higher than n–1, ruling out (A). The ml number, which describes the chemical’s magnetic properties, can only be an integer value between –l and l, and it cannot be equal to 1 if l = 0, ruling out (C) and (D).
3. C
The correct answer is (C). For any value of n there will be a maximum of 2n2 electrons, i.e., two per orbital.
4. B
This formula describes the number of electrons in terms of the azimuthal quantum number l, which ranges from 0 to n–1, n being the principal quantum number.
Subshell Azimuthal Quantum Number (l) Number of Electrons s 02p 16d 210f 314
Using the data in the table, the quickest way to solve this problem is to choose a subshell and plug in its l value to solve for the number of electrons. If more than one option works, discard the others and test the remaining possibilities using another subshell.
5. D
Sulfur is diamagnetic as opposed to ferromagnetic (iron, cobalt) or paramagnetic (hydrogen). Ferromagnetism refers, loosely, to the ability of a surface to attract an external magnetic field. It is characteristic of iron (Fe), from which it derives its name. More specifically, paramagnetism describes the tendency of valence electrons to align with the same spin in the presence of a strong magnetic field. Strongly paramagnetic materials, including transition metals, are usually called ferromagnetic. Transition metals like iron are characterized by a “sea” of electrons moving freely about the surface, which makes it easier for all these electrons to align in one direction. (This electron “sea” is an imprecise model, but it’s good enough for the MCAT.) It is harder for more stable elements (e.g., oxygen, halogens, noble gases) to align their electrons in one orientation because their orbitals are nearly filled; these substances are known as diamagnetic. Sulfur has a similar atomic structure to oxygen, so it is also diamagnetic.
6. C
The problem requires the MCAT favorite equation E = hf, where h = 6.626 × 10-34 (Planck’s constant) and f is the frequency of the photon. (Memorize Planck’s constant!) One can calculate the frequency of the photon using the provided wavelength, 500 nm, with the equation f = c/
7. B
There is not enough information in the problem to determine how the velocity of the electron will change. There will be some energy change, however, as the electron must lose energy to return to the minimum energy ground state. That will require emitting radiation in the form of a photon, (B).