Kaplan MCAT General Chemistry Review

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/ , where c = 3 × 10⁸ m/s, the speed of light. Here, f = (3 × 10⁸ m/s)/500 × 10^-9 m, or 6 × 10¹⁴ s^-1 (1 Hz = 1 s^-1). That leads to E = hf, or E = (6.626 × 10^-34) × (6 × 10¹⁴ Hz) = 3.98 × 10^-19 J. (Don’t worry about memorizing the units of Planck’s constant—energy is always in joules!) However, the problem includes an additional trick, in that the answer must account for a mole of photons. The E = hf equation works for a single photon only. Thus, the answer must account for this using Avogadro’s number, 6.022 × 10²³ photons. Multiply: (3.98 × 10^-19 J/photon) × (6.022 × 10²³ photons) = 2.39 × 10⁵ J.

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).

8. A

Recall that the superscript (i.e., the A in ^AC) refers to the mass number of an atom, which is equal to the number of protons plus the number of neutrons present in an element. (Sometimes a text will list the atomic number, Z, or total number of protons, under the mass number A.) According to the periodic table, carbon contains 6 protons; therefore, its atomic number (Z) = 6. An isotope contains the same number of protons and a different number of neutrons as the element. Carbon is most likely to have an atomic number of 12, for 6 protons and 6 neutrons. (C) and (D) are possible isotopes that would have more neutrons than does ¹²C. The ⁶C isotope is unlikely. It would mean that there were 6 protons and 0 neutrons, and it would probably collapse under the stress of the positive charge.

9. C

The Heisenberg uncertainty principle states that you cannot know the position and momentum of a particle simultaneously, which eliminates (A) and (D). Momentum depends on velocity (recall from Newtonian mechanics that p = mv), so in order to calculate momentum, velocity must be known. (C) explains this.

10. B

For the electron to gain energy, it must absorb photons to jump up to a higher energy level. This eliminates (A) and (D). Between (B) and (C), there is a bigger jump between n = 2 and n = 6 than there is between n = 3 and n = 4. Therefore, (B) represents the greatest energy gain.

11. A

The MCAT covers qualitative topics more often than quantitative topics in this unit. It is critical to be able to distinguish the fundamental principles that determine electron organization, which are usually known by the names of the scientists who discovered them. The Heisenberg uncertainty principle refers to the momentum and position of a single electron, and the Bohr model was an early attempt to describe the behavior of the single electron in a hydrogen atom. (D) is tempting, but (A) is more complete and therefore the correct answer. The element shown here, nitrogen, is often used to demonstrate Hund’s rule because it is the smallest element with a half-filled p subshell. Hund’s rule explains that electrons fill empty orbitals first, and in fact, the three p-electrons in this image each occupy a separate orbital. Hund’s rule is really a corollary of the Pauli exclusion principle, in that the Pauli exclusion principle suggests that each orbital contains two electrons of opposite spin. Additional electrons must fill new orbitals so the compound remains stable in its ground state.

12. A