Kaplan MCAT General Chemistry Review

The subatomic particles include the proton, which has a positive charge; the neutron, which has no charge; and the electron, which has a negative charge. The nucleus contains the protons and neutrons, while the electrons reside in regions of space. The element’s atomic number is its number of protons, while the sum of an electron’s protons and neutrons is its mass number.

Isotopes are atoms of a given element that have different mass numbers because they have different numbers of neutrons in their nuclei. Because they have the same atomic number, they are all of the same elemental type. Most isotopes of elements are identified by the element followed by the mass number (e.g., carbon-12, carbon-13, carbon-14). The three isotopes of hydrogen go by different names: protium, deuterium, and tritium.

Bohr proposed a model of the atom with a dense, positively charged nucleus surrounded by electrons revolving around the nucleus in defined pathways of distinct energy levels called orbits.

The energy of an electron is quantized, which is to say that there is not an infinite range of energy levels available to an electron. Electrons can exist only at certain energy levels, and the energy of an electron increases the farther it is from the nucleus. The energy difference between energy levels is called a quantum.

For an electron to jump from a lower energy level to a higher one, it must absorb an amount of energy precisely equal to the energy difference between the two levels. Every element has a characteristic atomic absorption spectrum. When electrons return from the excited state to the ground state, they emit an amount of energy that is exactly equal to the energy difference between the two levels. Every element has a characteristic atomic emission spectrum. Sometimes the electromagnetic energy emitted corresponds to a frequency in the visible light range.

The quantum mechanical model posits that electrons do not travel in defined orbits but rather in complex patterns called orbitals. An orbital is a region of space around the nucleus defined by the probabilities of finding an electron in that region of space. The Heisenberg uncertainty principle states that it is impossible to know at the same time both an electron’s position and its momentum.

There are four quantum numbers. These numbers completely describe any electron in an atom. The principal quantum number, n, describes the average energy of an orbital. The azimuthal quantum number, l, describes the subshells within a given principal energy level. The magnetic quantum number, m_l , specifies the particular orbital within a subshell where an electron is likely to be found at a given moment in time. The spin quantum number, m_s, indicates the spin orientation of an electron in an orbital.

The system of designating the placement of electrons into the principal energy levels, subshells, and orbitals is electron configuration. For example, 1s²2s²2p⁶3s² is the electron configuration for magnesium. A neutral magnesium atom has 12 electrons: two in the s-orbital of the first energy level, two in the s-orbital of the second energy level, 6 in the p-orbitals of the second energy level, and 2 in the s-orbital of the third energy level. The two electrons in the s-orbital of the third energy level are the valence electrons for the magnesium atom.

Electrons fill the principle energy levels and subshells according to increasing energy, which can be determined by the (n + l) rule. Electrons fill orbitals according to Hund’s rule, which states that electrons prefer to be unpaired with parallel spins.

Valence electrons are those electrons in the outermost shell and/or those available for interaction (bonding) with other atoms. For the representative elements, the valence electrons are found in s- and/or p-orbitals. For the transition elements, the valence electrons are found in s-, d-, and f-orbitals. Many atoms interact with other atoms to form bonds so as to complete the octet in the valence shell.

EQUATIONS TO REMEMBER

Practice Questions

1. Which of the following is the correct electron configuration for Zn²⁺?

A. 1s²2s²2p⁶3s²3p⁶4s⁰3d¹⁰

B. 1s²2s²2p⁶3s²3p⁶4s²3d⁸

C. 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰

D. 1s²2s²2p⁶3s²3p⁶4s⁰3d⁸

2. Which of the following quantum number sets describes a possible element?

A. n = 2; l = 2; m_l = 1; m_s = +½

B. n = 2; l = 1; m_l = -1; m_s = +½

C. n = 2; l = 0; m_l = -1; m_s = -½

D. n = 2 ; l = 0; m_l = 1; m_s = -½

3. What is the maximum number of electrons allowed in a single atomic energy level in terms of the principal quantum number n ?

A. 2n

B. 2n + 2

C. 2n²

D. 2n² + 2

4. Which of the following equations describes the maximum number of electrons that can fill a subshell?

A. 2l + 2

B. 4l + 2

C. 2l²

D. 2l² + 2

5. Which of the following substances is most likely to be diamagnetic?

A. Hydrogen

B. Iron

C. Cobalt

D. Sulfur