Kaplan MCAT General Chemistry Review

For a given atom or ion, the pattern by which subshells are filled and the number of electrons within each principal energy level and subshell are designated by its electron configuration. In this notation, the first number denotes the principal energy level, the letter designates the subshell, and the superscript gives the number of electrons in that subshell. For example, 2p⁴ indicates that there are four electrons in the second ( p) subshell of the second principal energy level. By definition, this also implies that the energy levels below 2p (that is, 1s and 2s) have already been filled (see Figure 1.3).

MCAT Expertise

Remember that the shorthand used to describe the electron configuration is derived directly from the quantum numbers.

To write out an atom’s electron configuration, you need to know the order in which subshells are filled. They are filled from lower to higher energy, and each subshell will fill completely before electrons begin to enter the next one. You don’t really need to memorize this ordering because there are two very helpful ways of recalling this. The (n + l ) rule can be used to rank subshells by increasing energy. This rule states that the lower the sum of the values of the first and second quantum numbers (n + l ), the lower the energy of the subshell. This is a very helpful rule to remember for Test Day. If two subshells possess the same (n + l ) value, the subshell with the lower n value has a lower energy and will fill with electrons first. The other helpful way to recall this ordering is the flow diagram in Figure 1.3, which also gives the order in which electrons will fill the shells and subshells. We recommend that you quickly write out this flow diagram on the scratch material provided at the testing center at the start of your Physical Science section for quick and easy reference throughout the section.

MCAT Expertise

Remember this chart for Test Day—being able to re-create it quickly on your scratch paper may save you some time and get you that higher score!

Figure 1.3

Example: Which will fill first, the 3d subshell or the 4s subshell?

Solution: For 3d, n = 3 and = 2, so (n + ) = 5. For 4s, n = 4 and = 0, so (n + ) = 4. Therefore, the 4s subshell has lower energy and will fill first. This can also be determined from the chart by examination.

If you are asked to determine which subshells are filled, you must know the number of electrons in the atom. In the case of a neutral (uncharged) atom, the number of electrons equals the number of protons, which can be found by the atomic number of the element. If the atom is charged, the number of electrons is

• equal to the atomic number plus the extra electrons if the ion is negatively charged; or

• equal to the atomic number minus the missing electrons if the ion is positively charged.

In subshells that contain more than one orbital, such as the 2p subshell with its three orbitals, the orbitals will fill according to Hund’s rule, which states that within a given subshell, orbitals are filled such that there are a maximum number of half-filled orbitals with parallel spins. Electrons are somewhat curmudgeonly and misanthropic in that they don’t like the company of other electrons. They prefer to spend as much time alone as possible. Therefore, an electron will be happier (at a lower energy level) if it is placed into an empty orbital rather than being forced to share its living quarters with another electron. (If you ever had to share a bedroom with a sibling, you know exactly what we’re talking about.) Of course, the basis for this preference is the fact that all electrons have a negative charge and, as like charges, they exert repelling forces against each other; these forces must be overcome for two electrons to be in the same orbital.

Mnemonic

Hund’s rule is like sitting down on a bus. Everyone wants an empty seat, but we have to start pairing up after all the seats are half filled.

Example: What are the written electron configurations for nitrogen (N) and iron (Fe) according to Hund’s rule?

Solution: Nitrogen has an atomic number of 7. Thus, its electron configuration is 1s² 2s² 2p³. According to Hund’s rule, the two s-orbitals will fill completely, while the three p-orbitals will each contain one electron, all with parallel spins.

Iron has an atomic number of 26, and its 4s subshell fills before the 3d. Using Hund’s rule, the electron configuration will be as follows:

Iron’s electron configuration is written as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁶ 4s². Subshells may be listed either in the order in which they fill (e.g., 4s before 3d) or with subshells of the same principal quantum number grouped together, as shown here. Both methods are correct.

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