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Chemistry LibreTexts

2.3: Electron Configurations

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If you keep your papers in manila file folders, you can pick up a folder and see how much it weighs. To identify the number of different papers (articles, bank records, or whatever else is kept in a folder), you have to take everything out and count. A computer directory, on the other hand, will specify exactly how many files are in each folder. We can get the same information on atoms using similar tactics. If we use an orbital filling diagram, we have to count arrows. When we look at electron configuration data, we simply add up the numbers.

Electron Configurations

Learning Objectives

  • Represent the organization of electrons by an electron configuration and orbital diagram.

Electron configuration notation eliminates the boxes and arrows of orbital filling diagrams. Each occupied sublevel designation is written followed by a superscript that is the number of electrons in that sublevel. For example, the hydrogen configuration is 1s1, while the helium configuration is 1s2. Multiple occupied sublevels are written one after another. The electron configuration of lithium is 1s22s1. The sum of the superscripts in an electron configuration is equal to the number of electrons in that atom, which is in turn equal to its atomic number.

Example 2.3.1

Draw the orbital filling diagram for carbon and write its electron configuration.

Solution

Step 1: List the known quantities and plan the problem.

Known

  • Atomic number of carbon, Z=6

Use the order of fill diagram to draw an orbital filling diagram with a total of six electrons. Follow Hund's rule. Write the electron configuration.

Step 2: Construct diagram.

CK12 Screenshot 5-17-1.png

Orbital filling diagram for carbon.

Electron configuration 1s22s22p2

Step 3: Think about your result.

Following the 2s sublevel is the 2p, and p sublevels always consist of three orbitals. All three orbitals need to be drawn even if one or more is unoccupied. According to Hund's rule, the sixth electron enters the second of those p orbitals, with the same spin as the fifth electron.

Second Period Elements

Periods refer to the horizontal rows of the periodic table. The first period of the periodic table contains only the elements hydrogen and helium. This is because the first principal energy level consists of only the s sublevel, and so only two electrons are required in order to fill the entire principal energy level. Each time a new principal energy level begins, as with the third element lithium, a new period is started on the periodic table. As one moves across the second period, electrons are successively added. With beryllium (Z=4), the 2s sublevel is complete and the 2p sublevel begins with boron (Z=5). Since there are three 2p orbitals and each orbital holds two electrons, the 2p sublevel is filled after six elements. Table 2.3.1 shows the electron configurations of the elements in the second period.

Element Name Symbol Atomic Number Electron Configuration
Table 2.3.1: Electron Configurations of Second-Period Elements
Lithium Li 3 1s22s1
Beryllium Be 4 1s22s2
Boron B 5 1s22s22p1
Carbon C 6 1s22s22p2
Nitrogen N 7 1s22s22p3
Oxygen O 8 1s22s22p4
Fluorine F 9 1s22s22p5
Neon Ne 10 1s22s22p6

Summary

Electron configuration notation simplifies the indication of where electrons are located in a specific atom. Superscripts are used to indicate the number of electrons in a given sublevel.

Contributors and Attributions


2.3: Electron Configurations is shared under a CC BY-NC license and was authored, remixed, and/or curated by LibreTexts.

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