Electron Configurations
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 1s^{1}, while the helium configuration is 1s^{2}. Multiple occupied sublevels are written one after another. The electron configuration of lithium is 1s^{2}2s^{1}. 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.
Sample Problem: Orbital Filling Diagrams and Electron Configurations
Draw the orbital filling diagram for carbon and write its electron configuration.
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
Electron configuration 1s^{2}2s^{2}2p^{2}
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 and with the same spin as the fifth electron.
Second Period Elements
Periods refer to the horizontal rows of the periodic table. Looking at a periodic table you will see that the first period 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. The Table below shows the electron configurations of the elements in the second period.
Element Name | Symbol | Atomic Number | Electron Configuration |
Lithium | Li | 3 | 1s^{2}2s^{1} |
Beryllium | Be | 4 | 1s^{2}2s^{2} |
Boron | B | 5 | 1s^{2}2s^{2}2p^{1} |
Carbon | C | 6 | 1s^{2}2s^{2}2p^{2} |
Nitrogen | N | 7 | 1s^{2}2s^{2}2p^{3} |
Oxygen | O | 8 | 1s^{2}2s^{2}2p^{4} |
Fluorine | F | 9 | 1s^{2}2s^{2}2p^{5} |
Neon | Ne | 10 | 1s^{2}2s^{2}2p^{6} |
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.