Student Resources

Figure 2.10 The Periods of the Periodic Table Represent Electron Shells (A) Each electron shell is represented by a row or period on the the periodic table. (B) A representation of the electron shells of the sodium atom. Sodium has an atomic number of 11, thus it has 11 protons and 11 electrons in the elemental form. The first and innermost electron shell can hold a total of 2 electrons, as indicated by the gray circles in the diagram. The second electron shell can hold 8 electrons and is completely full. The third electron shell has the capacity to hold 18 electrons, however, in sodium, there is only one more electron that needs to be placed after filling the first and second electron shells. Thus, the third electron shell of sodium only has 1 electron. 
Figure 2.10B adapted from Pumbaa
The electron shells are further subdivided into subshells that are represented by the second quantum number. There are four major types of subshells that are used to house electrons in known elements. These are termed the s, p, d, and f subshells. The first electron shell (n = 1) is so small that it only contains an s-subshell. The second electron shell (n = 2) is large enough to house an s- and a p-subshell. The third electron shell (n = 3) contains s-, p-, and d-subshells. The fourth shell and above (n = 4, 5, 6, and 7) contain all four electron subshells (s, p, d, and f). Shells 5-7 are predicted to have additional subshells, but elements using these subshells have never been discovered, so we will limit our discussion with the s, p, d, and f subshells.

Each subshell, in turn, contains a specified number of electron orbitals arranged in unique shapes (Figure 2.11). Each orbital can then contain a total of 2 electrons rotating in opposite spin states. The s-subshell only contains 1 circular orbital that can house a total of two electrons. The p-subshell contains a total of 3 dumbbell-shaped orbitals that can house a total of 6 electrons. The d-subshell contains 5 uniquely shaped orbitals that can house a total of 10 electrons and the f-subshell contains 7 orbitals with a potential of housing 14 electrons. While the 5th – 7th shells are predicted to be able to house more electrons in even stranger g, h, and i subshells, there have never been any atoms discovered that contain enough electrons to utilize these locations. Atoms appear to become too large and unstable after the filling of the f-subshell orbitals in the 4th and 5th shell.