The common hydrogen atom contains one proton, one electron, and no
The electrons revolve or orbit around the nucleus. They are arranged in shells or
orbits much like the planets revolve around the sun (see figure 2-2). The first shell (K
shell) can hold one or two electrons but no more. The second shell, or L shell, may
contain up to 8 electrons and the third, or M shell, may contain as many as 18 electrons.
Some atoms have numerous electrons. To determine the maximum number of
electrons in any given shell, the calculations in Table 2-1 can be used. However, no
outer shell actually contains more than eight electrons (see figure 2-3).
Figure 2-2. Diagram of K, L, and M shells.
2 X 12
2 (K shell)
2 X 22
8 (L shell)
2 X 32
18 (M shell)
Additional shells follow the same progression. For example, the N shell can
contain up to 32 (2 X 42 ) electrons.
Table 2-1. Determining the maximum number of electrons in any given shell.