You have already seen that the electrons are arranged in shells and that the valence electrons involved in bonding are in incomplete outer shells.
Each shell contains one or more
subshells.
The first twenty elements have all of their electrons in two different types of subshell that are designated by the letters
s and
p.
These are from the crystallographic terms sharp and principal.
The number of subshells in each shell and their letter designations are shown below.
As shown in the summary table below,
the number of the shell and the number of subshells that it contains are the same.
both
shell 1 and shell 2 (and indeed all shells) have an
s subshell.
shells
beyond shell 1 have both an
s and a p subshell | Shell | Number of Subshells in the Shell | Letter Designation of Subshell |
| 1 | 1 | s |
| 2 | 2 | s and p |