Final answer:
The subshells within a shell are designated by the azimuthal quantum number, with s, p, d, f being the most common subshell designations.
Step-by-step explanation:
The subshells within a shell are designated by the azimuthal quantum number. The subshells are usually denoted as s, p, d, f, and so on. Each shell has subshells equal to the shell number. For example, the 1st shell (n = 1) has only one subshell, which is the s subshell. The 2nd shell (n = 2) has two subshells, 2s and 2p. The 3rd shell (n = 3) has three subshells, 3s, 3p, and 3d. And the 4th shell (n = 4) has four subshells, 4s, 4p, 4d, and 4f.
The energy order of the subshells is as follows: 1s < 2s < 2p < 3s < 3p < 4s, and so on.