Every electron shell, designated by "n", has a specific number of electron subshells. The shell number has values equivalent to natural numbers (1, 2, 3, 4...), while the subshells are s, p, d and f.
The subshell that does not exist within the third shell (n = 3) is the f subshell. The energy of the f subshell is too high and cannot be accommodated in the third shell.
