Final answer:
The principal quantum number, denoted as 'n' in quantum number sequences, represents the principal energy level (PEL) of an electron within an atom.
Step-by-step explanation:
The number in the quantum number sequence that represents the principal energy level (PEL) is the principal quantum number, symbolized by n. It indicates the main energy level occupied by an electron and corresponds to the row number of an atom on the periodic table.
The other important quantum number is the angular momentum quantum number, designated by l, which can have integer values from 0 to n - 1. The letter (s, p, d, f) indicates the type of subshell, each corresponding to a specific value of l: 0 for s, 1 for p, 2 for d, and 3 for f.