Final answer:
No, the principal quantum number must be a positive integer starting at 1. It defines the energy level of an electron and is associated with a principal electron shell.
Step-by-step explanation:
The student asks if the principal quantum number can be 0 or less than zero. The answer is No, the principal quantum number cannot be 0. In quantum mechanics, the principal quantum number (n) is used to characterize the energy level in which an electron resides and it can only be a positive integer. The values for n start at 1 and increase in value (n = 1, 2, 3, ...), with each value of n defining a principal electron shell. The secondary quantum number, or angular momentum quantum number (l), is dependent on n, and can have integer values from 0 up to n-1.