1 Principal quantum number (n)

Used already by Bohr, it:

1. determines the order of the principal energy levels (shells) and their number: in the heaviest known atom in the ground state there are seven shells (n = 1 to 7: K, L, M, N, O, P, Q);
2. determines the maximum number of electrons that fill a shell = 2n²:
   • K (n = 1): 2(1)² = 2 electrons;
   • L (n = 2): 2(2)² = 8;
   • M (n = 3): 2(3)² = 18;
   • N (n = 4): 2(4)² = 32.

• The rule does not apply to levels beyond the fourth (an atom becomes unstable if a level has more than 32 electrons).
• n is always a positive integer: 1, 2, 3, …; it can never be zero or fractional.
• A larger n means a larger and higher-energy shell.