- Electron shells (also called energy levels) surround the nucleus of an atom. They are denoted by the principal quantum number, n (n=1, 2, 3, ...), where n=1 is the shell closest to the nucleus. - The maximum number of electrons that can occupy a given shell is determined by the formula 2n², where n is the shell number. - For the first shell (n=1), the maximum number of electrons is 2 (1)² = 2 1 = 2. - For the second shell (n=2), the maximum number of electrons is 2 (2)² = 2 4 = 8. - For the third shell (n=3), the maximum number of electrons is 2 (3)² = 2 9 = 18, and so on.