The 2, 8, 8, 18 rule describes the maximum electron capacity of electron shells: 1st shell holds 2, 2nd holds 8, 3rd holds 8 (initially) or 18 (maximum), and 4th holds 18.
Maximum Electrons Per Shell:
- Formula: 2n² (where n = shell number)
- 1st shell (n=1): 2 × 1² = 2 electrons
- 2nd shell (n=2): 2 × 2² = 8 electrons
- 3rd shell (n=3): 2 × 3² = 18 electrons (but fills to 8 first)
- 4th shell (n=4): 2 × 4² = 32 electrons
Why 3rd Shell Complexity:
- Initially fills to 8 electrons (stable octet)
- After 4th shell starts, 3rd can expand to 18
- Transition metals fill 3d orbitals later
Importants Points:
- Formula: 2n² for maximum capacity
- Common pattern: 2, 8, 8, 18 for first 36 elements
- Shell filling: Not always sequential