1.3.3—Main energy levels
- Syllabus
- First assessment 2025
- Objective
- 1.3.3
- Level
- SL
Main Energy-Level Capacity
A main energy level, or shell, is identified by the principal quantum number n = 1, 2, 3, and so on.
maximumelectrons=2n2
| n | Maximum electrons |
|---|---|
| 1 | 2 |
| 2 | 8 |
| 3 | 18 |
| 4 | 32 |
Substitute the stated n value into 2n²; do not confuse the shell number with the capacity.
For n = 3, the theoretical capacity is 2(3²) = 18 electrons. This is a capacity, not a claim that every third shell is full: the actual occupancy depends on the atom and the relative energies of available sublevels.
Calculating Shell Capacity
[ 1 ]
What is the maximum number of electrons that can occupy the n=3 main energy level?
Electron Configurations Summary
Retrieve the chain: emission lines reveal discrete levels; capacities, sublevels, orbitals, and spin rules build configurations; first and successive ionization energies then reveal how electrons are held and arranged.
When checking an answer, ask: Did I link a line to a transition? Did I use 2n² and the filling rules? Did I explain an ionization trend or count electrons before a successive-IE jump?