Shells and the 2, 8, 8, 2 rule (spec 1.21)
Fill the lowest shell first; capacities are 2, 8, 8 for the first 20 elements.
Electrons are arranged in shells (also called energy levels) around the nucleus. The shell closest to the nucleus has the lowest energy, so electrons always fill the inner shells first before starting a new one.
For the first 20 elements the maximum number of electrons each shell can hold is:
- 1st shell: holds up to 2 electrons.
- 2nd shell: holds up to 8 electrons.
- 3rd shell: holds up to 8 electrons (for the first 20 elements).
- 4th shell: just starts to fill (with the last 1 or 2 electrons) for potassium and calcium.
So you fill electrons in the order 2, 8, 8, 2. The number of electrons to place equals the atomic number (because a neutral atom has equal protons and electrons).
How to write a configuration from the atomic number. Take the atomic number, then "spend" the electrons shell by shell:
- Sodium, Na (Z = 11): 2 in shell 1, 8 in shell 2, 1 left β 2,8,1.
- Chlorine, Cl (Z = 17): 2, then 8, then 7 left β 2,8,7.
- Argon, Ar (Z = 18): 2, then 8, then 8 β 2,8,8 (full).
- Calcium, Ca (Z = 20): 2, 8, 8, then 2 left β 2,8,8,2.
Write the numbers separated by commas with the inner shell first. The numbers must add up to the atomic number β always check this.
- Shell capacities for the first 20 elements: 2, 8, 8 (then 4th shell starts).
- Always fill the lowest (inner) shell first.
- Number of electrons = atomic number (neutral atom).
- Write inner shell first, comma-separated: e.g. 2,8,1.
See the full worked example for electronic configurations β