Electron Configuration Order
1s 2s 2p 3s 3p 4s 3d 4p 5s 4d 5p 6s 4f 5d 6p 7s 5f 6d 7p
The aufbau principle filling order can be remembered with a diagonal diagram. Electrons fill orbitals in order of increasing energy, following s < p < d < f within similar energy levels.
Breakdown
First shell
Holds 2 electrons max
Second shell
s holds 2, p holds 6
Third shell
Same as second
Fourth/Third
4s fills before 3d!
Continue pattern
Follow diagonal rule
f-orbitals
Lanthanides and actinides
Example
Iron (Fe, 26 electrons): 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁶. Or in noble gas notation: [Ar] 4s² 3d⁶.
When to Use This
- ✓Writing electron configurations
- ✓Understanding periodic table organization
- ✓Predicting magnetic properties
- ✓Explaining periodic trends
FAQs
Common questions about this mnemonic
The 4s orbital has lower energy than 3d due to better penetration toward the nucleus. However, when removing electrons from transition metals, 4s electrons are removed first because 3d is then lower in energy.
Chromium is [Ar] 4s¹ 3d⁵ and Copper is [Ar] 4s¹ 3d¹⁰ because half-filled and fully-filled d subshells have extra stability. These are the most common exceptions to memorize.