Aufbau principle: the order of filling
Why 4s fills before 3d
1. Aufbau principle (order of filling): from zero
2. Where do electrons 19 and 20 go?
Argon's 18 electrons fill 1s, 2s, 2p, 3s and 3p. So where do calcium's electrons 19 and 20 go?
Pick the right answer
Predict:
- A. Into 3d
- B. Into 4s
- C. Into 4p
Explanation
Electrons 19 and 20 go into 4s. By the Aufbau principle, 4s fills before 3d, so calcium is [Ar]\ 4s²; 3d starts filling at scandium.
3. The Aufbau principle
Aufbau principle (building-up principle)
Electrons fill the sublevels in order, starting with the lowest-energy sublevel, 1s.
gathered1s → 2s → 2p → 3s → 3p \\ → 4s → 3d → 4pgathered
4. Build the atoms from Ar to V
[Ar] means argon's 18 electrons: 1s²\ 2s²\ 2p⁶\ 3s²\ 3p⁶. Change the atomic number from Ar to V and watch where each new electron goes.
Atomic number (Z) 19
↑
4sFrom Sc to V, each new electron goes into 3d. Beyond V, Cr and Cu break this pattern; you will meet them in Ch.1.
True or false?
3d fills before 4s, because 3 is less than 4.
Explanation
The statement is false. The level number alone does not set the filling order: in K and Ca the new electrons go into 4s, and 3d starts filling only at Sc.
5. Use the order
Put in order
Put these sublevels in the order they are filled.
- 3s
- 3p
- 4s
- 3d
- 4p
Fill in the blanks
Gallium (Z = 31) has 13 electrons after [Ar]. Write how many go into each sublevel: 4s _____, 3d _____, 4p _____.
Explanation
4s² (2) + 3d¹⁰ (10) + 4p¹ (1) = 13.
Need a hint?
- Follow the order: 4s, then 3d, then 4p.
- In gallium, 4s and 3d fill to their maximum before 4p. Whatever is left goes to 4p.
6. Summary
- Aufbau: electrons fill the sublevels in order, starting with 1s.
- Order: 1s 2s 2p 3s 3p 4s 3d 4p.
- Back to the hook: electrons 19 and 20 go into 4s; 3d starts at Sc.
| Element | Electronic configuration |
|---|---|
| ₁₉K | [Ar]\ 4s¹ |
| ₂₀Ca | [Ar]\ 4s² |
| ₂₁Sc | [Ar]\ 4s²\ 3d¹ |
Using calcium, can you explain why '3 comes before 4' is the wrong way to find the filling order?