Let's solve the
Aufbau diagrams for the first four elements: hydrogen (H), helium (He), lithium (Li), and beryllium (Be). We’ll fill in the electron orbitals according to the
Aufbau principle, which states that electrons fill orbitals starting from the lowest energy level.
---
🔷 Key Rules:
- Electrons fill orbitals in order of increasing energy.
- The order is:
1s → 2s → 2p → 3s → 3p → 4s → 3d → ...
- Each orbital holds a maximum of
2 electrons (with opposite spins).
- For p, d, f subshells: each box represents one orbital (e.g., 2p has 3 orbitals).
---
✔ a) Hydrogen (H)
- Atomic number = 1 → 1 electron
- Electron configuration:
1s¹
Aufbau Diagram:
```
4s [ ] 4p [ ] [ ] [ ]
3s [ ] 3p [ ] [ ] [ ]
2s [ ] 2p [ ] [ ] [ ]
1s [↑] ← Only 1 electron in 1s
```
> ✔️ One arrow (↑) in the 1s orbital.
---
✔ b) Helium (He)
- Atomic number = 2 → 2 electrons
- Electron configuration:
1s²
Aufbau Diagram:
```
4s [ ] 4p [ ] [ ] [ ]
3s [ ] 3p [ ] [ ] [ ]
2s [ ] 2p [ ] [ ] [ ]
1s [↑↓] ← Two electrons (opposite spins) in 1s
```
> ✔️ Two arrows (↑↓) in the 1s orbital.
---
✔ c) Lithium (Li)
- Atomic number = 3 → 3 electrons
- Electron configuration:
1s² 2s¹
Aufbau Diagram:
```
4s [ ] 4p [ ] [ ] [ ]
3s [ ] 3p [ ] [ ] [ ]
2s [↑] 2p [ ] [ ] [ ]
1s [↑↓]
```
> ✔️ 1s filled (↑↓), 2s has one electron (↑)
---
✔ d) Beryllium (Be)
- Atomic number = 4 → 4 electrons
- Electron configuration:
1s² 2s²
Aufbau Diagram:
```
4s [ ] 4p [ ] [ ] [ ]
3s [ ] 3p [ ] [ ] [ ]
2s [↑↓] 2p [ ] [ ] [ ]
1s [↑↓]
```
> ✔️ Both 1s and 2s orbitals are fully filled.
---
🌟 Bonus: Electron Configurations for Elements 1–21
Here’s the full list:
| Element | Symbol | Electron Configuration |
|--------|--------|------------------------|
| 1 | H | 1s¹ |
| 2 | He | 1s² |
| 3 | Li | 1s² 2s¹ |
| 4 | Be | 1s² 2s² |
| 5 | B | 1s² 2s² 2p¹ |
| 6 | C | 1s² 2s² 2p² |
| 7 | N | 1s² 2s² 2p³ |
| 8 | O | 1s² 2s² 2p⁴ |
| 9 | F | 1s² 2s² 2p⁵ |
| 10 | Ne | 1s² 2s² 2p⁶ |
| 11 | Na | 1s² 2s² 2p⁶ 3s¹ |
| 12 | Mg | 1s² 2s² 2p⁶ 3s² |
| 13 | Al | 1s² 2s² 2p⁶ 3s² 3p¹ |
| 14 | Si | 1s² 2s² 2p⁶ 3s² 3p² |
| 15 | P | 1s² 2s² 2p⁶ 3s² 3p³ |
| 16 | S | 1s² 2s² 2p⁶ 3s² 3p⁴ |
| 17 | Cl | 1s² 2s² 2p⁶ 3s² 3p⁵ |
| 18 | Ar | 1s² 2s² 2p⁶ 3s² 3p⁶ |
| 19 | K | 1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ |
| 20 | Ca | 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² |
| 21 | Sc | 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹ |
> ⚠️ Note: After argon (Ar), the 4s fills before 3d — this is why potassium (K) and calcium (Ca) have 4s electrons before scandium (Sc) starts filling 3d.
---
✔ Summary:
You should now fill in the diagrams as follows:
-
Hydrogen: ↑ in 1s
-
Helium: ↑↓ in 1s
-
Lithium: ↑↓ in 1s, ↑ in 2s
-
Beryllium: ↑↓ in 1s, ↑↓ in 2s
All higher orbitals (2p, 3s, etc.) remain empty.
Let me know if you’d like help with the
last page or drawing the
full Aufbau diagram!
Parent Tip: Review the logic above to help your child master the concept of electron configuration worksheet and lots more.
