Where the Electrons Live
In the previous lesson, we learned that electrons orbit the nucleus. But they don't just float around randomly - they're organized into specific energy levels and sublevels.
Understanding electron configuration helps us:
Electrons are arranged in principal energy levels (also called shells), numbered 1, 2, 3, 4, etc.
Each principal energy level contains one or more sublevels. These are named s, p, d, and f.
| Sublevel | Shape | # of Orbitals | Max Electrons |
|---|---|---|---|
| s | Spherical | 1 | 2 |
| p | Dumbbell/Figure-8 | 3 | 6 |
| d | Cloverleaf | 5 | 10 |
| f | Complex | 7 | 14 |
| Level | Available Sublevels | Max Electrons |
|---|---|---|
| n = 1 | 1s | 2 |
| n = 2 | 2s, 2p | 8 |
| n = 3 | 3s, 3p, 3d | 18 |
| n = 4 | 4s, 4p, 4d, 4f | 32 |
Electrons fill sublevels in order of increasing energy, not necessarily by energy level number! This is called the Aufbau Principle (German for "building up").
1s → 2s → 2p → 3s → 3p → 4s → 3d → 4p → 5s → 4d → 5p → 6s → 4f → 5d → 6p → 7s → 5f → 6d → 7p
1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f
6s 6p 6d
7s 7p
Follow the arrows diagonally: ↗
Problem: Write the electron configuration for calcium.
Solution:
20 electrons to place. Follow the filling order:
Ca: 1s²2s²2p⁶3s²3p⁶4s²
Problem: Write the electron configuration for phosphorus.
Solution:
15 electrons to place:
P: 1s²2s²2p⁶3s²3p³
Problem: Write the electron configuration for lead.
Solution:
82 electrons - follow the filling order completely:
Pb: 1s²2s²2p⁶3s²3p⁶4s²3d¹⁰4p⁶5s²4d¹⁰5p⁶6s²4f¹⁴5d¹⁰6p²
Check: 2+2+6+2+6+2+10+6+2+10+6+2+14+10+2 = 82 ✓
Writing full configurations for heavy elements is tedious! We can use noble gas shorthand to abbreviate.
The Noble Gases:
Full: 1s²2s²2p⁶3s²3p⁶4s²
Noble gas before Ca: Ar (Z = 18)
Shorthand:
Ca: [Ar]4s²
Problem: Write the noble gas configuration for vanadium.
Solution:
Noble gas before V: Ar (18 electrons)
Remaining electrons: 23 - 18 = 5
After [Ar], fill: 4s² (2 electrons), then 3d³ (3 electrons)
V: [Ar]4s²3d³
Solution:
Noble gas before Na: Ne (10 electrons)
Remaining: 11 - 10 = 1 electron → goes in 3s
Na: [Ne]3s¹
Orbital diagrams show electrons as arrows in boxes. Each box represents one orbital.
Number of boxes per sublevel:
Each orbital can hold at most 2 electrons, and they must have opposite spins (↑↓).
❌ ↑↑ (same spin - WRONG!)
✅ ↑↓ (opposite spin - correct)
When filling orbitals of equal energy, put one electron in each orbital first (all with same spin), then pair them up.
❌ [↑↓][↑][ ] (paired too early)
✅ [↑][↑][↑] (spread out first)
Configuration: 1s²2s²2p³
Orbital Diagram:
Notice: The 3 electrons in 2p spread out (Hund's Rule), each in its own orbital with the same spin.
Unpaired electrons: 3
Configuration: [Ne]3s²3p¹
Orbital Diagram (valence only):
Is Al a metal or nonmetal? Metal
Will it give or take electrons? Give (metals lose electrons)
Ion charge? Al³⁺ (loses the 3s² and 3p¹ electrons)
Mistake 1: Pairing too early (violates Hund's Rule)
Mistake 2: Same spin in one orbital (violates Pauli)
Unpaired electrons are electrons that are alone in an orbital (not paired with another electron of opposite spin).
Problem: How many unpaired electrons are in [Ar]4s¹3d⁵?
Solution:
Unpaired electrons: 6 (1 in 4s + 5 in 3d)
Compare: F, S, Cu, N
Answer: N has the most unpaired electrons (3)
Problem: What element has the configuration 1s²2s²2p⁶3s²3p⁵?
Solution:
Count electrons: 2 + 2 + 6 + 2 + 5 = 17
Atomic number 17 = Chlorine (Cl)
Problem: What element has the configuration [Xe]6s¹?
Solution:
Xe has 54 electrons + 1 more = 55 electrons
Atomic number 55 = Cesium (Cs)
| s sublevel | 1 orbital, 2 electrons max |
| p sublevel | 3 orbitals, 6 electrons max |
| d sublevel | 5 orbitals, 10 electrons max |
| f sublevel | 7 orbitals, 14 electrons max |
| Aufbau Principle | Fill lowest energy first |
| Pauli Exclusion | Max 2 electrons per orbital, opposite spins |
| Hund's Rule | Spread out before pairing in same sublevel |