Electron configuration
Electron configuration
- Electrons sit in shells, split into sub-shells, built from orbitals.
- They fill from lowest energy up.
- One surprise: 4s fills before 3d.
Shells, sub-shells and orbitals
- Each shell has a principal quantum number $n = 1, 2, 3, \dots$ (bigger $n$ = further out, higher energy).
- Sub-shells are s, p, d; each is made of orbitals holding 2 electrons each.
| Sub-shell | Orbitals | Max electrons |
|---|---|---|
| s | 1 | 2 |
| p | 3 | 6 |
| d | 5 | 10 |
Practice
How many electrons can a p sub-shell hold?
A p sub-shell has 3 orbitals, each holding 2 electrons, so 6 in total (s = 2, d = 10).
Order of filling
$$1\text{s} < 2\text{s} < 2\text{p} < 3\text{s} < 3\text{p} < 4\text{s} < 3\text{d} < 4\text{p}$$
- 4s is lower than 3d, so 4s fills first.
Practice
Which sub-shell fills first?
4s lies just below 3d in energy, so 4s fills before 3d.
Writing configurations
- Iron (Fe, $Z=26$): $1\text{s}^2\,2\text{s}^2\,2\text{p}^6\,3\text{s}^2\,3\text{p}^6\,3\text{d}^6\,4\text{s}^2$, or shorthand $[\text{Ar}]\,3\text{d}^6\,4\text{s}^2$.
- For positive ions, a transition metal loses its 4s electrons first: $\text{Fe}^{3+}$ is $[\text{Ar}]\,3\text{d}^5$.
Practice
The ground-state configuration of iron (Z = 26) is:
After argon (18), iron adds 4s² then 3d⁶: [Ar] 3d⁶ 4s².
Practice
When iron forms Fe³⁺, which electrons are lost first?
Transition metals lose 4s before 3d, so Fe³⁺ is [Ar] 3d⁵.
You've got it
Key idea
- shells ($n$) → sub-shells (s/p/d) → orbitals (2 electrons each); s=2, p=6, d=10
- fill lowest energy first; 4s fills before 3d
- Fe: $[\text{Ar}]\,3\text{d}^6\,4\text{s}^2$
- ions lose 4s before 3d: $\text{Fe}^{3+} = [\text{Ar}]\,3\text{d}^5$