Aufbau principle
Electrons fill the lowest energy subshell first. Lower (n + l) first; for equal (n + l), lower n first.
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CrChromium, Z = 24, 51.996 u
[Ar] 3d5 4s1
In full: 1s² 2s² 2p⁶ 3s² 3p⁶ 3d⁵ 4s¹
- Blockd block
- Period4
- Group6
- Valence6
- Unpaired6
- MagneticPara
Last electron 3d
- n3shell
- l2d subshell
- m+2orbital
- s+½spin up
Why Cr is an exception
Plain filling would give [Ar] 3d⁴ 4s². A half-filled d subshell (d⁵) is extra stable: five electrons with parallel spins in five orbitals give the largest exchange energy and a symmetrical charge cloud. So one s electron moves into d.
Order of filling
- 1s1
- 2s2
- 2p3
- 3s3
- 3p4
- 4s4
- 3d5
- 4p5
- 5s5
- 4d6
- 5p6
- 6s6
- 4f7
- 5d7
- 6p7
- 7s7
- 5f8
- 6d8
- 7p8
The small number is n + l. 4s (4 + 0 = 4) fills before 3d (3 + 2 = 5).
Aufbau principle: quick answers
What is the aufbau principle?
In the ground state, electrons fill the orbitals of lowest energy first. The order of energy follows the (n + l) rule: the lower the value of (n + l), the lower the energy, and for equal (n + l) the orbital with lower n has the lower energy.
Why does 4s fill before 3d?
For 4s, n + l = 4 + 0 = 4. For 3d, n + l = 3 + 2 = 5. The lower value fills first, so 4s fills before 3d. Once 3d holds electrons, 4s is the outer shell, which is why ions lose 4s electrons first.
What are Hund’s rule and the Pauli exclusion principle?
Pauli: no two electrons in an atom have the same four quantum numbers, so an orbital holds at most two electrons with opposite spins. Hund: in orbitals of equal energy, electrons go in singly with parallel spins before any pairing starts.