What is the electron configuration of molybdenum?

Molybdenum (Mo, Z=42) has the ground-state electron configuration 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d⁵ 5s¹, or in noble-gas shorthand [Kr] 4d⁵ 5s¹. This is anomalous — the Aufbau principle would predict [Kr] 4d⁴ 5s².

Why is molybdenum's electron configuration anomalous?

A half-filled 4d subshell (4d⁵) is especially stable: each orbital holds exactly one electron, minimizing electron-electron repulsion and maximizing exchange energy. This stability gain is large enough that one 5s electron shifts into 4d, overriding the normal Aufbau filling order.

What is the noble-gas shorthand for molybdenum?

The condensed notation is [Kr] 4d⁵ 5s¹, where [Kr] represents the filled krypton core (1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶, Z=36).

How is molybdenum's anomaly different from chromium's?

They are structurally identical — both are Group 6 elements that gain one electron from the outer s orbital to achieve a half-filled d subshell. Chromium does it with 3d⁵ 4s¹ (Period 4) and molybdenum with 4d⁵ 5s¹ (Period 5). The driving force is the same exchange-energy stabilization.

What are molybdenum's common oxidation states?

Molybdenum commonly exhibits +4 (e.g., MoO₂, MoS₂) and +6 (e.g., MoO₃, molybdate MoO₄²⁻). It can also appear as +2, +3, and +5 in coordination compounds. The +6 state is most common in aqueous chemistry.

How many valence electrons does molybdenum have?

Molybdenum has 6 valence electrons — one in 5s and five in 4d. All six can participate in bonding, explaining the wide range of oxidation states from +1 to +6.

What block and period is molybdenum in?

Molybdenum is in Period 5, Group 6 (VIB), and belongs to the d-block (transition metals) of the periodic table. It sits directly below chromium and above tungsten.

What is exchange energy and why does it stabilize half-filled d subshells?

Exchange energy is stabilization gained when electrons of the same spin can "exchange" positions between degenerate orbitals. With five singly occupied 4d orbitals, the number of possible same-spin electron pairs is maximized (10 pairs), giving the maximum exchange-energy benefit and making the half-filled configuration unusually stable.

Molybdenum Electron Configuration

Name: Molybdenum Electron Configuration
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Author: Nham Vu

Molybdenum (Mo, Z=42) has the anomalous electron configuration [Kr] 4d⁵ 5s¹ — not [Kr] 4d⁴ 5s² as Aufbau predicts. The half-filled 4d subshell provides extra stability, making Mo a classic exception alongside chromium.

Z = 42 Mo Molybdenum

Molybdenum — Electron Configuration

Atomic number 42 · Transition metal · Period 5, Group 6 · d-block

[Kr] 4d⁵ 5s¹ Anomalous 42 electrons 6 valence e⁻

Anomalous Configuration — Half-Filled d-Shell Exception

The Aufbau principle predicts [Kr] 4d⁴ 5s² for Mo. The actual configuration is [Kr] 4d⁵ 5s¹ because one 5s electron shifts into 4d, achieving a half-filled (4d⁵) subshell. Five singly occupied d orbitals maximize exchange energy, making this arrangement more stable than the Aufbau prediction.

Subshell Breakdown

Subshell	Type	Electrons	Max	Notation	Note
1s	s, n=1	2	2	1s²
2s	s, n=2	2	2	2s²
2p	p, n=2	6	6	2p⁶
3s	s, n=3	2	2	3s²
3p	p, n=3	6	6	3p⁶
3d	d, n=3	10	10	3d¹⁰
4s	s, n=4	2	2	4s²
4p	p, n=4	6	6	4p⁶
4d	d, n=4	5	10	4d⁵	Half-filled — anomalous
5s	s, n=5	1	2	5s¹	Aufbau predicts 5s²
Total		42

Full Configuration

1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d⁵ 5s¹

All subshells written explicitly.

Noble-Gas Shorthand

[Kr] 4d⁵ 5s¹

[Kr] = 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ (36 electrons).

Aufbau Prediction vs. Actual Configuration

Aufbau Prediction

[Kr] 4d⁴ 5s²

Fills 5s first, then 4d — gives 4 electrons in 4d, 2 in 5s.

Incorrect for Mo

Actual Ground State

[Kr] 4d⁵ 5s¹

One 5s electron shifts to 4d — five singly occupied d orbitals, maximizing exchange energy.

Correct

Shell Fill Summary

Shell 1 (n=1) — 1s² 2 / 2 electrons (100%)

Shell 2 (n=2) — 2s² 2p⁶ 8 / 8 electrons (100%)

Shell 3 (n=3) — 3s² 3p⁶ 3d¹⁰ 18 / 18 electrons (100%)

Shell 4 (n=4) — 4s² 4p⁶ 4d⁵ 13 / 32 electrons (41%)

Shell 5 (n=5) — 5s¹ 1 / 50 electrons (2%)

Shell 4 can hold up to 32 electrons (4s + 4p + 4d + 4f). Mo uses 13 of those 32 slots. Shell 5 holds 1 of a possible 50 electrons.

Quantum Numbers for All 42 Electrons

n = principal · l = azimuthal (0=s, 1=p, 2=d) · mℓ = magnetic · mΰ = spin

e⁻ #	n	l	mℓ	mΰ	Subshell
1	1	0	0	+½	1s
2	1	0	0	−½	1s
3	2	0	0	+½	2s
4	2	0	0	−½	2s
5	2	1	-1	+½	2p
6	2	1	-1	−½	2p
7	2	1	0	+½	2p
8	2	1	0	−½	2p
9	2	1	1	+½	2p
10	2	1	1	−½	2p
11	3	0	0	+½	3s
12	3	0	0	−½	3s
13	3	1	-1	+½	3p
14	3	1	-1	−½	3p
15	3	1	0	+½	3p
16	3	1	0	−½	3p
17	3	1	1	+½	3p
18	3	1	1	−½	3p
19	3	2	-2	+½	3d
20	3	2	-2	−½	3d
21	3	2	-1	+½	3d
22	3	2	-1	−½	3d
23	3	2	0	+½	3d
24	3	2	0	−½	3d
25	3	2	1	+½	3d
26	3	2	1	−½	3d
27	3	2	2	+½	3d
28	3	2	2	−½	3d
29	4	0	0	+½	4s
30	4	0	0	−½	4s
31	4	1	-1	+½	4p
32	4	1	-1	−½	4p
33	4	1	0	+½	4p
34	4	1	0	−½	4p
35	4	1	1	+½	4p
36	4	1	1	−½	4p
37	4	2	-2	+½	4d
38	4	2	-1	+½	4d
39	4	2	0	+½	4d
40	4	2	1	+½	4d
41	4	2	2	+½	4d
42	5	0	0	+½	5s

Electrons 37–42 (highlighted) are the anomalous 4d⁵ 5s¹ valence shell. Note all five 4d electrons share mΰ = +½ — the maximum-multiplicity arrangement per Hund's rule.

Summary

How it works

The Aufbau principle normally fills 5s before 4d, predicting [Kr] 4d⁴ 5s² for molybdenum.
Instead, one 5s electron migrates to 4d, yielding [Kr] 4d⁵ 5s¹ — a half-filled d subshell.
A half-filled subshell (all five 4d orbitals singly occupied) is unusually stable: reduced electron-electron repulsion and maximized exchange energy.
The exchange-energy gain outweighs the small energy cost of promoting one 5s electron.
The result: 42 electrons arranged as 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d⁵ 5s¹.
Chromium (Z=24) shows an identical pattern one period above — [Ar] 3d⁵ 4s¹ instead of [Ar] 3d⁴ 4s².

Use cases

Verify molybdenum's actual ground-state configuration vs. the Aufbau prediction.
Understand why molybdenum is a classic exception to the Aufbau principle.
Study exchange energy and half-filled d-subshell stability concepts.
Compare molybdenum's anomaly with chromium (Z=24) in the same Group 6.
Review oxidation states (+4, +6) and how the d⁵ configuration relates to Mo chemistry.
Use the quantum-number table for all 42 electrons in exam preparation.
Teaching aid for inorganic chemistry courses on d-block electronic structure.
Quick reference for periodic trends across Period 5 transition metals.

Frequently Asked Questions

Last updated: 2026-07-23 · Reviewed by Nham Vu