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

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

Molybdenum (Mo) has atomic number 42 and a ground-state electron configuration of 1s² 2s² 2p⁶ 3s² 3p⁶ 3d¹⁰ 4s² 4p⁶ 4d⁵ 5s¹. This deviates from the Aufbau-predicted [Kr] 4d⁴ 5s² because a half-filled 4d⁵ subshell achieves extra electrostatic stability — one electron from 5s shifts into 4d, giving five singly occupied d orbitals. This page provides the full orbital diagram, subshell breakdown, quantum numbers for all 42 electrons, and molybdenum's key physical and chemical properties.

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: 2026-06-19 · Reviewed by Nham Vu

Molybdenum Electron Configuration

Molybdenum — Electron Configuration

Subshell Breakdown

Aufbau Prediction vs. Actual Configuration

Shell Fill Summary

Orbital Box Diagram

Quantum Numbers for All 42 Electrons

Identity

Physical Properties

Electronic Properties

Context & Uses

Summary

How it works

Use cases

Frequently Asked Questions

Molybdenum Electron Configuration

Use the Molybdenum Electron Configuration

Molybdenum — Electron Configuration

Subshell Breakdown

Aufbau Prediction vs. Actual Configuration

Shell Fill Summary

Orbital Box Diagram

Quantum Numbers for All 42 Electrons

Identity

Physical Properties

Electronic Properties

Context & Uses

Summary

How it works

Use cases

Frequently Asked Questions