Elements

Electron configuration of Molybdenum (Mo)

Read the Molybdenum electron configuration, noble-gas shorthand, valence shell, orbital filling logic, and the mistakes to avoid when writing Mo. Includes the Mo 4d5 5s1 exception.

Guide: Guía práctica Read: 6 min Updated: 2026-07-07

Element guide

Electron configuration of Molybdenum (Mo)

The electron configuration of Molybdenum is 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d5 5s1. In shorthand form it is written as [Kr] 4d5 5s1.

Use this page to connect the notation with atomic number 42, the d-block, and the valence pattern 4d5 5s1.

How to write the configuration

Configuration at a glance

Molybdenum has atomic number 42, so a neutral atom has 42 electrons. The configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d5 5s1 is a compact record of where those electrons sit by subshell.

The shorthand [Kr] 4d5 5s1 replaces the filled inner core with the previous noble gas. That notation keeps attention on the electrons that usually explain bonding, ions, and periodic trends.

The Molybdenum exception

Molybdenum is a classic Aufbau exception. Based on the expected pattern for group 6, you would predict [Kr] 4d4 5s2 (following Cr: [Ar] 3d5 4s1 would suggest half-filled d-subshell stability). The measured ground state is [Kr] 4d5 5s1 — a half-filled d-subshell (4d5) with one electron in 5s.

The half-filled 4d subshell (one electron in each of the five d orbitals) provides exceptional stability through exchange energy. Every d orbital has one electron with parallel spin, which maximizes exchange energy and minimizes electron-electron repulsion.

This is the same principle that drives the Chromium exception ([Ar] 3d5 4s1). Molybdenum is one of the most reliable Aufbau exceptions to remember.

Step-by-step filling walkthrough: 1. Start with atomic number 42. 2. Fill through krypton: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 (36 electrons). 3. Fill 5s2 (38 electrons). 4. Fill 4d4 (42 electrons) naively. 5. For the correct ground state: shift one 5s electron to the 4d subshell, giving 4d5 5s1. 6. Verify: add every superscript and confirm the total equals 42.

Real-world context

Molybdenum is an essential trace element for most life forms. Its 4d5 5s1 configuration supports multiple oxidation states (most commonly +6, +4, +3), which is why Mo is a key component of nitrogenase and other metalloenzymes.

Common mistakes

The first mistake is writing [Kr] 4d4 5s2 instead of [Kr] 4d5 5s1. Always check: Mo is in group 6, same as Cr, and both favor half-filled d-subshell configurations. The second mistake is forgetting to count the 4d electrons correctly — Mo has 5 unpaired d electrons.

A third common error is writing the configuration without accounting for the exception. When an exam question asks for the ground-state configuration, the exception is what they are testing.

Molybdenum reference table

Item Value
Atomic number 42
Full configuration 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d5 5s1
Noble-gas notation [Kr] 4d5 5s1
Valence clue 4d5 5s1
Block d-block

Use the table as a learning reference, then verify advanced oxidation-state questions with your course material.

Before you submit an answer

  • The superscripts add up to 42 for a neutral Molybdenum atom.
  • The notation follows the filling order used in your course.
  • The last occupied subshell matches the periodic-table block.
  • Any transition-metal or actinide exception is checked against a reference.
  • The shorthand core is a noble gas, not a random previous element.

Common questions

What is the electron configuration of Molybdenum?

The neutral Molybdenum atom is written as 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d5 5s1. The shorthand form is [Kr] 4d5 5s1.

Is Molybdenum an exception to the Aufbau principle?

Yes. The expected configuration is [Kr] 4d4 5s2, but the ground state is [Kr] 4d5 5s1. The half-filled 4d subshell provides extra stability.

How many valence electrons does Molybdenum have?

Start from the outer-shell clue 4d5 5s1. For transition metals, inspect both s and d electrons carefully. Molybdenum has 6 valence electrons.

Why is Molybdenum important in biology?

Molybdenum's ability to access multiple oxidation states (+6, +5, +4, +3) makes it an essential cofactor in enzymes like nitrogenase and sulfite oxidase. This versatility comes from its 4d5 5s1 configuration.

Study notes and practice checks

From answer to understanding

Molybdenum's half-filled d-subshell exception is one of the most tested concepts in general chemistry. Unlike Chromium, which achieves 3d5 4s1, Mo achieves 4d5 5s1. The principle is identical: a half-filled d-subshell is energetically favorable.

Practice comparison

Compare Molybdenum (Mo) with Chromium (Cr) and Tungsten (W). Cr: [Ar] 3d5 4s1, Mo: [Kr] 4d5 5s1, W: [Xe] 4f14 5d4 6s2. Note that Tungsten reverts to the expected d4 s2 pattern, unlike its lighter congeners.

Teacher and self-study notes

The trend across group 6 is worth understanding: Cr and Mo both adopt d5 s1 configurations, but W does not. This is because the 5d and 6s orbitals have a larger energy difference, making the d4 s2 arrangement more stable for W.

Elena Marquez

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Elena Marquez

Chemistry educator and study-guide editor

Elena explains electron configurations, orbital diagrams, and periodic trends for learners who need clear steps before memorization. Her guides focus on counting, notation, and common classroom mistakes.

Credenciales: Editorial specialization in introductory chemistry, periodic trends, and student reference material.

Experiencia: More than eight years preparing practical chemistry explainers, worksheets, and reference material for independent learners.

Actualizado: 2026-07-07 Politica editorial Correcciones

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Guides use a repeatable count-and-check method, flag common exceptions, and avoid collecting personal or student data.

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