Element guide
Electron configuration of Chromium (Cr)
The electron configuration of Chromium is 1s2 2s2 2p6 3s2 3p6 3d5 4s1. In shorthand form it is written as [Ar] 3d5 4s1.
Use this page to connect the notation with atomic number 24, the d-block, and the valence pattern 4s1.
How to write the configuration
Configuration at a glance
Chromium has atomic number 24, so a neutral atom has 24 electrons. The configuration 1s2 2s2 2p6 3s2 3p6 3d5 4s1 is a compact record of where those electrons sit by subshell. The number before each letter is the principal energy level, the letter names the subshell, and the superscript tells you how many electrons are assigned there.
The shorthand [Ar] 3d5 4s1 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 Chromium exception you need to know
Chromium is one of the most frequently tested exceptions in general chemistry. Based on the Aufbau principle, you would expect the configuration to be [Ar] 3d4 4s2 (4s fills before 3d). However, the measured ground state of chromium is [Ar] 3d5 4s1. A half-filled 3d subshell (3d5) is more stable than a 4s2 3d4 arrangement because the exchange energy gained from having all five d orbitals half-occupied outweighs the energy cost of a half-empty 4s orbital. The same principle applies to molybdenum (Mo) in group 6.
For Chromium, apply the exception: after filling 1s2 2s2 2p6 3s2 3p6 4s2 3d4, shift one 4s electron to 3d to get 3d5 4s1.
Step-by-step filling walkthrough: 1. Start with atomic number 24. 2. Follow the Aufbau sequence: fill 1s, 2s, 2p, 3s, 3p, then continue through 4s, 3d, 4p in order. 3. Keep a running total of electrons after each subshell. 4. Write the noble-gas shorthand by replacing the filled core with the previous noble gas in brackets. 5. Verify: add every superscript and confirm the total equals 24.
Real-world applications
Chromium's half-filled d-shell contributes to its hardness and corrosion resistance. The 4s1 electron participates in metallic bonding, giving chromium a high melting point (1907°C). Chromium is widely used in stainless steel production, chrome plating, and as a corrosion-resistant coating. The Cr3+ ion is an essential trace nutrient for humans.
Common oxidation states are +3 (removing 4s1 and two 3d electrons) and +6 (removing all six valence electrons for compounds like chromate and dichromate). Cr2+ is also possible but less stable in water.
Common mistakes
- Writing 3d4 4s2: This follows the basic Aufbau order but ignores the half-filled d-subshell stabilization. The correct ground state is 3d5 4s1.
- Counting 25 electrons instead of 24: Double-check that superscripts sum to 24, not 25.
- Forgetting the exception applies to Cr and Mo but not W: Tungsten follows the expected 5d4 6s2 pattern due to relativistic effects.
- Confusing Cr2+ with Cr: The Cr2+ ion removes the 4s1 electron first, giving [Ar] 3d4.
Chromium reference table
| Item | Value |
|---|---|
| Atomic number | 24 |
| Full configuration | 1s2 2s2 2p6 3s2 3p6 3d5 4s1 |
| Noble-gas notation | [Ar] 3d5 4s1 |
| Valence clue | 4s1 |
| 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 24 for a neutral Chromium 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 Chromium?
The neutral Chromium atom is written as 1s2 2s2 2p6 3s2 3p6 3d5 4s1. The shorthand form is [Ar] 3d5 4s1.
How many valence electrons does Chromium have?
Start from the outer-shell clue 4s1. For main-group elements this usually gives a direct valence count; for transition metals, inspect both s and d electrons carefully.
Should I memorize this configuration?
Memorize the filling pattern first. Individual configurations become easier when you can rebuild them from atomic number, noble-gas cores, and periodic-table position.
What are the common oxidation states of Chromium?
The d-block position and outer-shell configuration 4s1 determine the most stable ions. Cr commonly forms +3 and +6, with +2 appearing in some conditions.
Practice nearby concepts
Study notes and practice checks
Element-specific practice problems
Problem 1: Write the full electron configuration for neutral Chromium and verify the electron count.
Problem 2: Convert the full configuration to noble-gas shorthand notation.
Problem 3: Predict the electron configurations of Cr3+ and Cr6+. Explain which electrons are removed first.
Problem 4: Compare the configuration of Chromium with Vanadium directly before it and Manganese directly after it. What changes in the d-subshell between V (3d3 4s2), Cr (3d5 4s1), and Mn (3d5 4s2)?
Problem 5: Without looking at the periodic table, derive the configuration from atomic number 24. Fill each subshell in Aufbau order and keep a running total. Check your answer against the configuration shown on this page.
Siguiente lectura
Guias relacionadas para continuar
Si quieres seguir con el mismo tema, aqui tienes paginas cercanas que amplian la tecnica, comparan variantes o te llevan a la siguiente rutina.