Elements

Electron configuration of Cobalt (Co)

Read the Cobalt electron configuration, noble-gas shorthand, valence shell, orbital filling logic, and the mistakes to avoid when writing Co.

Guide: Guía práctica Read: 5 min Updated: 2026-06-21

Element guide

Electron configuration of Cobalt (Co)

The electron configuration of Cobalt is 1s2 2s2 2p6 3s2 3p6 3d7 4s2. In shorthand form it is written as [Ar] 3d7 4s2.

Use this page to connect the notation with atomic number 27, the d-block, and the valence pattern 4s2.

How to write the configuration

Configuration at a glance

Cobalt has atomic number 27, so a neutral atom has 27 electrons. The configuration 1s2 2s2 2p6 3s2 3p6 3d7 4s2 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] 3d7 4s2 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.

Cobalt follows the standard filling order

Cobalt is a period 4 transition metal that follows the standard Aufbau sequence: 1s2 2s2 2p6 3s2 3p6 4s2 3d7. Note that 4s fills before 3d, but when cobalt forms ions, the 4s electrons are lost first.

Step-by-step filling walkthrough: 1. Start with atomic number 27. 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 27.

Position in the transition series

Cobalt sits between Iron (Z=26, 3d6) and Nickel (Z=28, 3d8). Adding one electron at a time across these neighbours changes the d-electron count predictably. The 3d7 configuration has three unpaired electrons, giving cobalt significant magnetic properties.

Real-world applications

  • Cobalt is a key component in lithium-ion battery cathodes (LiCoO2), providing high energy density.
  • Cobalt-chrome alloys are used in medical implants and dental prosthetics due to their hardness and biocompatibility.
  • Cobalt-60 is a radioactive isotope used in radiation therapy for cancer treatment.
  • Cobalt-based catalysts are important in hydrodesulfurization and Fischer-Tropsch synthesis.

Common ions: Co2+ ([Ar] 3d7) and Co3+ ([Ar] 3d6) are both common. Co2+ is more stable in aqueous solution, while Co3+ is stabilized in complex compounds like [Co(NH3)6]3+.

Cobalt reference table

Item Value
Atomic number 27
Full configuration 1s2 2s2 2p6 3s2 3p6 3d7 4s2
Noble-gas notation [Ar] 3d7 4s2
Valence clue 4s2
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 27 for a neutral Cobalt 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 Cobalt?

The neutral Cobalt atom is written as 1s2 2s2 2p6 3s2 3p6 3d7 4s2. The shorthand form is [Ar] 3d7 4s2.

How many valence electrons does Cobalt have?

Start from the outer-shell clue 4s2. For transition metals, inspect both s and d electrons since both can participate in bonding.

What are the common oxidation states of Cobalt?

Co2+ ([Ar] 3d7) is most common in aqueous chemistry. Co3+ ([Ar] 3d6) is a strong oxidizing agent in water but is stabilised by ligands like ammonia.

Is Cobalt magnetic?

Yes, cobalt is ferromagnetic with the highest Curie temperature of all elements (1121 degrees Celsius). Its three unpaired 3d electrons give it strong magnetic properties used in permanent magnets.

Study notes and practice checks

Element-specific practice problems

Problem 1: Write the full electron configuration for neutral Cobalt and verify the electron count against 27.

Problem 2: Convert the full configuration to noble-gas shorthand notation.

Problem 3: Compare the configuration of Cobalt (Z=27) with Iron (Z=26) and Nickel (Z=28). What pattern emerges as you move right across the first transition series?

Problem 4: Predict the electron configurations of Co2+ and Co3+. Why is Co3+ a strong enough oxidising agent to oxidise water?

Problem 5: Cobalt-60 is used in radiotherapy. How does the electron configuration of neutral cobalt relate to the properties that make this isotope useful?

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: 2026-06-21 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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