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Cobalt

Element 27 · Transition metal

Specimen

Atomic number
27
Atomic mass
58.933
State at room temperature
Solid
Melting point
1,495 °C
Boiling point
2,927 °C
Density
8.86 g/cm³
Discovered
1735

Atomic mass from IUPAC.1 Other physical data from PubChem.2

Can I eat it?

In food and water
Your body needs it. The body needs cobalt only as part of vitamin B12 (cobalamin). ATSDR says that swallowed cobalt in other forms can harm the blood and the thyroid. 34

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The atom

Co
ShellElectrons
1 (K) 2
2 (L) 8
3 (M) 15
4 (N) outermost 2
Total27

Configuration [Ar] 3d⁷ 4s²

3d ↑ ↓ ↑ ↓ ↑ ↑ ↑
4s ↑ ↓
A shell diagram: each ring is an electron shell and each dot is one electron. The nucleus holds 27 protons. The outermost shell is shown in red. Electrons do not travel on rings: they fill orbitals, regions where an electron is likely to be found. The boxes are those orbitals for the electrons outside the Ar core, with one arrow per electron.

Related elements

Group 9
Rh Rhodium Ir Iridium Mt Meitnerium

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Sources

  1. 1

    IUPAC Commission on Isotopic Abundances and Atomic Weights (CIAAW), 2024. Abridged Standard Atomic Weights 2024 .

    The atomic mass shown for the 84 elements that have a standard atomic weight.

  2. 2

    National Center for Biotechnology Information (PubChem), 2026. Periodic Table of Elements .

    Electron configuration, melting and boiling points, density and discovery year for all 118 elements.

  3. 3

    European Food Safety Authority (EFSA), 2017. Dietary Reference Values for nutrients. Summary report (amended 2019) .

    Summaries of the EFSA NDA Panel opinions on each vitamin and mineral, section 4 (minerals) and section 5 (cobalamin).

    • Cobalamin is a metal complex with a central cobalt atom bonded to six ligands
  4. 4

    Agency for Toxic Substances and Disease Registry (ATSDR), 2024. ToxFAQs for Cobalt .

    • Oral exposure to cobalt may cause adverse effects in the blood and thyroid.
  5. 5

    National Institute of Standards and Technology (NIST), 2026. Atomic Weights and Isotopic Compositions for All Elements .

    Isotopic composition of each element; the most abundant isotope sets the neutron count in the nucleus view.