What Is the Heaviest Element?

Why It's Hard to Identify the Element with Highest Density

This is a photo of a crystal of ultrapure osmium metal.
This is a photo of a crystal of ultrapure osmium metal. The osmium crystal was produced by chemical transport reaction in chlorine gas. Alchemist-hp, Creative Commons License

Are you wondering which element is the heaviest? There are three possible answers to this question, depending on how you define "heaviest" and the conditions of the measurement. Osmium and iridium are the elements with the highest density, while oganesson is the element with the largest atomic weight.

Key Takeaways: Heaviest Element

  • There are different ways of defining the heaviest chemical element.
  • The heaviest element, in terms of atomic weight, is element 118 or oganesson.
  • The element with the highest density is osmium or iridium. Density depends on temperature and crystal structure, so which element is most dense varies according to conditions.

Heaviest Element in Terms of Atomic Weight

The heaviest element in terms of heaviest per a given number of atoms is the element with the highest atomic weight. This is the element with the largest number of protons, which is presently element 118, oganesson or ununoctium. When a heavier element is discovered (e.g., element 120), then that will become the new heaviest element. Ununoctium is the heaviest element, but it is man-made. The heaviest naturally-occurring element is uranium (atomic number 92, atomic weight 238.0289).

Heaviest Element in Terms of Density

Another way to look at heaviness is in terms of density, which is mass per unit volume. Either of two elements can be considered the element with the highest density: osmium and iridium. The density of the element depends on many factors, so there isn't a single number for density that would allow us to identify one element or the other as the most dense. Each of these elements weigh approximately twice as much as lead. The calculated density of osmium is 22.61 g/cm3 and the calculated density of iridium is 22.65 g/cm3, though the density of iridium has not been experimentally measured to exceed that of osmium.

Why Osmium and Iridium Are So Heavy

Even though there are many elements with higher atomic weight values, osmium and iridium are the heaviest. This is because their atoms pack together more tightly in solid form. The reason for this is that their f electron orbitals are compacted when n=5 and n=6. The orbitals feel the attraction of the positive-charged nucleus because of this, so the atom size contracts. Relativistic effects also play a role. The electrons in these orbitals go around the atomic nucleus so fast their apparent mass increases. When this happens, the s orbital shrinks.


  • KCH: Kuchling, Horst (1991) Taschenbuch der Physik, 13. Auflage, Verlag Harri Deutsch, Thun und Frankfurt/Main, German edition. ISBN 3-8171-1020-0.
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Helmenstine, Anne Marie, Ph.D. "What Is the Heaviest Element?" ThoughtCo, Aug. 25, 2020, thoughtco.com/what-is-the-heaviest-element-606627. Helmenstine, Anne Marie, Ph.D. (2020, August 25). What Is the Heaviest Element? Retrieved from https://www.thoughtco.com/what-is-the-heaviest-element-606627 Helmenstine, Anne Marie, Ph.D. "What Is the Heaviest Element?" ThoughtCo. https://www.thoughtco.com/what-is-the-heaviest-element-606627 (accessed March 4, 2021).