What Is the Densest Element on the Periodic Table?

Osmium Crystals

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Have you ever wondered which element has the highest density or mass per unit volume? While osmium is generally cited as the element with the highest density, the answer isn't always true. Here's an explanation of density and how the value is determined.

Most Dense Element

  • There are two chemical elements with claims to the title of "most dense element." They are osmium and iridium.
  • Under ordinary conditions of temperature and pressure, osmium is the element with the highest density. Its density is 22.59 g/cm3.
  • At high pressure, iridium becomes the densest element, with a density of 22.75 g/cm3.
  • Osmium and iridium are both metals. The reason they are so dense is because of their electron configuration. Specifically, the f-orbitals contract because they aren't well-shielded from the atomic nucleus.

Density is mass per unit volume. It can be measured experimentally or predicted based on properties of matter and how it behaves under certain conditions. As it turns out, either of two elements can be considered the element with the highest density: osmium or iridium. Both osmium and iridium are very dense metals, each weighing approximately twice as much as lead. At room temperature and pressure, the calculated density of osmium is 22.61 g/cm3 and the calculated density of iridium is 22.65 g/cm3. However, the experimentally measured value (using x-ray crystallography) for osmium is 22.59 g/cm3, while that of iridium is only 22.56 g/cm3. Normally, osmium is the densest element.

However, the density of the element depends on many factors. These include the allotrope (form) of the element, the pressure, and the temperature, so there isn't a single value for density. For example, hydrogen gas on earth has a very low density, yet that same element in the Sun has a density surpassing that of either osmium or iridium on Earth. If both osmium and iridium density are measured under ordinary conditions, osmium takes the prize. Yet, slightly different conditions could cause iridium to come out ahead.

At room temperature and a pressure above 2.98 GPa, iridium is denser than osmium, with a density of 22.75 grams per cubic centimeter.

Overall, metals tend to have a higher density than metalloids and nonmetals. The other elements only have a chance of coming out ahead when enormous pressures are applied. That being said, some metals are very light. For example, sodium has such a low density that it floats on water.

Why Osmium Is Densest When Heavier Elements Exist

Assuming osmium has the highest density, you might be wondering why elements with a higher atomic number aren't denser. After all, each atom weighs more. But, density is mass per unit volume. Osmium (and iridium) have a very small atomic radius, so the mass is packed into a small volume. The reason this happens is the f electron orbitals are contracted at the n=5 and n=6 orbitals because the electrons in them aren't well-shielded from the attractive force of the positive-charged nucleus. Also, the high atomic number of osmium brings relativistic effects into play. The electrons orbit the atomic nucleus so fast their apparent mass increases and the s orbital radius decreases.

Confused? In a nutshell, osmium and iridium are denser than lead and other elements with higher atomic numbers because these metals combine a large atomic number with a small atomic radius.

Other Materials With High-Density Values

Basalt is the type of rock with the highest density. With an average value around 3 grams per cubic centimeter, it's not even close to that of the metals, but it's still heavy. Depending on its composition, diorite might also be considered a contender.

The densest liquid on Earth is the liquid element mercury, which has a density of 13.5 grams per cubic centimeter.

Sources

  • Grigoriev, Igor S.; Meilikhov, Evgenii Z. (1997). Handbook of Physical Quantities. Boca Raton: CRC Press.
  • Serway, Raymond; Jewett, John (2005). Principles of Physics: A Calculus-Based Text. Cengage Learning. ISBN 0-534-49143-X.
  • Sharma, P. V. (1997). Environmental and Engineering Geophysics. Cambridge University Press. ISBN 9781139171168. doi:10.1017/CBO9781139171168
  • Young, Hugh D.; Freedman, Roger A. (2012). University Physics with Modern Physics. Addison-Wesley. ISBN 978-0-321-69686-1.
  • Zumdahl, Steven S.; Zumdahl, Susan L.; Decoste, Donald J. (2002). World of Chemistry. Boston: Houghton Mifflin Company.
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Helmenstine, Anne Marie, Ph.D. "What Is the Densest Element on the Periodic Table?" ThoughtCo, May. 6, 2022, thoughtco.com/densest-element-on-the-periodic-table-606626. Helmenstine, Anne Marie, Ph.D. (2022, May 6). What Is the Densest Element on the Periodic Table? Retrieved from https://www.thoughtco.com/densest-element-on-the-periodic-table-606626 Helmenstine, Anne Marie, Ph.D. "What Is the Densest Element on the Periodic Table?" ThoughtCo. https://www.thoughtco.com/densest-element-on-the-periodic-table-606626 (accessed July 6, 2022).