The Densest Element on the Periodic Table

Which Element Has the Highest Density?

This is a photo of a crystal of ultrapure osmium metal.
Alchemist-hp, Creative Commons License

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.

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.

Why Is Osmium Most Dense When There Are Heavier Elements?

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, right? Yes, 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.

Source: 

Johnson Matthey, "Is Osmium Always the Densest Metal?" Technol. Rev., 2014, 58, (3), 137 doi:10.1595/147106714x682337