Have you ever wondered about the heaviest or densest element on the periodic table? The densest element is osmium, a blue-gray metal with atomic number 76. The density of osmium is about 22.59 g/cm3. Osmium is about twice as dense as lead, 1.2 times heavier than gold, and about 23 times heavier than water. Iridium follows osmium on the periodic table and is nearly as dense, with a density of 22.56 g/cm3. The heaviest element, in terms of atomic weight, is oganesson (atomic number 118).
Osmium is the densest element on the periodic table, with a density of 22.59 g/cm3 at room temperature.. Oganesson is the heaviest element, in terms of atomic weight. However, oganesson’s density is predicted to be between 6.6 and 74 g/cm3.
Why Is Osmium So Dense?
Osmium and iridium don’t have the highest atomic numbers of the elements, so you may be wondering what makes them so dense. Remember, density describes how matter packs together in a volume. The reason osmium and iridium are dense is because their atoms have a very small atomic radius. This is because the f electron orbitals at n=5 and n=6 are contracted because their electrons aren’t well-shielded from the attractive pull of the positive-charged atomic nucleus. In other words, the protons of the nucleus pull the electrons closer than usual. Relativistic effects also play a role in high density. Electrons in any element with a high atomic number orbit the nucleus so quickly that the atom’s apparent mass increases and s orbital radius decreases.
So, each osmium and iridium atom is small for the number of protons, neutrons, and electrons it contains. The atoms pack together tightly because metals participate in metallic bonds, where atoms essentially share their electrons with neighbor atoms.
High density goes hand-in-hand with related properties. Osmium and iridium and both very hard and display extremely low compressibility.
Other Dense Materials
Osmium is the densest element on the periodic table, but other elements have their own claims to fame. Mercury is densest liquid element (13.5 g/cm3), while radon is the densest gas (9.73 g/L).
The densest rock is basalt (~3 g/cm3). The densest objects in the universe are neutron stars (3.7×1017 to 5.9×1017 kg/m3) and black holes (theoretically infinite density at the singularity).
Densest Element vs Heaviest Element
Normally, when you talk about how much something weighs, what you’re really discussing is its density. Density is mass per unit volume and is reported in units of grams per cubic centimeter (g/cm3) or kilograms per cubic meter (kg/m3).
Atomic weight is another types of weight that describes the average mass of atoms. It is the sum of the number of protons and neutrons in a sample of an element (electrons don’t contribute enough mass to make a difference) and its expressed as a unitless quantity or sometimes in atomic mass units (amu) or grams per mole. It isn’t actually “weight” at all, because its value doesn’t depend on the force of gravity, so atomic weight is more properly called atomic mass. In any event, the heaviest element, in terms of atomic weight, is oganesson. Oganesson is a synthetic element with 118 protons. The number of neutrons (and thus, the atomic weight) depend on the isotope synthesized by the lab. A ballpark atomic weight for oganesson is 294.
References
- Arblaster, J. W. (1989). “Densities of osmium and iridium: recalculations based upon a review of the latest crystallographic data” . Platinum Metals Review. 33 (1): 14–16.
- Arblaster, J. W. (1995). “Osmium, the Densest Metal Known“. Platinum Metals Review. 39 (4): 164.
- Cynn, Hyunchae; Klepeis, J. E.; Yeo, C. S.; Young, D. A. (2002). “Osmium has the Lowest Experimentally Determined Compressibility”. Physical Review Letters. 88 (13): 135701. doi:10.1103/PhysRevLett.88.135701
- Haynes, William M., ed. (2011). CRC Handbook of Chemistry and Physics (92nd ed.). CRC Press. ISBN 978-1439855119.
- Sahu, B. R.; Kleinman, L. (2005). “Osmium Is Not Harder Than Diamond”. Physical Review B. 72 (11): 113106. doi:10.1103/PhysRevB.72.113106