What Is the Lightest Metal?

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Lithium is the lightest metal in the world.
Lithium is the lightest metal in the world. Here, it is stored in argon because it is reactive in air. (photo: 2×910)

Can you name the lightest metal? Most metals are heavy or dense, yet some metals can float on water while others are nearly as light as air. Learn about the lightest elemental metal and the lightest alloy.

Lightest Metal That Is an Element

When we talk about a material being lightweight, what we’re really discussing is density. Density is the amount of matter per unit of volume. The less matter there is in a space, the lighter the substance is. So, the lightest metallic element is the one with the lowest density. The lightest or least dense element that is a metal is lithium. Lithium is atomic number 3 on the periodic table, with a density of 0.534 g/cm3. This is comparable to the density of pine wood. The density of water is about 1 g/cm3, so lithium floats on water. But, like other alkali metals, lithium is highly reactive, so it rapidly reacts to release heat and form lithium hydroxide and hydrogen gas. It also burns in air, so the pure metal is stored under oil (where it also floats) or in an inert atmosphere.

Lithium floats in oil.
Lithium also floats in oil. ( W. Oelen )

Under ordinary conditions, only two other metallic elements are light enough to float on water: potassium and sodium. The density of potassium is 0.862 g/cm3, while the density of sodium is 0.971 g/cm3. Lithium, potassium, and sodium are all alkali metals. The reason they are so light is because these elements are the first atoms in their period on the periodic table to gain an additional electron shell. The shell makes the atoms large for their mass.

What about hydrogen? It’s in the alkali metal group and is even lighter than lithium. Normally, hydrogen behaves as a nonmetallic gas. However, under certain conditions hydrogen forms a solid metal with a density of 0.0763 g/cm3. It would be the lightest metal if it existed naturally on Earth!

Lightest Metal Alloy

Metallic microlattice is so light that a dandelion head can support it without being crushed.
Metallic microlattice is so light that a dandelion head can support it without being crushed. (Dan Little, HRL Laboratories)

The lightest metal on Earth is an alloy rather than an element. It is a lattice of nickel phosphorus tubes (Microlattice) that was invented by researchers at the University of California at Irvine. The density of Microlattice is 0.0009 g/cm3 or 9 mg/cm3. When the air is removed, the true density of the metal structure is only 2.1 mg/cm3. It is one hundred times lighter than polystyrene foam and about 1.76 times heavier than air at 25 °C. A piece of the alloy can rest on a dandelion without bending it.

A chunk of nickel phosphorus has a density of 9.0 g/cm3. The reason the lattice is so light is because it consists of hollow tubes in a lattice that is 99.99% air and only 0.01% metal. Researchers coated a template with alloy and then etched away the template, leaving hollow tubes only around 100 nanometers thick (a thousand times thinner than a human hair). Microlattices show tremendous promise as thermal insulators and acoustic and vibration dampeners.

The nickel-phosphorus alloy microlattice is the lightest metal because it is consists of hollow tubes. It’s 99.99% air!

References

  • Brandes, E.A.; Brook, G.B. (eds) (1998). Light Metals Handbook. Butterworth Heinemann. Oxford. ISBN 0-7506-3625-4.
  • Lide, D. R., ed. (2005). CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton (FL): CRC Press. ISBN 0-8493-0486-5.
  • Polmear, I. (2006). Light Alloys: From Traditional Alloys to Nanocrystals (4th ed.). Butterworth Heinemann. Oxford. ISBN 0-7506-6371-5.
  • Schaedler, T. A.; Jacobsen, A. J.; Torrents, A.; Sorensen, A. E.; Lian, J.; Greer, J. R.; Valdevit, L.; Carter, W. B. (October 12, 2011). “Ultralight Metallic Microlattices”. Science. 334 (6058): 962–5. doi:10.1126/science.1211649