Lithium, potassium and sodium are all less dense than water in their elemental form. Water has a defined density of 1 gram per cubic centimeter, while the three lightest metals are between 0.534 gram per cubic centimeter and 0.971 gram per cubic centimeter.
Though only three metals are lighter than water in their pure form, lightweight alloys exist that are lighter still. As of 2015, the lightest metal alloy ever developed is a compound of nickel and phosphorous that has been organized into a lattice of hollow, thin-walled tubes. This lattice has a density of only 0.9 mg per cubic centimeter, which makes it slightly lighter than silica aerogel.
To create a block of this super-light metal, the nickel-phosphorous alloy must be applied to a synthetic scaffold that has been produced via 3-D printer and is later etched away. The resulting ingot is able to withstand a compression strain of over 50 percent in tests, after which it recovers 98 percent of its former vertical height. This flexibility is present only in the processed ingot, however, as the bulk nickel-phosphorous alloy is brittle and difficult to work with on its own. After processing, the microfiber ingot is 99.99 percent air by volume.