What is the lightest known material?

The lightest known material among the given options is aluminum. This metal is renowned for its low density, which makes it significantly lighter than steel, chromium, and mercury. Aluminum has an approximate density of about 2.7 grams per cubic centimeter, which contributes to its lightweight nature. This characteristic is one of the primary reasons for its widespread use in applications where weight reduction is essential, such as in aerospace, automotive, and construction industries.

Steel, on the other hand, is a much denser material, typically ranging from 7.75 to 8.05 grams per cubic centimeter. This makes it considerably heavier than aluminum, although it is favored for its strength and durability.

Chromium also has a higher density, approximately 7.19 grams per cubic centimeter, which places it above aluminum in terms of weight.

Mercury, while it is a liquid metal that is relatively dense at about 13.6 grams per cubic centimeter, does not fit the context of lightweight materials compared to aluminum's solid state and applications where weight is critical.

Thus, considering the density and the practical usage of the materials listed, aluminum is definitively the lightest option among them.