Which of these metals is the lightest per unit of volume?

Aluminum is a chemical element; it has the symbol Al and atomic number 13. The density of aluminium is 2.70 g/cm³, about 1/3 that of steel, much lower than other commonly encountered metals, making aluminium parts easily identifiable through their lightness. Aluminium's low density compared to most other metals arises from the fact that its nuclei are much lighter, while the difference in the unit cell size does not compensate for this difference. The only lighter metals are the metals of groups 1 and 2, which, apart from beryllium and magnesium, are too reactive for structural use (and beryllium is very toxic). Aluminium is not as strong or stiff as steel, but the low density makes up for this in the aerospace industry and for many other applications where lightweight and relatively high strength are crucial.

Source: Wikipedia