Aluminium has a density lower than those of other common metals, at approximately one third that of steel. It has a great affinity towards oxygen, and forms a protective layer of oxide on the surface when exposed to air.

Aluminium visually resembles silver, both in its color and in its great ability to reflect light. It is soft, non-magnetic and ductile. It has one stable isotope, 27Al; this isotope is very common, making aluminum the twelfth most common element in the Universe. The radioactivity of 26Al is used in radio-dating.

Chemically, aluminum is a weak metal in the boron group; as it is common for the group, aluminum forms compounds primarily in the +3 oxidation state. The aluminum cation Al3+ is small and highly charged; as such, it is polarizing, and bonds aluminum forms tend towards covalency. The strong affinity towards oxygen leads to aluminum's common association with oxygen in nature in the form of oxides; for this reason, aluminum is found on Earth primarily in rocks in the crust, where it is the third most abundant element after oxygen and silicon, rather than in the mantle, and virtually never as the free metal.

Despite its prevalence in the environment, no living organism is known to use aluminum salts metabolically, but aluminum is well tolerated by plants and animals. Because of the abundance of these salts, the potential for a biological role for them is of continuing interest, and studies continue.