What makes some aluminium alloys so strong while remaining light? Compared to cast iron, for example, certain alloys of aluminum have both strength and weight advantages. How can aluminum be so strong while being so light?
 A: ONE thing which makes Aluminum better than steel in SOME applications is its higher Strength/Weight ratio.  A big contributor to this better ratio is the much lower density of Aluminum
There are some classes of aluminum alloys which are stronger than steel alloys, but, in general, aluminum is actually a bit weaker than steel.
However, aluminum is MUCH lighter (less dense) than steel: $2.8g/cm^3$ for Al as opposed to $7.8g/cm^3$ for steel, making Al a clear winner in the strength/weight arena.
The lower density of Al is due to its lower atomic weight.
A: The density of aluminium is driven by two factors, the atomic mass of the atoms and the volume they occupy when in solid form.
Here is a chart showing how density varies across the periodic table rows and columns.

Aluminium is row 3 column 13(group XIII).
I understand the properties of each element are principally due to the form of their outermost electron shells. Some are spherical, s shells, others have lobes. Aluminium has three electrons in its outer shell, and can form three bonds.
Aluminium has a face-centered-cubic (fcc) structure which means the density is related to the cube of inter-atomic separation. For element of similar atomic mass this arrangement results in lower densities.
There are a number of elements with similar density to aluminium, but their other properties make them very different. Aluminium is metallic and therefore malleable, it is less likely to break than to bend. It is relatively inert in our environment and solid at standard temperature and pressure. 
