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?
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.