What is it that makes an element denser than the another?

Question

yes gravity acts on it, but what is it which makes say gold denser than aluminum?

Answer 1

Density is defined as mass per unit volume.  A gold atom has more protons and neutrons (and electrons) in roughly the same volume.  Actually, the volume of a gold atom is larger than an aluminum atom, but there is much more mass for a slightly larger volume.  

For molecules, the principle is the same. Non-polar molecules have a larger minimum distance between the molecules than polar molecules and that is why oil floats above water when the two are mixed.