Here's a breakdown:

* Water (H₂O): Water molecules are polar – they have a slightly positive end and a slightly negative end due to the uneven sharing of electrons between the oxygen and hydrogen atoms. This polarity allows water molecules to form strong hydrogen bonds with each other. These bonds are relatively strong and hold the molecules close together, making water a dense liquid.

* Oil (Hydrocarbons): Oil is primarily composed of hydrocarbons, which are nonpolar molecules. These molecules have weak Van der Waals forces between them. These forces are much weaker than hydrogen bonds, resulting in less attraction between oil molecules and a lower density.

In summary:

* Stronger intermolecular forces (hydrogen bonds) in water lead to a closer packing of molecules and higher density.

* Weaker intermolecular forces (Van der Waals forces) in oil result in a looser packing of molecules and lower density.

This difference in density is why oil floats on water. The less dense oil will rise to the surface, while the denser water sinks to the bottom.