Elements and compounds are pure substances. A compound consists of two or more elements that are chemically bonded in definite proportion. Based on the constituent elements, we can fairly accurately determine the type of compound it is.
Each compound has its own set of physical and chemical properties, which depend largely on the type of compound. Ionic substances tend to have a high melting point, can be soluble in water and tend to be fairly dense. Physical properties of covalent compounds range widely based on the size and polarity of the molecule.
Density is a measure of how tightly packed the particles are in a substance. On the macroscopic level, density is the mass of a sample in a given amount of space (volume).