Density and mass are mutually dependent physical properties, with density being equal to the amount of mass per unit of volume of any particular object or substance. The mass of any object depends entirely on its overall density, with denser objects having greater mass, for the same volume, than less-dense objects. The mass of an object with uniform composition changes if pieces are added or removed, but its density remains constant. Meanwhile, barring any melting or evaporation, the density of an object tends to change with changes in temperature, while the mass remains the same.
The density of a given substance is constant, given constant conditions, regardless of the amount. Mass, on the other hand, is dependent on amount, as it is meaningless to try to define the mass of a substance without defining its volume and its environmental conditions.
Mass and density vary greatly between various elements and their compounds. An atom of hydrogen, the lightest element, has only one-quarter the mass of helium, the second lightest. Hydrogen tends to form molecules with two hydrogen atoms, but one of these molecules still weighs only half as much as a single helium atom. Because of the nature of gases, under identical conditions a given volume of hydrogen gas is about half as dense as a volume of helium gas. Density relationships are more complex in solids and liquids, however, because their atomic or molecular components are in contact, and their size and bonding geometry alter their densities.