A property of any substance is its density $\rho$(Greek letter rho), defined as the amount of mass contained in a unit volume, which we usually express as mass per unit volume: $$ \rho = {m \over V}$$
The SI unit of density is kilogram per cubic meter ($kg/m^3$).
Fig. 1: Both of the material have similar volumes but lead is more denser than aluminum due to high number of proton and neutron in the nucleus.
For example, aluminum has a density of $2.70 g/cm^3$, and lead has a density of $11.3 g/cm^3$. Therefore, a piece of aluminum of volume $10.0 cm^3$ has a mass of $27.0 g$, whereas an equivalent volume of lead has a mass of $113 g$. The difference of densities between aluminum and lead is due to their difference of atomic masses. The unit for atomic mass is the atomic mass unit (u), where $1 u = 1.660 5402x10^{-27} kg$.
Density is the number of things—which could be people, animals, plants, or objects—in a certain area. To calculate density, you divide the number of objects by the measurement of the area. The population density of a country is the number of people in that country divided by the area in square kilometers or miles.

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