DENSITY

 DENSITY

A property of any substance is its density  (Greek letter rho), defined as the amount of mass contained in a unit volume, which we usually, expressed as mass per unit volume

For example, aluminum has a density of 2.70 g/cm3, and lead has a density of 11.3 g/cm3. Therefore, a piece of aluminum of volume 10.0 cm3 has a mass of 27.0 g, whereas an equivalent volume of lead has a mass of 113 g.

The difference in density between aluminum and lead is due, in part, to their different atomic masses. The atomic mass of an element is the average mass of one atom in a sample of the element that contains all the element’s isotopes, where the relative amounts of isotopes are the same as the relative amounts found in nature. The unit for atomic mass is the atomic mass unit (u), where

1 u = 1.660 540 2 * 10raise to power-27 kg. The atomic mass of lead is 207 u, and that of aluminum is 27.0 u. However, the ratio of atomic masses, 207 u/27.0 u  7.67, does not correspond to the ratio of densities, (11.3 g/cm3)/(2.70 g/cm3)  4.19. The discrepancy is due to the difference in atomic separations and atomic arrangements in the crystal structure of these two substances.

 The mass of a nucleus is measured relative to the mass of the nucleus of the carbon-12 isotope, often written as 12C. (This isotope of carbon has six protons and six neutrons. Other carbon isotopes have six protons but different numbers of neutrons.) Practically all of the mass of an atom is contained within the nucleus. Because the atomic mass of 12C is defined to be exactly 12 u, the proton and neutron each have a mass of about 1 u. One mole (mol) of a substance is the amount of the substance that contains as many particles (atoms, molecules, or other particles) as there are atoms in 12 g of the carbon-12 isotope. One mole of substance A contains the same number of particles as there are in 1 mol of any other substance B. For example, 1 mol of aluminum contains the same number of atoms as 1 mol of lead.