Matter & Energy

Matter is composed of atoms or groups of atoms called molecules. The arrangement of particles in a material depends on the physical state of the substance. In a solid, particles form a compact structure that resists flow. Particles in a liquid have more energy than those in a solid. They can flow past one another, but they remain close. Particles in a gas have the most energy. They move rapidly and are separated from one another by relatively large distances.

Avogadro’s Number


Avogadro’s Number, the number of molecules that exist in one mole, or gram molecular weight, of any substance. One gram molecular weight is the weight of a substance, in grams, that is numerically equivalent to the dimensionless molecular weight of that substance (see Periodic Law). The number of molecules in one gram molecular weight has been determined to be approximately molecules, as established by various methods currently available to physical chemists.

The Avogadro number is named in honor of the Italian physicist Amedeo Avogadro, who postulated in 1811 that equal volumes of gases, at equivalent temperatures and pressures, contain the same number of molecules (see Avogadro's Law). The theory was significant in the development of chemistry, but the number itself was not calculated until the later 19th century, when the concept was extended to include not only gases but all chemicals. Volume considerations do not apply to liquids or solids, but Avogadro's number itself holds true for all substances, whatever their state.