Molecules and Moles

Learn about molecules, moles, and Avogadro's number

The number of molecules and atoms in a human being is mind-boggling.
The number of molecules and atoms in a human being is mind-boggling. Richard Newstead, Getty Images

Molecules and moles are important to understand when studying chemistry and physical science. Here's an explanation of what these terms mean, how they relate to Avogadro's number, and how to use them to find molecular and formula weight.


A molecule is a combination of two or more atoms that are held together by chemical bonds, such as covalent bonds and ionic bonds. A molecule is the smallest unit of a compound that still displays the properties associated with that compound.

Molecules may contain two atoms of the same element, such as O2 and H2, or they may consist of two or more different atoms, such as CCl4 and H2O. A chemical species consisting of a single atom or ion is not a molecule. So, for example, an H atom is not a molecule, while H2 and HCl are molecules. In the study of chemistry, molecules are usually discussed in terms of their molecular weights and moles.

A related term is a compound. In chemistry, a compound is a molecule consisting of at least two different types of atoms. All compounds are molecules, but not all molecules are compounds! Ionic compounds, such as NaCl and KBr, do not form traditional discrete molecules like those formed by covalent bonds. In their solid state, these substances form a three-dimensional array of charged particles. In such a case, molecular weight has no meaning, so the term formula weight is used instead.

Molecular Weight and Formula Weight

The molecular weight of a molecule is calculated by adding the atomic weights (in atomic mass units or amu) of the atoms in the molecule.

The formula weight of an ionic compound is calculated by adding its atomic weights according to its empirical formula.

The Mole

A mole is defined as the quantity of a substance that has the same number of particles as are found in 12.000 grams of carbon-12. This number, Avogadro's number, is 6.022x1023.

Avogadro's number may be applied to atoms, ions, molecules, compounds, elephants, desks, or any object. It's just a convenient number to define a mole, which makes it easier for chemists to work with very large numbers of items.

The mass in grams of one mole of a compound is equal to the molecular weight of the compound in atomic mass units. One mole of a compound contains 6.022x1023 molecules of the compound. The mass of one mole of a compound is called its molar weight or molar mass. The units for molar weight or molar mass are grams per mole. Here is the formula for determining the number of moles of a sample:

mol = weight of sample (g) / molar weight (g/mol)