A molecule is a combination of two or more atoms that are held together by covalent 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. In the study of chemistry, molecules are usually discussed in terms of their molecular weights and moles.
Ionic compounds, such as NaCl and KBr, do not form true molecules. 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.
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. 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 1 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)