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 O2and 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 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 unitsatomic weights according to its empirical formula.
or amu) of the atoms in the molecule. The formula weight of an ionic compound is calculated by adding its 
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