Final answer:
Molecular compounds are formed when two nonmetals combine, while ionic compounds are formed when a metal and a nonmetal combine.
Step-by-step explanation:
When two nonmetals combine, they form molecular compounds. For example, the combination of hydrogen and oxygen forms the molecular compound water (H2O). Molecular compounds consist of individual molecules and are named using prefixes to indicate the number of atoms present in each element. On the other hand, when a metal and a nonmetal combine, they form ionic compounds. For example, the combination of sodium and chlorine forms the ionic compound sodium chloride (NaCl). Ionic compounds do not consist of individual molecules but instead form a three-dimensional array of alternating positive and negative ions.