# What is a pure substance that contains two or more elements?

Compounds can be ionic or covalent . Many ionic compounds are composed of a metal and a nonmetal, such as sodium chloride, $\text{NaCl}$, common table salt. Ionic compounds can also contain polyatomic ions, such as the nitrate ion, $\text{NO"_3^(-)}$. An example would be potassium nitrate, $\text{KNO"_3}$.
Covalent compounds are formed when atoms share their valence electrons. Such compounds contain one or more covalent bonds. They include water, $\text{H"_2"O}$ and carbon dioxide, $\text{CO"_2}$.