When two elements are combined through a chemical reaction then a compound forms. For example water is a compound formed from the combination of hydrogen and oxygen.
2H2 + O2 → 2H2O.
Complex molecules such as proteins are formed by a series of reactions involving elements and simple compounds.
Another example is A reaction between sodium and chlorine giving the compound sodium chloride. A new substance is formed when two atoms combines to form a compound. Compounds are written with formulas showing which elements from the periodic table are combined.