Elements are a pure chemical substance that consists of just one type of atom. When elements come together to form different substances they are called compounds. Compounds are two are more different chemical elements forming a pure substance.
Chemical reactions happen when one or more elements are changed into one or more other chemicals. During chemical reactions, elements can be joined together and behave like one substance. When elements are joined together to form a new substance, they are called a compound.
When the element hydrogen and the element oxygen come together to form a compound, they sometimes form the substance known as water.