A material that contains only one type of atom is called an element. There are 98 elements known to occur naturally, along with another 20 that have been synthesized in laboratories, as of 2015. Hydrogen, the lightest element, is also the most abundant chemical substance in the universe.
A substance made from two or more elements is called a compound. Compounds consist of chemically bonded atoms in a fixed ratio. For example, water is always made up of two hydrogen atoms bonded to one oxygen atom. Matter often occurs as a mixture of various compounds and elements, such as air, which is comprised of more than a dozen different elements and compounds.