In Ancient Greece, philosophers believed that all things on Earth were made from four elements — Earth, Air, Fire, and Water. Later a fifth element, Ether, was added as a transparent, massless medium assumed to fill empty space. Today we know this not to be the case. All of the visible matter in the universe is believed to be made up of the 92 naturally-occurring elements that have been identified, plus a smaller number of artificially-made elements on Earth.
An element cannot be broken down or divided into anything else by chemical reactions. Elements are substances with distinctly different atomic structures. The elements exist in solid, liquid or gas form; all are indivisible. All of the known elements are listed on the Periodic Table of Elements first described in 1869 by the Russian chemist Dmitri I. Mendeleev.
Matter can be either a pure substance or a mixture. A pure substance or compound is made of either a single element such as copper or two or more elements in a fixed ratio. Examples of compounds include: table salt (NaCl — 1-sodium:1-chlorine), methane gas (CH4 — 1-carbon:4-hydrogens), water (H2O — 2-hydrogens:1-oxygen).
Collections of pure substances may form mixtures and solutions when mixed together. Examples are milk, apples, concrete, coffee, soil, people and animals. Believe it or not, 99% of your body is made up of six elements: oxygen, carbon, hydrogen, nitrogen, calcium and phosphorus. These are arranged into compounds and mixtures to form all of the tissues needed to make your skin, hair, lungs, blood and so on. The other 1% includes elements such as chlorine, sulfur, iron and potassium — a naturally-occurring radioactive element.
Chemistry is the study of how elements, compounds and mixtures interact and react with each other. Chemists use their knowledge of compounds and mixtures, to create new materials and medicines or to study our environment.