A a chemical compound is any substance that consists of two or more different elements which are chemically bonded in a fixed ratio to form molecules. Compounds can be converted to different compounds or into their constituent elements by interaction with another substance via a chemical reaction. In this process, bonds between the atoms may be broken in either or both of the interacting substances and new bonds formed. Many elements form compounds with other elements readily, or even explosively, such as fluorine and cesium, while others are very resistant to forming compounds, such as helium and gold. Composites and mixtures differ from compounds in that they contain two or more elements or compounds which are not chemically bonded with each other.
The number of possible compounds is probably unlimited because of the large number of elements (118, of which 94 occur naturally on the earth) and the countless ways in which they can be combined, particularly for organic compounds. Organic compounds are compounds that contain carbon, with some exceptions such as carbon dioxide and carbon monoxide.
Water may be the most abundant compound in the known universe, because it consists of the most common element, hydrogen, and the third most common element, oxygen. The most abundant compounds in the earth's crust are silicon dioxide, at close to 43 percent by weight, followed by magnesium oxide, ferrous oxide, aluminum oxide and calcium oxide.
Water is also the most common compound in living organisms, accounting for 70 to 90 percent of the mass of individual cells and about 70 percent of the mass of the human body. Humans and other organisms also contain a huge number of other compounds in small quantities, both organic and inorganic. The main categories of the former are carbohydrates, fats, proteins and nucleic acids. Inorganic compounds found in living organisms in addition to water include molecular oxygen, carbon dioxide, and various acids, bases and salts.