Elements and compounds make up the universe.
The smallest unit of an element is an atom.
The properties of atoms are described by the atomic theory first proposed in 1803. Elements occur as single atoms, as groups or molecules of atoms of a single element, or in compounds. Compounds are pure substances formed by the chemical combination of atoms of more than one element. Compounds have fixed properties and composition.
We now know of more than 100 elements. Each has a name, a symbol, and a number. The elements are very unevenly distributed throughout the universe.
A chemical reaction occurs when atoms of different elements combine to form compounds or when a compound decomposes to element or to simpler compounds. Chemical equations can be written to describe these events. Equations must contain correct formulas and must be balanced by mass.