The smallest unit of matter is the atom. In the fifth century B.C., Democritus proposed that matter can only be cut up to a certain point. At that point, that smallest particle still retains the properties of that substance.
In the early 19th century, John Dalton discovered that each type of chemical called an element was comprised of the same type of atom. Dalton also found that compounds, which are composed of two or more types of elements, contain exact ratios of different atoms.
Matter can exist in several states: solid, liquid, gas or plasma. In a solid, atoms are constricted in a tight space and cannot move around much; in a gas, atoms travel quite freely in space. Changes in heat or pressure can change matter from one state to another.