An
atom is the smallest constituent unit of ordinary
matter that has the properties of a
chemical element. Every
solid,
liquid,
gas, and
plasma is composed of neutral or
ionized atoms. Atoms are very small; typical sizes are around 100 pm (a ten-billionth of a meter, in the short scale). However, atoms do not have well defined boundaries, and there are different ways to define their size which give different but close values.