An atom is a million times smaller than the thickest human hair. All the atoms of an element are not alike, however. For example, a second kind of hydrogen exits which is present in every sample of the gas no matter where it is obtained. It weighs twice as much as the more common hydrogen and is called deuterium or heavy hydrogen. The diameter of an atom for this type of hydrogen differs from the more common type.
Atoms vary greatly in weight, but they are all about the same size.
The World book Encyclopedia . The diameter of an atom ranges from about 0.1 to 0.5 nanometer. According to Rutherford, the radius of the entire atom was 0.00000001 cm. Baylis, William E. Macmillan Encyclopedia of Physics. An atom is one of the basic units of matter.