Matter is defined as anything that has mass and volume. Atoms are the smallest unit of matter. This means that atoms are the building blocks of all other substances on Earth- both organic and inorganic.
Atoms are composed of three subatomic particles - positively charged protons, neutrons that are neutral (have neither positive nor negative charge), and negatively charged electrons. The positive protons and neutral neutrons make up the central nucleus of the atoms.
The mass of an electron is 9.1 X 10 ^ -31 amu. The mass of protons and neutrons are each roughly 1.67 x 10 -27 amu. The mass of the electrons are considered insignificant compared to that of protons and neutrons. Thus, the sum of protons and neutrons is used to calculate the atomic mass.
The negative electrons circle the nucleus in either orbits or electrons clouds. This provides most of the volume of an atom.