The word atom is comes from the Greek word atom which means indivisible. The Greeks thought that matter could be broken down into pieces too small to be seen. These particles were called atoms. Atoms are composed of three types of particles: protons, neutrons, and electron. Protons and neutrons are responsible for most of the atomic mass e.g. in a 150 person 149 lbs, 15 oz are protons and neutrons while only 1 oz. is electrons. The mass of an electron is very small (9.108 X 10-28 grams). Both the protons and neutrons reside in the nucleus. Protons have a postive (+) charge, neutrons have no charge --they are neutral. Electrons reside in orbitals around the nucleus. They have a negative charge (-).The electrons of an atom are bound to the nucleus by an electromagnetic force. Likewise, a group of atoms can remain bound to each other, forming a molecule. An atom containing an equal number of protons and electrons is electrically neutral, otherwise it has a positive or negative charge and is an ion. An atom is classified according to the number of protons and neutrons in its nucleus: the number of protons determines the chemical element, and the number of neutrons determines the isotope of the element.