The chemical nature of an atom, that is , the chemical properties of a specific element, is determined by the number of protons in the nucleus<span>. This number of protons is called the </span>atomic number<span>. The mass of the atom, its atomic mass, depends upon both the number of protons and upon the number of neutrons present in the nucleus (remember that the mass of an electron is so small that it is simply ignored for the purpose of establishing the atomic mass).</span>The difference between atoms, ions and isotopes is the number of subatomic particles.<span> An atom is the basic building block of matter, the smallest molecule of an element that exists and that cannot be chemically divided by ordinary means. Each atom is comprised of protons, neutrons and electrons. In ions, the number of electrons differs, and in isotopes. </span>
