A Helium Atom
As you can see, helium is larger than hydrogen. As you add electrons, protons and neutrons, the size of the atom increases. We can measure an atom's size in two ways: using the atomic number (Z) or using the atomic mass (A, also known as the mass number). The atomic number describes the number of protons in an atom. For hydrogen the atomic number, Z, is equal to 1. For helium Z = 2. Since the number of protons equals the number of electrons in the neutral atom, Z also tells you the number of electrons in the atom. The atomic mass tells you the number of protons plus neutrons in an atom. Therefore, the atomic mass, A, of hydrogen is 1. For helium A = 4.
Ions and Isotopes
So far we have only talked about electrically neutral atoms, atoms with no positive or negative charge on them. Atoms, however, can have electrical charges. Some atoms can either gain or lose electrons (the number of protons never changes in an atom). If an atom gains electrons, the atom becomes negatively charged. If the atom loses electrons, the atom becomes positively charged (because the number of positively charged protons will exceed the number of electrons). An atom that carries an electrical charge is called an ion. Listed below are three forms of hydrogen; 2 ions and the electrically neutral form.