| The atomic number of an atom represents the number of protons in its nucleus. This number remains constant for a given element. The number of neutrons may vary, however, creating isotopes that have the same chemical behavior, but different mass. The isotopes of hydrogen are protium (no neutrons), deuterium (one neutron), and tritium (two neutrons). Hydrogen always has one proton in its nucleus. These illustrations are not to scale—the nucleus is approximately 10,000 times smaller than the average orbital radius of the electron, which defines the overall size of the atom. |
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