| The elements are substances that cannot be broken | | | | The number of neutrons is variable within an element |
| down into other matter by chemical means, and an | | | | precisely because they exert no charge, and thus |
| atom is the fundamental particle in an element. As of | | | | while their addition or removal changes the mass, it |
| 2009, there are 118 known elements, 88 of which | | | | does not affect the electric charge of the atom. |
| occur in nature; the rest were created in laboratories. | | | | Therefore, whereas the importance of the proton |
| Due to their high levels of radioactivity, they exist | | | | and the electron is very clear to anyone who studies |
| only for extremely short periods of time. Whatever | | | | atomic behavior, neutrons, on the other hand, might |
| the number of elements may be today it is obvious | | | | seem at first glance as though they are only "along |
| that this number will increase over time, as new | | | | for the ride." Yet they are all-important to the |
| elements are synthesized but the same number of | | | | formation of isotopes. Not surprisingly, given their lack |
| basic atomic structures will exist in the universe. | | | | of electric charge, neutrons were the last of the |
| What distinguishes one element from another is the | | | | three major subatomic particles to be discovered. |
| number of protons, subatomic particles with a | | | | English physicist J. J. Thomson (1856-1940) identified |
| positive electric charge, in the nucleus, or center, of | | | | the electron in 1897, and another English physicist, |
| the atom. The number of protons, whatever it may | | | | Ernest Rutherford (1871-1937), discovered the proton |
| be, is unique to an element. Thus if an atom has one | | | | in 1914. Rutherford's discovery overturned the old |
| proton, it is an atom of hydrogen, because hydrogen | | | | "plum pudding" model, whereby atoms were depicted |
| has an atomic number of 1, as shown on the periodic | | | | as consisting of electrons floating in a positively |
| table of elements. If an atom has 109 protons, on | | | | charged cloud, rather like raisins in an English plum |
| the other hand, it is meitnerium, the element | | | | pudding. As Rutherford showed, the atom must have |
| synthesized at a German laboratory in 1982. | | | | a nucleus—yet protons alone could not account |
| Together with protons in the nucleus are neutrons, | | | | for the mass of the nucleus. There must be |
| which exert no charge. The discovery of these | | | | something else at the heart of the atom, and in 1932, |
| particles is integral to the formation of isotopes. The | | | | yet another English physicist, James Chadwick |
| nucleus, with a diameter about 1/10,000 that of the | | | | (1891-1974), identified what it was. Working with |
| atom itself, makes up only a tiny portion of the | | | | radioactive material, he found that a certain type of |
| atom's volume, but the vast majority of its mass. | | | | subatomic particle could penetrate lead. All types of |
| Thus a change in the mass of a nucleus, as occurs | | | | radiation known at the time were stopped by the |
| when an isotope is formed, is reflected by a | | | | lead, and therefore Chadwick reasoned that this |
| noticeable change in the mass of the atom itself. Far | | | | particle must be neutral in charge. In 1932, he won |
| from the nucleus (in relative terms, of course), at the | | | | the Nobel Prize in physics for his discovery of the |
| perimeter of the atom, are the electrons, which have | | | | neutron. |
| a negative electric charge. Whereas the protons and | | | | Neutrons played a critical role in the development of |
| neutrons have about the same mass, the mass of an | | | | the atomic bomb during the 1940s. In nuclear fission, |
| electron is less than 0.06% of either a proton or | | | | atoms of uranium are bombarded with neutrons. The |
| neutron. Nonetheless, electrons play a highly | | | | result is that the uranium nucleus splits in half, |
| significant role in chemical reactions and chemical | | | | releasing huge amounts of energy. As it does so, it |
| bonding. Just as isotopes are the result of changes in | | | | emits several extra neutrons, which split more |
| the number of neutrons, ions—atoms that are | | | | uranium nuclei, creating still more energy and setting |
| either positive or negative in electric charge—are | | | | off a chain reaction. This explains the destructive |
| the result of changes in the number of electrons. | | | | power in an atomic bomb, as well as the constructive |
| Unless it loses or gains an electron, thus becoming an | | | | power—providing energy to homes and |
| ion, an atom is neutral in charge, and it maintains this | | | | businesses—in a nuclear power plant. Whereas the |
| electric-charge- neutrality by having an equal number | | | | chain reaction in an atomic bomb becomes an |
| of protons and electrons. There is, however, no law | | | | uncontrolled explosion, in a nuclear plant, the reaction |
| of the universe stating that an atom must have the | | | | is slowed and controlled. One of the means used to |
| same number of neutrons as it does protons and | | | | do this is by the application of "heavy water," which |
| electrons: some do, but this is far from being a | | | | is water made with a hydrogen isotope. |
| universal fact. | | | | |