Chapter 24: The Nucleus

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The nucleus was not understood until about the 1930s. The missing piece in the puzzle was the neutron. Because the neutron has no electric charge, it is invisible to most detection schemes which rely on "seeing" charged sub-atomic particles from their electrical interactions with matter. Nuclei are composed of and neutrons. Neutrons have almost exactly the same as protons, but neutrons have no electrical . Nuclei with the same number of protons, but different number of neutrons are called of one another. (Name of?) (1H2) and (name of?) (1H3) are isotopes of hydrogen. Nuclei are represented by symbols: 8O16. The subscript is called the " number." It is the electrical charge of the nucleus (equal to the number of protons) of a particle. The superscript is called the " number." The mass number is the number of nucleons in the nucleus. A "nucleon" is either a proton or a . Nuclei which emit particles are said to be " ." "Alpha", "beta" and "gamma" refer to different emitted particles. In radioactive decays, there are two important conserved quantities: atomic number (charge) and number. Alpha Decay: The alpha particle is the same as the nucleus of (what element?) . The alpha particle has an atomic number of (number?) and a mass number of (number?) . Beta Decay: The beta particle is a(n) . The beta particle has an atomic number (charge) of (+1, 0, -1?) and a mass number of (+1,0,-1?) . In addition to the beta particle, another extremely hard to observe particle called a is emitted in beta decays, but this particle has an atomic number (charge) of 0 and a mass number of 0. You may also think of a beta decay as a single in a nucleus transforming into a and emitting an electron and a neutrino. Gamma Decay: The gamma particle is a high-energy . A gamma particle (or gamma ray) has an atomic number (charge) of (+1, 0, -1?) and a mass number of (+1, 0, -1?) , but a photon is NOT a neutrino. Particles emitted by nuclei are said to be "high-energy" radiations. The term means that the emitted particles have sufficient energy to disrupt millions of molecules when they pass through matter, causing "ionization", "molecular damage", or "excitation." Molecular damage may result in cancer. Radioactive substances may be used as clocks. The " -life" of a radioactive substance is the amount of time required for one-half of the nuclei to undergo decay. Carbon-14 (6C14) has a half-life of about 5730 (hours, days, years?) for beta decay. Most carbon in living things is carbon-(number?) , which is not radioactive. Carbon-14 is constantly produced and replenished in the atmosphere by the bombardment of cosmic rays (primarily protons) from the sun. Both carbon-12 and carbon-14 are taken up to form living tissue, but when a living thing dies, the ratio of carbon-14 to carbon-12 in the tissue remnants (increases, decreases?) . A measurement of the status of this ratio is a measure of the time since the death of the tissue. One has great confidence in carbon-14 dating up to ages of about 5000 years because over this range it has been tested on objects whose age is known independently. Potassium-40 has a half-life of about 1.3 (thousand, million, billion?) years for electron capture. While carbon-14 dating is useful for measuring ages of several (thousand, million, billion?) years, potassium-40 is useful for determining ages of several (thousand, million, billion?) years.

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