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DMI 261 RADIATION BIOLOGY and PROTECTION UNIT 1 THE ATOM Nucleus Protons Neutrons Shells / orbits electrons Protons Nucleon + electrical charge 1.00728 mass Opposite charge of the electron Neutrons Nucleon No charge – neutral 1.00867 mass Shells / Orbits Electrons Exist in electron cloud Arranged in precise orbits or energy levels - electrical charge .000549 mass Maximum electrons / shell K=2 L=8 M = 18 N = 32 O = 50 P = 72 Q = 98 Binding energy of an atom Holds electrons in orbit Measured in electron volts (eV) Inner electrons are tightly bound Outer electrons are loosely bound and easily ionized Valence shell The outermost shell of loosely bound electrons. # of electrons in valence shell determines the atom’s combining ability. Ionization Ionize – to remove electrons from an atom. Occurs when an x-ray photon transfers its energy to an orbital electron and ejects it from its shell. An ion pair is formed. ION PAIR + charged atom free electron Atomic Number Number of protons in the nucleus “Z” number Written at the bottom of the chemical symbol Atomic Mass Sum of the protons and neutrons in the nucleus. “A” number Written at the top of the chemical symbol Isotopes Atoms that have the same atomic number but different mass number. Atoms that have the same # of protons but a different # of neutrons. Most elements have more than one stable isotope and some radioactive isotopes. Medical Radioisotopes Used in Nuclear Medicine Imaging Man-made in particle accelerators called cyclotrons Technetium 99m Iodine 123 Xenon 133 gas