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Chemistry LibreTexts

21.S: Nuclear Chemistry (Summary)

  • Page ID
    70551
  • 21.1: Radioactivity

      • nucleons – neutron and proton
      • all atoms of a given element have the same number of protons, atomic number
      • isotopes – atoms with the same atomic number but different mass numbers
      • three isotopes of uranium: uranium-233, uranium-235, uranium-238
      • clipboard_e369830715db1228003f55c12839f16a0.png(superscript is mass number, subscript atomic number)
      • radionuclides – nuclei that are radioactive
      • radioisotopes – atoms containing radionuclides

    21.1.1 Nuclear Equations

      • alpha particles – helium-4 particles
      • alpha radiation – stream of alpha particles
      • emission of radiation is one way that an unstable nucleus is transformed into a more stable one
      • clipboard_e4c7d5b916164f4cffdbe0a63fae7baba.png
      • superscript = mass number
      • subscript = atomic number
      • radioactive decay – when a nucleus spontaneously decomposes
      • sum of the mass numbers is the same on both sides of the equation
      • sum of the atomic numbers same on both sides of the equation
      • radioactive properties of the nucleus are independent of the state of chemical combination of the atom
      • chemical form does not matter when writing nuclear equations

    21.1.2 Types of Radioactive Decay

    • three most common type of radioactive decay: alpha(α), beta(β), and gamma(γ) radiation
    Types of Radiation
    Property α β γ
    Charge 2+ 1- 0
    Mass 6.64x10-24 g 9.11x10-28 g 0
    Relative penetrating power 1 100 10,000
    Nature of radiation \(\ce{^{2}_4 He} \, \text{nuclei}\) electrons High-energy photons
      • beta particles – high speed electrons emitted by an unstable nucleus
      • clipboard_e32ff9c313323f37c33fd1ddcfdf83c9a.png
      • beta decay results in increasing the atomic number
      • clipboard_ec2cbc87e1ab4e1f62d7a311f7be89688.png
      • gamma radiation – high-energy protons
      • gamma radiation does not change atomic number or mass number or a nucleus
      • almost always accompanies other radioactive emission
      • represents the energy lost when the remaining nucleons reorganize into more stable arrangements
      • positron – particle that has same mass as an electron but opposite charge
      • represented by clipboard_e3dc7b3cad0da98a9822a15f7d5f67f31.png
      • emission of a positron has effect of converting a proton to a neutron decreasing atomic number of nucleus by 1
      • electron capture – the capture by the nucleus of an inner-shell electron from the electron cloud surrounding the nucleus
      • has effect of converting a proton to neutron
      • clipboard_e7f536221d9e56122a481ce59e2cc4a7d.png
    Particle Symbol
    Neutron \(\ce{^{1}_0n}\)
    Proton \(\ce{^{1}_1H}\) or \(\ce{^{1}_1p}\)
    Electron \(\ce{^{0}_{-1}e}\)
    Alpha Particle \(\ce{^{4}_2 He}\) or \(\ce{^{4}_2 \alpha}\)
    Beta Particle \(\ce{^{0}_{-1} e}\) or \(\ce{^{0}_{-1} \beta}\)
    Positron \(\ce{^{0}_1e}\)

    21.2: Patterns of Nuclear Stability

    21.2.1 Neutron-to-Proton Ratio

      • strong nuclear force – a strong force of attraction between a large number of protons in the small volume of the nucleus
      • stable nuclei with low atomic numbers up to 20 have nearly equal number of neutrons and protons
      • for higher atomic numbers, the number of neutrons are greater than the number of protons
      • the neutron-to-proton ratio of stable nuclei increase with increasing atomic number
      • belt of stability – area where all stable nuclei are found
        • ends at bismuth
        • all nuclei with 84 or more protons are radioactive
        • an even number of protons and neutrons is more stable than an odd number
      • determining type of radioactive decay
        • 1) nuclei above the belt of stability
        • high neutron-to-proton ratios
        • move toward belt of stability by emitting a beta particle
        • decreases the number of neutrons and increases the number of protons in a nucleus
        • 2) nuclei below the belt of stability
        • low neutron-to-proton ratios
        • move toward belt of stability by positron emission or electron capture
        • increase number of neutrons and decrease the number of protons
        • positron emission more common with lower nuclear charges
        • electron capture becomes more common with increasing nuclear charge
        • 3) nuclei with atomic numbers 84
        • alpha emission
        • decreases both number of neutrons and protons by 2

    21.2.2 Radioactive Series

      • some nuclei cannot game stability by a single emission
      • radioactive series or nuclear disintegration series – series of nuclear reactions that begin with an unstable nucleus to a stable one
      • three types of radioactive series found in nature
        • uranium-238 to lead-206, uranium-235 to leat-207, and thorium-232 to lead-208

    21.2.3 Further Observations

      • nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons are more stable than with nuclei without these numbers
      • numbers called magic numbers
      • nuclei with even number of protons and neutrons more stable than with odd number of protons and neutrons
      • observations made in terms of the shell model of the nucleus
        • nucleons reside in shells
      • magic numbers represent closed shells in nuclei

    21.3: Nuclear Transmutations

      • nuclear transmutations – nuclear reactions caused by the collision of one nucleus with a neutron or by another nucleus
      • first conversion of one nucleus into another performed by Ernest Rutherford in 1919
      • converted nitrogen-14 to oxygen-17
      • clipboard_e39d8d5e3c67e2f6d82323eff4c82dc37.png
      • clipboard_ec55ce5c84ce156f9647603eb1fcaf4a9.png

    21.3.1 Using Charged Particles

      • particle accelerators – used to accelerate particles at very high speeds
      • cyclotron, and synchrotron

    21.3.2 Using Neutrons

    • neutrons do not need to be accelerated

    21.3.4 Transuranium Elements

    • transuranium elements – elements with atomic numbers above 92 that are produced by artificial transmutations

    21.4: Rates of Radioactive Decay

      • radioactive decay is a first-order process
      • has characteristic of half life, which is the time required for half of any given quantity of a substance to react
      • half-life unaffected by external conditions

    21.4.1 Dating

      • radiocarbon dating assumes that the ratio of carbon-14 to carbon-12 in the atmosphere has been constant for at least 50,000 years
      • age of rocks can be determined by ratio of uranium-238 to lead-206

    21.4.2 Calculations Based on Half-life

      • rate = kN
      • \(k\) = decay constant, N = nuclei
      • \[ \ln \dfrac{N_t}{N_o} = -k t\]
      • t = time interval of decay, k = decay constant, N0 = initial number of nuclei at time zero, Nt = number remaining after time interval
      • \[k= \dfrac{0.693}{t_{1/2}}\]

    21.5 Detection of Radioactivity

      • Geiger counter – device used to measure and detect radioactivity
      • Based on ionization of matter caused by radiation
      • Phosphors – substances that give off light when exposed to radiation
      • Scintillation counter – used to detect and measure radiation based on tiny flashes of light produced when radiation strikes a suitable phosphor

    21.5.1 Radiotracers

      • radioisotopes can be used to follow an element through its chemical reactions
      • isotopes of same element have same properties
      • radiotracer – radioisotopes used to trace an element

    21.6: Energy Changes in Nuclear Reactions

    \[E=mc^2\]

    E = energy, m = mass, c = speed of light

    If system loses mass, it loses energy (exothermic)

    If system gains mass, it gains energy (endothermic)

    21.6.1 Nuclear Binding Energies

      • masses of nuclei always less than masses of individual nucleons
      • mass defect – mass difference between a nucleus and its constituent nucleons
      • energy is needed to break nucleus into separated protons and neutrons, addition of energy must also have an increase in mass
      • nuclear binding energy – energy required to separate a nucleus into its individual nucleons
        • the larger to nuclear binding energy the more stable the nucleus toward decomposition
      • fission – energy produced when heavy nuclei split
      • fusion – energy produced when light nuclei fuse

    21.7: Nuclear Fission

    • fission and fusion both exothermic
    • chain reaction – reaction in which the neutrons produced in one fission cause further fission reactions
    • in order for a fission chain reaction to occur, the sample of fissionable material must have a certain minimum mass
    • critical mass – amount of fissionable material large enough to maintain the chain reaction with a constant rate of fission
    • supercritical mass – mass in excess of a critical mass

    21.7.1 Nuclear Reactors

    • nuclear reactors the fission is controlled to generate a constant power
    • reactor core consists of fissionable fuel, control rods, a moderator, and cooling fluid
    • fission products are extremely radioactive and are thus hard to store
    • about 20 half-lives needed for products to react acceptable levels for biological exposure

    21.8: Nuclear Fusion

    • fusion is appealing because of availability of light isotopes and fusion products are not radioactive
    • high energies needed to overcome attraction of nuclei
    • thermonuclear reactions – fusion reactions
    • lowest temperature required is about 40,000,000 K

    21.9: Biological Effects of Radiation

    • when matter absorbs radiation, the energy of the radiation can cause either excitation or ionization
    • ionization radiation more harmful than nonionization radiation
    • most of energy of radiation absorbed by water molecules
    • free radical – a substance with one ore more unpaired electrons
    • can attack other biomolecules to produce more free radicals
    • gamma rays most dangerous
    • tissues that take most damage are the ones that reproduce at a rapid rate
    • bone marrow, blood forming tissues, lymph nodes

    21.9.1 Radiation Doses

      • becquerel (Bq) – SI unit for activity of the radiation source; rate at which nuclear disintegrations are occurring
      • 1 (Bq) = 1 nuclear disintegration/s
      • curie (Ci) = 3.7x1010 disintegrations/s = rate of decay of 1g of radium
      • two units used to measure amount of exposure to radiation: gray (Gy) and rad
      • gray – SI unit of absorbed dose = absorption of 1 J of energy per kilogram of tissue
      • rad (radiation absorbed dose) – absorption of 1x10-2 J of energy per kilogram of tissue
      • 1 Gy = 100 rads
      • relative biological effectiveness – RBE
        • 1 for gamma and beta radiation, 10 for alpha radiation
        • exact value varies with dose rate, total dose, and type of tissue affected
        • rem (roentgen equivalent for man) – product of the radiation dose in rads and the RBE of the radiation gibes the effective dosage
        • rem is unit of radiation damage that is usually used in medicine
        • number of rems = (number of rads)(RBE)
      • Sievert (Sv) – SI unit for dosage
        • 1 Sv = 100 rem
        • annual exposure = 360mrem

    21.9.2 Radon

      • radon exposure estimated to account for more than half annual exposure
      • half-life of radon is 3.82 days
      • decays into radioisotope polonium
      • atoms of polonium can be trapped in lungs giving out alpha radiation causing lung cancer
      • recommended levels of radon-222 in homes is to be less than 4 pCi per liter of air