21.S: Nuclear Chemistry (Summary)

A general chemistry Libretexts Textmap organized around the textbook
 Chemistry: The Central Science
by Brown, LeMay, Bursten, Murphy, and Woodward

Template:HideTOC

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
  • (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
  • 
  • 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		electrons	High-energy photons

• • beta particles – high speed electrons emitted by an unstable nucleus
  • 
  • beta decay results in increasing the atomic number
  • 
  • 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
  • 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
  • 

Particle	Symbol
Neutron
Proton
Electron
Alpha Particle
Beta Particle
Positron

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
  • 
  • 

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
  • 
  • t = time interval of decay, k = decay constant, N₀ = initial number of nuclei at time zero, N_t = number remaining after time interval
  • 

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 = 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