Nuclear Chemistry Chapter 21
Stable vs. Unstable Nuclei • Most nuclei are stable – do not change • Some nuclei are unstable (radioactive) • Change into a different nucleus • Spontaneous process – happens naturally, by itself • Releases radiation Only nuclear reactions can change a nucleus. No chemical process can
Radium Radon + Radiation • The radium was unstable (radioactive) • Turned into a different element (decayed) • The lost mass was turned into radiation
Nuclear Radiation • Is spontaneously emitted from a radioactive nucleus • Can not be seen, smelled, heard • Can be detected using a Geiger counter or photographic film
Uses of Radiation • Nuclear fuel (235U and 239Pu) • Nuclear Weapons • Irradiated Food • Smoke Alarms (Amercium-241) • Cancer treatment (Cobalt-60) • Medical Tracers
Types of Nuclear Radiation 2 p+ 2 n e-
The Electromagnetic Spectrum Light Dangerous (ionizing) Safe radiation (non-ionizing) Produced by nuclear decay
What Stops Radiation Al Foil Wood Lead. Iron, Concrete Paper Alpha (a) Beta (b) Gamma (g)
Decay Equations Alpha Decay 23892U 42He + 23490Th Beta Decay 23490Th 0-1e + 23491Pa
Decay Equations Gamma Decay Occurs with alpha and beta decay No change in atomic mass (gamma radiation has no mass 00g)
Decay: Ex 1 What product is formed when radium-226 undergoes alpha decay? 22688Ra 42He +
Decay: Ex 2 What element undergoes alpha decay to form lead-208? 42He + 20882Pb
Decay: Ex 3 What isotope is produced when thorium-231 beta decays? 23190Th 0-1e +
Positron Emission • Same mass an electron, but opposite charge • Form of anti-matter 01e Electron Capture • Nucleus captures a core electron • electron is added rather than lost
Decay: Ex 4 Write the equation that describes oxygen-15 undergoing positron emission. Write the equation that describes mercury-201 undergoing electron capture
Which nuclei are radioactive (unstable) • All elements have at least one radioactive isotope • All isotopes of elements heavier than Lead (element 82) are radioactive • All elements heavier than 92 (U) are man-made and radioactive 82 Pb 207.2 At least one radioactive isotope All isotopes are radioactive
Belt of stability – based on neutron:proton ratio • Below ~20 = 1:1 ratio stable • Ratio increases with increasing # protons • Isotopes outside the belt try to decay and get on the belt
Decay Modes • Atomic # >84 • Alpha Decay • Above belt • Too many neutrons • Beta emission • Below belt • Too few neutrons • electron capture or positron emission
Most heavy isotopes (above 84) decay by alpha emission • Slide down to lead-206
Decay Modes: Ex 1 Predict the decay mode for carbon-14 8n : 6p Too many n’s, prefers 1:1 146C
Decay Modes: Ex 2 Predict the decay mode for xenon-118 64n : 54p =1.2 Too few n’s (check graph) 11854Xe Or 11854Xe
Decay Modes: Ex 3 Predict the decay mode for plutonium-239 Predict the decay mode for indium-120
Further Observations • Magic #’s - Nuclei with 2, 8, 20, 28, 50 or 82 protons or 2, 8, 20, 28, 50 or 126 neutrons are especially stable. • Nuclei with even #s of both protons and neutrons are more stable than those with odds numbers. Ex: 63Cu and 65Cu are abundant, but 64Cu is not. Why?
Transmutation • Rutherford(1919) – First successful alchemist 147N + 42He 178O + 11H 147N(a,p) 178O • Modern methods • Particle Accelerators (Cyclotrons) • Use neutrons or other elements (creation of transuranium elements)
Transmutation: Ex 1 Write the balanced nuclear equations for the process : 2713Al(n, a) 2411Na
Transmutation: Ex 2 Write the shorthand notation for: 168O + 11H 137N + 42He
Transmutation: Neutrons • Neutrons produced from radioactive decay • Cobalt-60 is used in radiation therapy 5826Fe + 10n 5926Fe 5926Fe 5927Co + 0-1e 5927Co + 10n 6027Co
Transmutation: Transuranium Elements 23892U + 10n 23992U 23993Np + 0-1e 23994Pu + 42He 24296Cm + 10n 20983Bi + 6428Ni 272111Rg + 10n
Half-Life • Half-life - The time during which one-half of a radioactive sample decays • Ranges from fraction of a second to billions of years. • You can’t hurry half-life.
Half-Life The polonium-214 will decay much sooner than the uranium. The uranium will be radioactive pretty much until the earth is destroyed when our sun goes out in 10 billion years.
Carbon-14 dating • Upon death, 14C radioactively decays. (half-life = 5730 y) • Reasonable to up to 50,000 years. • 15% margin of error • Mummies, the Dead Sea Scrolls, Shroud of Turin
Half-life: Example 1 Carbon-14 has a half-life of 5730 years and is used to date artifacts. How much of a 26 g sample will exist after 3 half-lives? How long is that?
Half-life: Example 2 Tritium undergoes beta decay and has a half life of 12.33 years. How much of a 3.0 g sample of tritium remains after 2 half-lives?
Half-life: Example 3 Radon-226 has a half-life of 1600 years? How much of a 30 gram sample remains after 6400 years?
Half-life: Example 4 Cesium-137 has a half-life of 30 years. If you start with a 200 gram sample, and you now have 25 grams left, how much time has passed?
Half-life: Example 5 Calcium-45 has a half-life of 160 days. If you start with a 500 gram sample, and you now have 31.25 grams left, how much time has passed?
Rate Law First order rate law Rate = kN (N is the initial concentration) Rate = -DN = dN = -kN Dt dt dN = -kN dt dN = -kdt N
∫dN = ∫-kdt N ∫dN = -k∫dt (Integrate left from N0 to Nt N and time from 0 to t) lnNt = -kt or Nt = Noe-kt N0
Calculating k or the half-life lnNt = -kt N0 ln1 = -kt½ 2 k = 0.693 t½
Rate Law: Ex 1 Uranium-238 has a half-life of 4.5 X 109 yr. If 1.000 mg of a 1.257 mg sample of uranium-238 remains, how old is the sample? k = 0.693 t½ k = 0.693 = 1.5 x10-10 yr-1 4.5 X 109 yr
lnNt = -kt N0 ln 1.000 = -(1.5 x10-10 yr)t 1.257 t = 1.5 X 109yr
Rate Law: Ex 2 A wooden object is found to have a carbon-14 activity of 11.6 disintegrations per second. Fresh wood has 15.2 disintegrations per second. If the half-life of 14C is 5730 yr, how old is the object?
Rate Law: Ex 2 A wooden object is found to have a carbon-14 activity of 11.6 disintegrations per second. Fresh wood has 15.2 disintegrations per second. If the half-life of 14C is 5730 yr, how old is the object? ANS: 2230 yr
Rate Law: Ex 3 After 2.00 yr, 0.953 g of a 1.000 g sample of strontium-90 remains. How much remains after 5.00 years? x =0.887 g