chapter 24 nuclear reactions and their applications
TRANSCRIPT
Types of Radioactive Decay: Balancing Nuclear Equations
Total ATotal Z Reactants = Total A
Total Z Products
Alpha decay - A decreases by 4 and Z decreases by 2. Every element heavier than Pb undergoes decay.
Beta decay - ejection of a particle from the nucleus from the conversion of a neutron into a proton and the expulsion of 0
-1. The product nuclide will have the same Z but will be one atomic number higher.
Positron decay - a positron (01) is the antiparticle of an electron. A
proton in the nucleus is converted into a neutron with the expulsion of the positron. Z remains the same but the atomic number decreases.
Electron capture - a nuclear proton is converted into a neutron by the capture of an electron. Z remains the same but the atomic number decreases.
Gamma emission - energy release; no change in Z or A.
Sample Problem 1 Writing Equations for Nuclear Reactions
PROBLEM: Write balanced equations for the following nuclear reactions:
(a) Naturally occurring thorium-232 undergoes decay.
(b) Chlorine-36 undergoes electron capture.
Nuclear Stability and Mode of Decay
•Very few stable nuclides exist with N/Z < 1.
•The N/Z ratio of stable nuclides gradually increases a Z increases.
•All nuclides with Z > 83 are unstable.
•Elements with an even Z usually have a larger number of stable nuclides than elements with an odd Z.
•Well over half the stable nuclides have both even N and even Z.
Predicting the Mode of Decay
•Neutron-rich nuclides undergo decay.
•Neutron-poor nuclides undergo positron decay or electron capture.
•Heavy nuclides undergo decay.
Sample Problem 2 Predicting Nuclear Stability
PROBLEM: Which of the following nuclides would you predict to be stable and which radioactive? Explain.
(a) 1810Ne (b) 32
16S (c) 23690Th (d) 123
56Ba
Sample Problem 3 Predicting the Mode of Nuclear Decay
PROBLEM: Predict the nature of the nuclear change(s) each of the following radioactive nuclides is likely to undergo:
(a) 125B (b) 234
92U (c) 7433As (d) 127
57La
Decay rate (A) = N/t
SI unit of decay is the becquerel (Bq) = 1d/s.
curie (Ci) =
number of nuclei disinegrating each second in 1g of radium-226 =
3.70x1010d/s
Nuclear decay is a first-order rate process.
Large k means a short half-life and vice versa.
Sample Problem 4 Finding the Number of Radioactive Nuclei
PROBLEM: Strontium-90 is a radioactive by-product of nuclear reactors that behaves biologically like calcium, the element above it in Group 2A(2). When 90Sr is ingested by mammals, it is found in their milk and eventually in the bones of those drinking the milk. If a sample of 90Sr has an activity of 1.2x1012 d/s, what are the activity and the fraction of nuclei that have decayed after 59 yr (t1/2 of 90Sr = 29 yr)
Sample Problem 5 Applying Radiocarbon Dating
PROBLEM: The charred bones of a sloth in a cave in Chile represent the earliest evidence of human presence in the southern tip of South America. A sample of the bone has a specific activity of 5.22 disintegrations per minute per gram of carbon (d/min*g). If the ratio of 12C:14C in living organisms results in a specific activity of 15.3 d/min*g, how old are the bones? (t1/2 of 14C = 5730 yr)
A linear accelerator
The linear accelerator operated by Stanford University, California
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Penetrating power of radioactive emissions
Penetrating power is inversely related to the mass and charge of the emission.
Nuclear changes cause chemical changes in surrounding matter by excitation and ionization.
The use of radio-isotopes to image the
thyroid gland
asymmetric scan indicates disease
normal
PET and brain activity
normal Alzheimer’s
The Interconversion of Mass and Energy
E = mc2
E = mc2
m = E / c2
The mass of the nucleus is less than the combined masses of its nucleons. The mass decrease that occurs when nucleons are united into a nucleus is called the mass defect.
The mass defect (m) can be used to calculate the nuclear binding energy in MeV.
1 amu = 931.5x106 eV = 931.5MeV
Sample Problem 6 Calculating the Binding Energy per Nucleon
PROBLEM: Iron-56 is an extremely stable nuclide. Compute the binding energy per nucleon for 56Fe and compare it with that for 12C (mass of 56Fe atom = 55.934939 amu; mass of 1H atom = 1.007825 amu; mass of neutron = 1.008665 amu).