Nuclear Chemistry
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Nuclear chemistry is the study of reactions involving changes in atomic nuclei.
This branch of chemistry began with the discovery of natural radioactivity by Antoine Becquerel followed by subsequent investigations by Pierre and Marie Curie and many others.
All elements (nuclei) have fundamental particles, called protons and neutrons.
Some nuclei are unstable; they emit particles and/or electromagnetic radiation spontaneously → phenomenon called radioactivity.
All elements having an atomic number greater than 83 (Bi) are radioactive.
Example: the isotope of polonium, polonium—210 (84 210Po), decays spontaneously to 82 206Pb by emitting an 𝛼 particle.
Another type of radioactivity, known as nuclear transmutation, results from the bombardment of nuclei by neutrons, protons, or other nuclei.
Example: conversion of atmospheric 7 14N to 6 14C and 1 1H, which results when the nitrogen isotope captures a neutron (from the sun).
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Mode of Decay – for Radiation
alpha (𝛼) particle emission
beta (𝛽) particle emission
gamma (𝛾) radiation emission
Positron (ℯ) emission
K–electron (e–) capture
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Alpha Radiation, 𝜶
Composed of helium nuclei 𝟐 𝟒𝐇𝐞, being ejected at high speeds from radioactive elements.
It has limited penetrating power and can be stopped by several sheets of paper.
Beta Radiation, 𝜷
Composed of negatively charged particles or electrons ejected at high speed from some radioactive nuclei.
They can penetrate several mm of living tissue or bone.
Stop by at least a 1/8” aluminum
Gamma Radiation, 𝛄
A form of electromagnetic radiation like x-rays.
𝛾 rays have no electrical charge.
The most penetrating, can pass completely through the human body.
Thick layers of lead or concrete are required to minimize penetration.
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Positron (𝓮) emission
Also called 𝜷+emission
A positron is identical to an electron except that it has a charge of +1.
K–electron capture
In which an electron in the innermost energy level (n=1).
Electron capture more common with heavy nuclei, presumably because the n=1 level is closer to the nucleus.
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Change in Mode Emission Decay Process
Alpha Decay
a(He2+) + -4 -2 -2 S a expelled U
Beta Decay
B-(_iB) + OB 0 +1 -1 -1 nucleus with nucleus with in expelled xp+ and yn° (x +1)p* and (y-1)n° Net: 1n o" 1p + OB M in nucleus in nucleus in expelled Chemical symbol for the element. Mass number = A Z + N A M
Positron (B+) emission
B+(CB) + OB 0 -1 +1 1 nucleus with nucleus with B+ expelled X xp+ and yn° (x-1)p+ and (y+1)n° Atomic number = number of protons Z A Net: 1p 1n + ,BO in nucleus in nucleus B+ expelled
Electron (e-) capture
(EC)+ x-ray orbital 0 -1 +1 nucleus with nucleus with xp+ and yn° (x - 1)p+ and (y+1)n° Y Net: oe + 1p -1 1n absorbed from in nucleus in nucleus low-energy orbital
Gamma (y) emission
y + Y 0 0 0 excited stable yphoton nucleus nucleus radiated
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Stability of Atomic Nuclei
All element beyond Bi (Z=83) is radioactive and mostly decay by ejecting 𝜶 particle.
Above the stability belt, the nuclei have higher neutron-to-proton ratios than those within the belt (same number of proton).
To lower the ratio: 𝜷 emission occur in isotopes that have too many neutron to be stable.
Neutron → proton + electron
Below the stability belt the nuclei have lower neutron-to-proton ratios than those in the belt (same number of protons)
To increase the ratio: Lighter isotopes that has few neutrons attains stability by positron emission or electron capture.
Plot of neutrons vs protons for various stable isotopes, represented by dots. The straight line represents the points at which the neutron to-proton ratio equals 1. The shaded area represents the belt of stability.
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140 a decay 209 130 Bi 83 stable undergoes a decay ( NZ 1.52) Z undergoes undergoes B+ emission and/or e- capture 120 B- decay 184 90 W 110 74 ( N = 1.49) 100 Region shown in B 90 85 B- decay 80 107 47 Ag 70 ( NIN N 1.28) 60 80 55 60 65 70 B Protons (Z) 50 N 56 =1 26 Fe 40 Nuclides with N/Z ratios on the high side NZ 1.15) Z of the band will under go B- decay 30 B + emission and/or Nuclides with N/Z ratios on the low side e capture of the band will undergo electron capture 20 (e-) or positron (B*) emission 20 10 10 Ne Heavy nuclei (Z > 83) beyond the band ( N 1.0) Z undergo a decay 0 10 20 30 40 50 60 70 80 90 A Protons (Z)
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The disintegration of a radioactive nucleus employing various radiation type is often the beginning of a radioactive decay series → which refer to a sequence of nuclear reactions that ultimately result in the formation of a stable isotope.
EXAMPLE: Uranium decay series
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Nuclear Reaction and Equations
Nuclear reactions also called bombardment reactions.
More than 1500 radioactive isotopes have been prepared in the laboratory → by bombardment reaction in which the nucleus is converted to one that is radioactive
Nuclear reaction results in a change in atomic number and mass number.
The total number of nuclear particles, or nucleons (protons + neutrons), REMAINS the same.
The sum of the mass numbers of reacting nuclei MUST equal the sum of the mass number of the nuclei produced.
The sum of the atomic numbers of the products MUST equal the sum of the reactants.
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Nuclear reactions general Equation:
Example: The two conservation principles demand:
M1 = M2 + M3
Z1 = Z2 + Z3
Where the M's are mass numbers,
And the Z's are atomic numbers.
Q R 3Y 3 2 2 1 1 M Z M Z M Z → +
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In the reaction involving 𝜶 particle �� In 𝛼 emission, the atomic no. decreases by TWO units and the mass no. decreases by FOUR units for each 𝛼 particle emitted.
Example: decay of uranium-238 to thorium-234, with the emission of an 𝛼 particle
The ejection of a 𝜷 particle always means that a new element is formed with an atomic number ONE unit greater than the decaying nucleus.
𝜷 emission → increase in the number of protons in the nucleus and a simultaneous decrease in the number of neutrons
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PRACTICE EXERCISE: Balancing an equation:
Question: Balance the following nuclear equations (that is, identify the X):
a) 53 135I → 54 135Xe + X
b) 27 59Co + 0 1n → 25 56Mn + X
c) 8 20O → 9 20F + X
d) 27 59Co + 1 2H → 27 60Co + X
e) 24 53U + X → 26 56Fe + 0 1n
f) 19 40K → X + 20 40Ca
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PRACTICE QUESTION 2: Predict whether the following nuclides would stable and or radioactive. Explain your answer.
a) 10 18Ne
b) 16 32S
c) 90 236Th
d) 56 123Ba
e) 15 31P
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Practice Question 3: Predict the mode(s) of decay of the following radioactive nuclides.
a) 5 12B
b) 92 234U
c) 33 81As
d) 10 17Ne
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Rate of Radioactivity
Kinetics of Radioactive Decay
All radioactive decays obey first-order kinetics.
The rate of radioactive decay at any time t is given by:
the number of radioactive at time zero (N0) and time t (Nt)
and its corresponding half-life of the reaction
The formula for the rate of decay is:
Rate of decay at time, t = kN
Nt = N0 * (1/2)^(t/t1/2)
k = t1/2 / ln(2)
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Half-life
Half-Life, t½ of a radioisotope is the time required for HALF of the sample to decay.
The half-lives of radioactive isotopes have been used as “atomic clocks” to determine the ages of certain objects → radioactive dating.
e.g. rocks in the earth and of extraterrestrial objects
Measuring radioactivity
One can use a device like Geiger counter to measure the amount of activity present in a radioactive sample.
The ionizing radiation creates ions, which conduct a current that is detected by the instrument.
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Example 1: Tritium 1 3H, a radioactive isotope of hydrogen, has a half-life of 12.3 years. If you begin with 1.5 mg of the isotope, how many mg remain after 49.2 years?
No. of half-life = 49.2 years / 12.3 years = 4.00
The initial quantity of 1.5 mg is reduced four times by half
So, 1.5 mg x (1/2)^4 = 0.094 mg remains
Example 2: A tiny piece of paper taken from the dead sea scroll, believed to date back to the first century A.D., was found to have an activity per gram of carbon of 12.1 atoms/min. Taking initial carbon disintegration activity to be 15.3 atoms/min, estimate the age (t) of the scrolls. [Given half-life of Carbon = 5730 years]
Calculate k
Calculate t
t = 1.94 x 10^3 years
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Estimate the age of an unknown object whose 14C activity is only 55% that of living wood.
Determine the rate constant for 14C
Determine the age of the object
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How much 60Co remains 15.0 years after it is initially made?
Given half-life of 60Co is 5.27 years
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A sample of radon initially undergoes 7.0 x 10^4 α particle disintegrations per second (dps). After 6.6 days, it undergoes only 2.1 x 10^4 α particle dps. What is the half-life of this isotope of radon?
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Nuclear Fission and Fusion
Splitting of nucleus into smaller fragments
Fissionable atoms break into two fragments roughly of the same size when struck by neutrons
When two nuclei combine to produce a nucleus of heavier mass
Release more energy than fission
Takes place at a temp. higher than 40,000,000 °C
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Nuclear Fission
Fission event is within an atom bomb is initiated by a neutron bombarding a heavy nucleus such as uranium
The nuclear fission of 92 236U that arises from the bombardment of 92 235U with a neutron
The nucleus breaks into smaller pieces, releasing a lot of energy
Fission chain reaction has three general steps:
Initiation: Reaction of a single atom starts the chain (e.g., 235U + neutron)
Propagation: 236U fission releases neutrons that initiate other fissions
Termination: Consumption of the fissionable material is completed
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This sequence of fission reactions can be an explosive chain reaction
But, if there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out (stop)
Therefore, there must be a certain minimum amount/mass of fissionable material present for the chain reaction to be sustained: Critical Mass
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Application related to Nuclear fission: The Atomic bomb
The first application of nuclear fission was in the development of the atomic bomb
The crucial factor in the bomb’s design is the determination of the critical mass for the bomb
For obvious reasons, an atomic bomb is never assembled with the critical mass already present. Instead, the critical mass is formed by using a conventional explosive, such as TNT, to force the fissionable sections
Uranium-235 was the fissionable material in the bomb dropped on Hiroshima, Japan, on August 6, 1945. Plutonium-239 was used in the bomb exploded over Nagasaki 3 days later. The fission reactions generated were similar in these two cases, as was the extent of the destruction
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The Nuclear Reactor
A peaceful but controversial application of nuclear fission is the generation of electricity using heat from a controlled chain reaction in a nuclear reactor
Currently, nuclear reactors provide about 20 percent of the electrical energy in the United States (>100 reactor plant)
There are a total of 350 reactor plants worldwide and in Malaysia, only one for research purposes (Reaktor Triga Puspati)
Reactor handling and control:
The fission reaction can be slowed down by limiting the number of neutrons available, and the energy can be derived safely and used as a heat source in a power plant
The rate of fission is controlled by inserting cadmium rods (neutron absorber) into the reactor
By withdrawing or inserting the rods, the rate of fission reaction can be increased or decreased
Advantage: abundant clean energy
Disadvantage: generate highly radioactive nuclear wastes
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Nuclear Fusion
In contrast to the nuclear fission process, nuclear fusion, the combining of small nuclei into larger ones, is largely exempt from the waste disposal problem
Nuclear fusion occurs constantly in the sun. The sun is made up mostly of hydrogen and helium. In its interior, where temperatures reach about 15 million degrees Celsius
Because fusion reactions take place only at very high temperatures, they are often called thermonuclear reactions
Example application: The tokamak design → which a helical magnetic field confines the plasma and prevents it from contacting the walls: generating energy from fusion
Page 29: APPLICATION OF RADIOACTIVITY
Radioactive and stable isotopes have many applications in science and medicine.
Food Irradiation:
Use of 𝛾 rays from 60Co & 137Cs sources to prolong shelf life by killing pests that would destroy food during storage.
Irradiated milk has a shelf life of 3 months without refrigeration.
USDA has approved irradiation of meats and eggs.
Benefits of food irradiation: reduces energy demand by eliminating the need for refrigeration and prolongs the shelf life of various foods, important for poor countries.
Page 30: Radiochemical Dating
All organisms contain 3 isotopes of carbon: 12C, 13C, and 14C.
Carbon-14 is continuously created by cosmic radiation and decays to nitrogen-14 by 𝛽 emission.
The 𝛽 activity of carbon-14 in living plants and in the air is constant.
When a plant dies, carbon-14 activity decreases with time.
Age of a sample can be determined by knowing the half-life of 14C, the amount originally present (N0), and the measured amount (N).
Dating Using Uranium-238 Isotopes:
Uranium decay series have very long half-lives, suitable for estimating the age of rocks on Earth and extraterrestrial objects.
Dating Using Potassium-40 Isotopes:
Important technique in geochemistry.
The accumulation of gaseous argon-40 is used to gauge the age of a specimen.
Page 31: Radioactive Tracer
Isotopes, especially radioactive isotopes, are used to trace the path of atoms in a chemical or biological process.
Tracers are easy to detect, even in small amounts, using photographic techniques or counters.
Applications of tracers:
Study of photosynthesis: 18O isotope used to determine the source of O2, radioactive 14C isotope used to determine the path of carbon.
Tag hydrocarbons: 3H isotope (half-life = 12.26 yrs).
Tag pesticides, measure air flow: 35S isotope (half-life = 87.9 days).
Measure phosphorus uptake: 32P isotope (half-life = 14.3 days).
Diagnostic radioisotopes provide information on the locations of abnormal tissue by imaging.
Page 32: BIOLOGICAL EFFECT OF RADIATION
Exposure to radiation is harmful to living beings.
Ionizing radiation (particles or gamma rays) can remove electrons from atoms and molecules, leading to the formation of ions and radicals.
Superoxide ions and other free radicals attack cell membranes and organic compounds in tissues.
High-energy radiation can induce cancer in humans and animals