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22 • Nuclear Chemistry
The People
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- Wilhelm Roentgen (1845-1923) discovered X-rays, a high energy form of light. (1895)
- Henri Becquerel (1852-1909) found that uranium ores emit radiation that can pass through objects (like x-rays) and affect photographic plates. (1896)
- Marie Sklodowska Curie (1867-1934) Marie and Pierre worked with Becquerel to understand radioactivity. The three shared a Nobel Prize in Physics in 1903. Marie won a second Nobel Prize in Chemistry in 1911 for her work with radium and its properties.
- E. O. Lawrence invented the cyclotron which was used at UC Berkeley to make many of the transuranium elements.
22 • Nuclear Chemistry
Terms I-- Radioactivity
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radioactivity the spontaneous breakdown of atomic nuclei, accompanied by the release of some form of radiation (also called radioactive decay )
half-life time required for half of a radioactive sample to decay
transmutation one element being converted into another by a nuclear change
nuclides isotopes of elements that are identified by the number of their protons and neutrons
22 • Nuclear Chemistry
Terms II--Radioactivity
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decay series the sequence of nuclides that an element changes into until it forms a stable nucleus
radioactive using half-life information to determine dating the age of objects. C-14/C-12 is common for organic artifacts. Uranium is common for rocks.
nuclear fission large nucleus breaking down into pieces of about the same mass
nuclear fusion two or more light nuclei blend to form one or more larger nuclei
22 • Nuclear Chemistry
Types of Radiation
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Alpha particles are the same as a helium nucleus,
4 2 He, with a mass of 4 amu. It travels about 1/10th^ the speed of light and is the most easily stopped of the three particles (a sheet of paper will stop them). It is the least dangerous. Beta particles are high speed electrons, (^) −^01 e, with a mass of 0.00055 amu and travel at nearly the speed of light. They can be stopped by a sheet of aluminum. It is more penetrating and therefore more dangerous than alpha. Gamma rays are extremely high energy light, γ, with no mass, and are the most penetrating (several cm’s of lead are needed to stop them). They can cause severe damage.
22 • Nuclear Chemistry
Half-Life Problems
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In each half-life problem there are basically four variables:
- total time • half-life
- starting amount • ending amount 64g 32g 16g 8g 4g 2g 1g 0.5g 0.25g
Question : If you have 0.25 g of a radioactive substance with a half life of 3 days, how long ago did you have 64 grams? Answer: Draw the chart to determine the number of half- lives to get from the ending amount to the starting amount… each half-life is worth 3 days… 24 days.
22 • Nuclear Chemistry
Half-Life
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Half-Life The time it takes for half of a radioactive substance to decay. The decay graph has a characteristic shape:
time
The time it takes for the amount of substance or the activity of the substance to drop to half is the same WHEREVER you start on the graph. This is a first-order reaction. Half-lives can range from microseconds to thousands of years and is characteristic of each substance.
22 • Nuclear Chemistry
Nuclear Equations
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Memorize the symbols for the important particles alpha beta positron neutron 4 2 He^
0 − 1 e^
0
1 0 n Decay means the particle is on the right side of the equation: example: alpha decay of U- 238 238 92 U^ →^
4 2 He +^
234 90 Th The 234 and 90 are calculated… the Th is found on the periodic table (find the element with atomic # = 90). Several neutrons can be shown together and written as… 3(^10 n) and would be counted as 30 n in the equation.
22 • Nuclear Che mistry
How Each Type of Decay Can Stabilize an
Unstable Nucleus
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Certain values of p+’s and n°’s in the nucleus are stable. A nucleus can be unstable (radioactive) for 3 reasons:
- the nucleus has too many protons compared to neutrons solution : positron decay (change a proton into a neutron and a positive electron… …a positron)
- the nucleus has too many neutrons compared to protons solution : beta decay (change a neutron into a proton and a negative beta particle)
- the nucleus is too big (too many protons and neutrons) solution : alpha decay (lose 2 p+^ and 2 n°)
22 • Nuclear Chemistry
Calculating Half-Lives
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When a problem involves whole numbers of half-lives, divide by 2 to determine the amounts involved. For other situations, the following equations are useful:
ln t
0
[A]
[A]
= kt and the special case for half-life, t½, where by
definition, [A]t = ½[A] 0 ln 2 = 0.693= kt½ [A] is the concentration (or activity) of the radioactive substance, t = time, k = the rate constant (the same that is in Rate Laws). Note: if you know the half-life, you can calculate the rate constant and vice-versa.
22 • Nuclear Chemistry
Radioactive Decay Series
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Once a nucleus decays, the daughter isotope is often unstable as well. Many decays may occur before a stable nucleus is formed. A classic example is U-238 that decays through 14 steps into stable Pb-206. Each step has a characteristic decay particle and half-life. This characteristic decay series is the method used to verify the identity of newly formed atoms. The fact that daughter products can be even more radioactive than the parent isotope adds to the problem of nuclear waste and its storage/disposal.
22 • Nuclear Chemistry
Geiger-Muller Tubes, Smoke Detectors, and
Brushes for Cleaning Negatives
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An useful characteristic of decay particles are that they ionize the air they pass through by striking atoms and knocking off electrons. Geiger counters use this idea. As radioactive particles pass through a chamber with two electrodes, ionized particles migrate to the + and - electrodes and complete the circuit. Smoke Detectors use a tiny piece of radioactive Am to keep a circuit flowing due to ionized particles. Smoke particles attract ionized particles, break the circuit, & set off the alarm. Brushes are kept ionized by tiny bits of radioactive material to more easily attract tiny bits of dust.
22 • Nuclear Chemistry
Extending the Periodic Table
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Uranium, Z=92, is the largest naturally-occurring element. Larger atoms were manufactured. Elements 93 and 94 were formed in atomic bomb tests and identified by Seaborg. Glenn Seaborg and Al Ghiorso at UC Berkeley were able to use E. O. Lawrence’s cyclotron to make larger atoms (95-103).
Some of these new elements have uses in the medical field as well as helping to further the understanding of the nucleus. For many of the larger elements, however, only a few atoms or even one atom formed. They were identified by their characteristic decay series.
As of July 2000, 118 is the largest element.
