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Nuclear Chemistry
Principles and Applications
Isotopes
• • • • Recall that most elements consist of a distribution of various isotopes.
Isotopes of the same element have the same atomic number but different atomic masses.
The atomic mass in the periodic table is a statistical average over all of the isotopes for the element.
Some isotopes are stable, and some decompose through radioactivity.
Radioactive isotopes
• • • • Change through 3 different processes Alpha ( a ) decay – nucleus emits an nucleus). Nucleus becomes element with an atomic number two less than original and isotopic mass 4 less than original.
Beta ( b ) decay – nucleus emits a b a particle (a helium particle (an electron). Nucleus becomes element with atomic number one more than original and same isotopic mass as original. Gamma ( emitted.
g ) decay – nuclear atomic number and isotopic mass unchanged. High energy photon
b
emission -- + or - ?
• • Normal b emission is an electron – negatively charged. Usually happens when there are excess neutrons. A neutron decomposes to give a proton and an electron (and a neutrino) Variant on b emission is a positron, positively charged anti-matter to an electron. Happens when there is an excess of protons in the nucleus. Proton gives a neutron and a positron (and a neutrino).
Sources of radioactive isotopes
• Naturally occurring, four series found: 1. Thorium (4n series) 2. Neptunium (4n+1 series) 3. Uranium – radium (4n+2 series) 4. Uranium – actinium (4n+3 series) Plus isotopes can be manufactured in nuclear reactors and in various kinds of accelerators.
More manufactured isotopes than those found in nature.
Radioactive shielding
• • • • • a particles – easily shielded if source is external. Normal skin protects body.
b particles – electrons are emitted. Clothing, gloves will protect body.
g rays – high energy, requires extensive shielding. (e.g., lead box) Damage to body is caused by creation of unstable chemical species in the body.
Limit time exposure, use shielding, protective clothing
Balancing nuclear equations
• • A nuclear equation is balanced when the sum of the mass numbers and the sum of atomic numbers of the particles and isotopes are the same on both sides of the equation.
Example – a proactinium decay of neptunium to 237
Np
93 233
Pa
91 2 4
He
Balancing nuclear equations
• Example – bismuth going to polonium 212
Bi
83 212
Po
84 0 1
e
• Example – manufacture of curium from plutonium 239 94
Pu
2 4
He
242 96
Cm
0 1
n
Examples to work out
• Alpha decay of 251
Cf
98 • Beta decay of 141 56
Ba
Examples of reactions
• Beta decay (with positron emission) of 20
Mg
12 • Bombardment reaction 27
Al
13 2 4
He
0 1
n
Measuring radiation - disintegrations
• • • Curie (Ci) – number of disintegrations per per second. Based on 3.7 x 10 10 atoms of radium (1 gram of radium) disintegrating per second. 1 Ci = 3.7 x 10 10 disintegrations per second.
Bequerel (Bq) – 1 disintegration per second.
1 Ci = 3.7 x 10 10 Bq
Measuring radiation -- absorption
• • • • • • • • Rad – amount of radiation absorbed per gram SI counterpart – gray (Gy) 1 Gy = 100 rads Rem (radiation equivalent in humans) – Rem = rad x factor The factor is an adjustment for the damage potential of the radiation.
SI counterpart – sievert (Sv) 1 Sv = 100 rems
Example
• The recommended dosage of iodine-131 is 4.20 microCi/kg of body weight. How many microcuries of iodine-131 are needed for a 70.0 kg patient with hyperthyroidism?
Half-life of isotopes
• • The half-life of a radioisotope is the amount of time it takes for one-half of a sample to decay.
Many uses in archaeology, paleontology, and geochemistry for assigning ages to artifacts, fossils, and mineral deposits.
Example on half-life
• • • • • Technetium-99m is an ideal radioisotope for scanning organs because it has a half-life of 6.0 hours and is a pure gamma emitter. Suppose that 80.0 mg were prepared in the technetium generator this morning. How many milligrams would remain after the following intervals?
One half-life Two half-lives 18 hours 24 hours