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 a particle (a helium
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 particle (an
electron). Nucleus becomes element with atomic
number one more than original and same isotopic
mass as original.
• Gamma (g) decay – nuclear atomic number and
isotopic mass unchanged. High energy photon
emitted.
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 decay of neptunium to
proactinium
237
93
Np 
233
91
Pa  He
4
2
Balancing nuclear equations
• Example – bismuth going to polonium
212
83
Bi 
212
84
Po  e
0
1
• Example – manufacture of curium from
plutonium
239
94
Pu  He 
4
2
Cm  n
242
96
1
0
Examples to work out
• Alpha decay of
• Beta decay of
141
56
251
98
Cf
Ba
Examples of reactions
• Beta decay (with positron emission) of 1220Mg
• Bombardment reaction
27
13
Al  He  ?  n
4
2
1
0
Measuring radiation -disintegrations
• Curie (Ci) – number of disintegrations per per
second. Based on 3.7 x 1010 atoms of radium
(1 gram of radium) disintegrating per second.
1 Ci = 3.7 x 1010 disintegrations per second.
• Bequerel (Bq) – 1 disintegration per second.
• 1 Ci = 3.7 x 1010 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