Reading Notes
for Chapter 21
These are Dr. Bodwin's reading notes for Chapter 21 of "Chemistry
2e" from OpenStax.
I am using a local .pdf copy that was downloaded in May 2020.
Chapter
Summary:
All of the chemistry we have talked about up to this point takes place
in the electrons. When we first talked about balancing equations, we
held the identity of the elements constant... everything that went in to a reaction, had to come out
of the reaction. Although this is true for the vast majority of
chemical transformations, there are cases where the nucleus does
change. It is still true that everything that goes in must come out
("matter cannot be created or destroyed, only changed"), but now we
must look deeper into the nuclear structure of atoms. Fortunately,
electron chemistry and nuclear chemistry occur at significantly
different energy scales, so when we are looking at electron chemistry,
we can hold the nuclei constant, and when we're looking at nuclear
chemistry, we can (mostly) ignore the electrons.
Nuclear Structure and Binding Energy:
Review subatomic particles and the numbers that describe them: atomic number, mass number, charge.
Although it's convenient to say "matter cannot be created or destroyed", it's probably a little more accurate to say that energy
cannot be created or destroyed. If we think in terms of Einstein's most
famous equation, matter is just another form of energy. WHen nuclear
particles bind together, they lose a tiny bit of mass that is converted
to energy.
Stable Nuclei:
Protons and neutrons have to be in balance for a nucleus to be stable.
Figure 21.2 in your textbook is a nice illustration of the relationship
between the number of protons and neutrons in the known elements and we
can see that the more protons there are, the more neutrons are required
to make a (relatively) stable nucleus. Note the deviation from a 1:1
relationship.
Types of Nuclear Decay/Reactions:
There are a limited number of reactions that can take place in a
nucleus. Most of these are classes nuclear "decay" and involve loss of
some sort of particle or energy (usually both...).
Alpha decay - Loss of a helium nucleus, 2 protons and 2 neutrons (an alpha particle). Atomic number goes down by 2, mass number goes down by 4.
Beta decay - Loss of a beta particle. A beta particle is a particle
that comes out of the nucleus that has all the properties of an
electron. One way to think of this is that a neutron is like a proton
and an electron that are intimately bound together. When beta decay
occurs, a neutron becomes a proton. The atomic number increases by 1, the mass number stays the same.
Gamma decay - there is no particle involved here, it's just the emission of gamma energy waves.
Balancing Nuclear Equations:
In
some ways, balancing nuclear equations is an easier "count the
particles" exercise than balancing electron chemistry equations. Write
all of the species with their mass number and atomic number, and make
sure that there is equal total mass number on the left and right, as
well as equal total atomic number on the left and right. That's why a
beta particle has a "-1" for its atomic number... it looks odd, but it
makes the nuclear reaction balancing work.
Fission vs. Fusion - In a "fission" reaction, nuclei are breaking apart
(forming fissures); in a fusion reaction, nuclei are being combined.
Half-life:
Because nuclear reactions occur at such a different energy scale, the
reactions behave a little differently than electron chemistry
reactions. One of the results of this is that nuclear decay processes
all follow 1st-order kinetics.
Half-life is the amount of time it takes for the concentration of a
substance to decrease by half. For 1st order processes, the half-life does not depend upon the amount of material that is present! We can derive a half-life expression from the 1st order integrated rate law expression.
The concept of half-life is why nuclear waste is such a problem. If we
start with 1000kg of dangerous waste that has a half life of 10 years,
then after 10 years we will have 500kg. After another 10 years, we'll
still have 250kg. Another 10 years, 125kg... so although the amount is
getting smaller, we'll never get all the way to zero.
Radioisotope Dating:
Because radioactive decay occurs at a predictable rate with 1st order
kinetics, it can be used to determine the age of objects. A number of
different isotopes can be used. Carbon-14 is used for living things
because allliving things (including you and me) accumulate a constant
amount of cabon-14 as long as we are alive. Once something dies, it no
longer accumulates carbon-14, so the amount of decay can be used to
determine the (approximate) time that has passed since the organism
died.
Imaging and Medical Applications:
Radiation is dangerous and damaging to living things, but we can also
use it to our advantage. By designing it to accumulate in specific
parts of an organism, we can get better images of those parts than we
might otherwise. We can also design targetted pharmaceuticals that
deliver a radiation-producing does to a very specific site to deliver
their deadly radioactive payload to kill cancers or other diseased
tissue.
Nuclear Power Generation:
Most conventional electrical power generation technologies use heat to
boil water to form steam that is used to spin a turbine to generate
electricity. Coal, oil, natural gas, and nuclear are all just different
sources of heat that are used to boil water, Some solar energy
technologies are also just ways to concentration solar energy as heat
to boil water. Nuclear offers a number of significant advantages over
fossil-fuel-based technologies, but there are also significant risks.
Everything is about balance. We could spend a couple weeks talking
about electricity generation, but that's a little outside the scope of
this course.
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