Transcript Nuclear Chemistry

Nuclear Chemistry
Just the basics….
By
J.M.Soltmann
What is Nuclear Chemistry
As its name implies, nuclear chemistry is the
study of the nucleus and reactions between
nuclei.
Remember that virtually all of the mass of an
atom resides in the nucleus, as does all of the
positive charge.
Nuclear energy is a much greater form of
energy than bond energy.
Radioactivity
 While most nuclei are
stable, many nuclei are
unstable and
spontaneously emit
particles and
electromagnetic
radiation.
 These nuclei are refered
to as radionuclides.
Nuclear Equations
 In a nuclear equation,
mass numbers and
atomic numbers are
balanced instead of
elements.
 The example here to
the right depicts a
radioactive decay;
specifically an alpha
decay.
 The helium ion is called
an alpha particle.
3 Common types of Radioactive
Decay
 Alpha decay
 Beta decay - a ß- particle is a
subatomic nuclear particle
essentially equivalent to an
electron and a ß+ particle is a
positively charge electron,
called a positron.
 Gamma decay - high energy
photons are emitted which
have virtually no mass nor
charge.
Nuclear electrons?
 Modern theory has
shown that a neutron is
actually comprised of a
proton and an electron.
 So, if a nucleus emits an
electron, it has really
transformed a neutron
into a proton.
 Also, if a nucleus
absorbs an electron, it
will convert a proton into
a neutron.
Common Particles in Nuclear Reactions
 Neutrons (10n)
 Protons (11p or 11H)
 Electrons (0-1e)
 Alpha Particles (42He or 42)
 Beta- Particles (0-1e or 0-1)
 Gamma (00) - Gamma radiation consists of highenergy photons, with a mass far too little for
consideration.
 Positron (01e) - A positron is a positively charged
electron. It has the mass of an electron but a
positive charge.
Differentiating the Radiations
Alpha emissions are the heaviest and thus
have the least penetrating power.
Beta emissions have masses much smaller
than protons or neutrons, so they have more
penetrating power. In terms of penetrating
power, =100* .
Gamma emissions have essentially no mass,
so they are the most powerful. In terms of
penetrating power.  = 100* .
Try this
Write a nuclear equation for the process
when mercury-201 undergoes electron
capture.
To answer this question:
First we have to understand what mercury201 is. Since mercury is always atomic
number 80, this isotope is 20180Hg.
Since we are capturing an electron, the
electron must be a reactant.
Now we add up mass numbers and atomic
numbers. (201 + 0 = 201 and 80 + -1 =79).
Element 79 is gold, so the answer is:
20180Hg + 0-1e -->20179Au
Try another
Thorium-231 decays into protactinium-231.
What is the balanced equation?
What other particle(s) is/are involved in the
reaction?
The answers are:
 23190Th -->
231 Pa
91
+
0 e
-1
The extra particle is an electron, but because
it is being emitted, it would be called a Beta
emission.
Nuclear Transformations
The first manmade conversion of one nucleus
into another was performed by Sir Ernest
Rutherford (1919).
Rutherford bombarded a nitrogen-14 atom
with alpha particles to produce an oxygen-17
atom plus a proton.
 147N +
4
2He -->
17
8O +
1
1H
The shorthand version of this reaction is
 147N(,p)178O
WHY???
Now try this one:
Write the balanced nuclear equation for the
process noted by the shorthand:
2713Al(n,)2411Na
Now try this one:
Write the balanced nuclear equation for the
process noted by the shorthand:
2713Al(n,)2411Na
2713Al +
1 n
0
-->
24 Na
11
+
4 He
2
Nuclear Stability
Why are some nuclei more stable than
others?
To be honest, there are several factors, most
of which are beyond the scope of this course.
However, there are a few easy to see
indications of nuclear stability.
Did you ever wonder…?
 We know that like charges repel each other, yet a
nucleus can have dozens of positively charged
protons held together. Why?
 Neutrons are a major reason. All nuclei with 2 or
more protons have neutrons. The neutrons and the
protons meld by a force of nature, different than
gravity or electromagnetism, called the strong
(nuclear) force.
 Because of the way this force binds the protons and
neutrons together, the ratio of protons to neutrons is
an issue.
Smaller Atoms vs Bigger Atoms
In smaller atoms, most stable atoms have
neutron to proton ratios of about 1.00.
As isotopes increase in atomic number, most
stable isotopes have increasingly larger ratios
of neutrons to protons.
To our knowledge, any isotope with an atomic
number greater than or equal to 84 would be
radioactive.
Some stability trends
Of the 265 known stable isotopes:
157 of them have even numbers of protons and
neutrons.
53 of them have an even number of protons but
an odd number of neutrons.
50 of them have an odd number of protons but an
even number of neutrons.
Only 5 of them have odd numbers of both protons
and neutrons.
Magic Numbers
For some reason, nuclei with 2,8,20,28,50 or
82 protons and/or 2,8,20,28,50,82, or 126
neutrons are generally more stable than
isotopes without these numbers.
When we think of substances that shield
radiation, we tend to think of lead. The most
common isotope of lead is 20882Pb; that
means it has 82 protons and 126 neutrons.
Decays and Half-lifes
 When a radioactive
substance decays, the
amount of that
particular isotope will
decrease.
 We call the rate of
decay the half-life,
because it is the time
needed for exactly 1/2
of the isotope to decay.
More on Half-life
 If we examine the graph to
the right, we see that we
started with 50 g of the
isotope. Each subsequent
point represents half of the
previous mass (50 to 25 to
12.5 to 6.25 to 3.125 to
1.5625 to .78125).
 Each point is approximately
24 days apart; The half-life
for this substance is 24
days.
Calculations with half-life
Although it is possible to determine the
amount remaining of a radioisotope using
natural logs { ln(Nt/N0)=-kt } we do not need
to do this.
We only work with whole number increments
of the half-life.
For example
The half-life of an isotope is 8 days. If we
start with 100 grams of the isotope, how
much is present in 32 days?
32 days/(8 days/half-life) = 4 half-lives.
Each half-life divides the previous mass in half.
100g/2 = 50g/2 = 25g/2 = 12.5g/2 = 6.25g
There would be 6.25 g of that isotope left.
You try one
The half-life of Bismuth-211 is 185 years.
How much time would it take for a 360 g
sample to decay to 11.25 g?
You try one
The half-life of Bismuth-211 is 185 years.
How much time would it take for a 360 g
sample to decay to 11.25 g?
 360g/2=180g/2=90g/2=45g/2=22.5g/2=11.25g.
That is 5 half-lives.
5 half-lives*185 years/half-life = 925 years.
Fusion vs. Fission
 Fission and Fusion are two types of highly exothermic
nuclear reactions, different than the decays covered
earlier.
 Fusion means to bring two smaller nuclei together to
make a larger nucleus.
 136C +
13
6C
--> 2511Na +
1
1p
 Fission means to break a larger nucleus into 2 or
more smaller nuclei.
 23592U + 10n -->
137
52Te
+
97
40Zr
+2
1
0n
How much Energy are we talking about?
 In fusion and fission, a
tiny, almost
meaningless mass of
each affected nucleus is
converted to energy.
 Einstein theorized that
the amount of energy
was dependent on the
mass lost and the
square of the speed of
light. E = mc2.
So, how much energy is that?
 Well if each uranium
atom in a given fission
process loses the mass
of one electron
(9.11x10-31 kg):
 E = mc2
 E = (9.11x10-31 kg)
*(3.0x108 m/s)2
 E = 8.2x10-14 J
But that seems like a small number!
8.2x10-14 J is a small amount, but that was for
just one atom or uranium. If we had 1 mg of
uranium (about the mass of a cystal of salt),
that would contain roughly 2.5 x 1018 atoms of
uranium.
8.2x10-14 J/atom * 2.5 x 1018 atoms =2.1x105 J
That is almost enough energy to handle the
electrical needs of this school for a day - from a
tiny starting mass.
Think about this
 If we could convert the .25 kg mass of a banana peel
(using our Mr. Fusion power supply) into pure energy,
 E = mc2
 E = (.25 kg)*(3.0x108 m/s)2
 E = 2.25x1016 J
 That’s enough energy to run New York City for a year
(with enough energy left over to go Back to the
Future)!