Nuclear Chemistry - Midland Public Schools
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Transcript Nuclear Chemistry - Midland Public Schools
Nuclear
Chemistry
.2 Chemistry
Midland High School
Mrs. Daniels 2007
Back to the Beginning
Recall the particles that make up an atom:
Proton (+1 charge)
Neutron (no charge)
Electron (-1 charge)
If you write out the symbol for an
element and include the atomic
number and the atomic mass, it should
look like this: 23
Na
11
For sodium:
the symbol is Na
the atomic mass is 23
and the atomic # is 11
What information can you take
from the following?
238
How many e-? P+? n0?
e-=92 p+=92 n0=146
U
92
Isotopes
What if I change the # of protons?
It would be a different element
What if I change the # of electrons
It would be an ion
What if I change the # of neutrons
It would be the same element, but a
different ISOTOPE of that element
Hydrogen
Let’s look at a couple of isotopes of
hydrogen
1
H
1
2
H
1
The one on the left is referred to as “light”
hydrogen and the one on the right is
“heavy”
Which one is the “normal” hydrogen that we
usually see
Variety of Isotopes
Even though there are
~110 different elements
listed on the periodic table,
there are nearly 1500
different known isotopes of
these elements
Some are stable and some
“decay” or break apart over
time
Nuclear Decay
All nuclear decay is accompanied
by the emission of radiation
Spontaneous emission of radiation
from an atom is called
radioactivity
All elements have isotopes that
are unstable and underdo decay
to become other element
Nuclear Decay
Radioactive isotopes can emit three
types of radiation:
Alpha particles: a helium nucleus (2
protons, 2 neutrons, with a charge of +2)
Not very fast; can be blocked by
something as thin as a piece of paper
Beta particles: fast moving electrons
created from the splitting of a neutron
(into a proton and an electron)
Requires aluminum foil 3mm thick to
Nuclear Decay
Gamma rays: radiation that is NOT
particles at all, but are invisible rays
of energy with no mass or electrical
charge
Very penetrating; need several cm of
lead or several meters of concrete to stop
Emitting alpha or beta particles
changes the element into a new
element
This is called nuclear transformation
Detection
How do we know that radiation is being
released or emitted?
There are several types of “counters”
used to detect radiation:
Geiger counter- uses Argon to transfer
the radiation into a temporary electric
pulse
Scintillation counter - uses sodium
iodide to produce flashes of light when
in contact with radiation
Half - Life
We can also go larger scale
and look at the half life of
various isotopes
Half life is defined as the
time it takes for HALF of the
sample of element to decay
For example, the half life of
carbon-14 is 5,730 years
Half - Life
Calculate how many years it would
take to decay 100g of carbon-14
into 12.5g.
Think on this: how many times
was 100 cut in half to get to 12.5?
100 --> 50
50 --> 25
25 --> 12.5
So… 3 half lives
If each half life takes 5,730 years
and we cut our sample in half
three times, how long did it take?
5,730 x 3 = 17,190 years
Roughly how much of a 100.0g
sample would be left after 1 year?
Well, 50g will take 5,730 years to
decay
A good estimate would be that
.0087g would decay each year
So… 100.0-.0087 = 99.99g
We’d actually have to graph it to
determine this more accurately
Radioactive Dating
Carbon-14, potassium-40, and others are
isotopes can be used for dating objects
from the past
We need to make the following
assumptions for carbon dating:
All living organisms contain the same ratio of
carbon-14 atoms and decay begins upon death
Remains of organisms or items created from
once living organisms contain the remaining
amount of carbon-14, which can be measured
Radioactive Dating
If we know the half life of carbon-14 is 5,730
years and we make the above assumptions,
then we can compare the amount of
carbon-14 in the sample with the amount of
carbon-14 in a living organism
Then, we simply calculate how many half-lives
the material underwent and multiply by
5,730 years per half life
Ta Daa! Now, we know how old it is…roughly
Fission and Fusion
With all the discussion of nuclear
power…we HAVE to talk about
fission and fusion.
Nuclear fusion: combining
(FUSING together) two lighter
nuclei to form a heavier nucleus
Nuclear fission: splitting a heavy
nucleus into two smaller nuclei with
smaller mass numbers
Nuclear Fission
Bombarding various isotopes with
neutrons can cause an isotope to split
into two lighter elements
The splitting is not always equal, so two
different elements may be produced
Also, excess neutrons fly off during the
splitting process and hit other atoms of
the isotope
This begins several other fission
reactions in the CHAIN of events
Fission Continued
A huge amount of energy can
be released from nuclear fission
reactions
For example, splitting one mole
of uranium-235 is 26 million
times the energy released from
the combustion of one mole of
methane
Chain Reaction
Fission Continued
If no neutrons go flying off and cause
the chain reaction to keep going, then
the reaction stops
If more than one neutron causes a new
“chain” in the reaction, a build up of
heat and an explosion can happen
The “critical mass” of fissionable
material is needed to maintain a
productive and constant fission
reaction
Nuclear Fusion
Produces even more energy than nuclear
fission; however, initiating the fusion reaction is
much more difficult
Protons don’t want to come together because
they repel each other
Temperatures of ~40 million K are estimated to
be necessary to overcome the repulsion forces
Figure out a way to do it at more manageable
temps (ie cold fusion) and you’ll be very rich
and famous
Don’t forget to thank your high school
chemistry teacher if this happens