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