Nuclear Chemistry

1
Nuclear Chemistry

• .2 Chemistry
• Midland High School
• Mrs. Daniels 2007

2
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
3

• For sodium
• the symbol is Na
• the atomic mass is 23
• and the atomic is 11
• What information can you take from the following?
• How many e-? P? n0?
• e-92 p92 n0146

238 U 92
4
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

5
Hydrogen

• Lets look at a couple of isotopes of hydrogen
• 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

1 H 1
2 H 1
6
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

7
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

8
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 stop

9
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

10
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

11
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

12
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 --gt 50
• 50 --gt 25
• 25 --gt 12.5
• So 3 half lives

13

• 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
• Wed actually have to graph it to determine this
  more accurately

14
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

15
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 isroughly

16
Fission and Fusion

• With all the discussion of nuclear powerwe 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

17
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

18
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

19
Chain Reaction
20
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

21
Nuclear Fusion

• Produces even more energy than nuclear fission
  however, initiating the fusion reaction is much
  more difficult
• Protons dont 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 youll be very rich
  and famous
• Dont forget to thank your high school chemistry
  teacher if this happens