Chapter 23 Nuclear Chemistry

external image atomic-bomb.jpgexternal image dn11436-1_450.jpgexternal image Uranium
rashidfaridi.wordpress.com/www.newscientist.com/www.brianbeutler.com

Nuclear Chemistry- The study of reactions involving changes in atomic nuclei.

Chemical Equations
Nuclear Equations
Bonds are broken and rearranged
Elements are converted from one to another through isotopes
Electrons are the only subatomic particle involved in the reactions
Protons, electrons, and neutrons are all used in nuclear reactions
Relatively small amounts of energy consumed or given
Massive amounts of energy consumed or given off
Reaction rates can be easily effected
Reaction rates normally go unchanged


The Basic Concept:


Due to nuclear decay known as radiation elements that are unstable will lose electrons and other particles such as protons, neutrons, and positrons. As these elements go through radioactive decay they become isotopes and then another element entirely. The other type of radioactivity is a transmutation, where an atom can capture stray neutrons from the sun, this only happens naturally in space.

The Elementary Particles:
Proton- The proton has a +1 charge and weighs one atomic mass unit. It has the atomic number 1 because it has a proton, and a mass number of one.

Neutron- The neutron carries no charge and weighs one atomic mass unit. It has the atomic number zero because it has no protons, and a mass number of one.

Electron- The electron carries a charge of -1 and has no virtually no weight. It has no atomic number zero and the mass number zero. It is denoted by β if from the nucleus of an atom through decay, but normally denoted by e.

Positron- The positron is opposite of the electron, carrying a +1 charge and no weight. It also has the atomic and weight number zero. When coming from the nucleus of an atom through decay it is denoted by a β, but normally denoted by e.

α Particle- This particle carries a +2 charge and contains two protons and two neutrons, thus having the atomic number 2 and the mass number 4.
http://phet.colorado.edu/simulations/sims.php?sim=Alpha_Decay
Use the simulation to enhance your grasp on the concept of alpha decay.
Balancing Nuclear Equations:
Unlike the more familiar chemical equation, nuclear equations look slightly different and are balanced accordingly. When balancing chemical equations isotopes decay into more isotopes and one needs to find which particle the given isotopes are decaying by.
Examples:

Example
The decay mode of gold-198 is β- therefore, Au-198 transmutes naturally into Hg-198 by giving off a beta particle.
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Example
The decay mode of Ca-37 is β+ therefore, Ca-37 transmutes naturally into K-37 by giving off a positron.
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Example
The decay mode of Fr-220 is α therefore, Fr-220 transmutes naturally into At-216 by giving off an alpha particle.

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(http://www.chemcool.com/regents/nuclearchemistry/aim3.htm)

external image Decay_chain(4n%2B2,_Uranium_series).PNGUranium Decay (Series.http://upload.wikimedia.org/wikipedia/commons/a/a1/Decay_chain(4n%2B2,_Uranium_series).PNG)

Nuclear Stability:


With nuclear stability one must first know that the nucleus of an atom takes up a very small part of the area yet contains almost all of the mass. With that said the nucleus of an atom is enormously dense, trillions times more so than even the densest element. The nucleus is held so tightly together by the attractive forces between proton-proton, proton-neutron, and neutron-neutron. Because these subatomic particles are so close together there is an attractive force between all of them. However, since protons are all positively charged they all also repel each other. This is where the stability comes in to play the nucleus will either hold together with the attractive forces or it will decay when the forces of repulsion over take the forces of attraction. This causes particles to be emitted from the nucleus and the phenomenon of radioactivity occurs.
There are a few rules nuclear stability which follow:
  • The neutron-to-proton ratio required for nuclear stability varies with atomic number. For the lighter elements (up to about 20), the ratio is close to 1:1 as indicated by both the red and blue graph segments. As the atomic number increases beyond 20, the ratio of neutrons to protons increases as indicated by the blue graph.
  • All elements beyond 83Bi are radioactive.
  • Nuclei with an even number of nucleons, protons and neutrons, are more stable than those with an odd number of nucleons.
  • The unstable region resulting from a nucleus having too many neutrons (above the blue graph) undergoes spontaneous beta decay to become more stable.
  • The unstable region resulting from a nucleus with too many protons (below the red graph) undergoes spontaneous positron decay or electron capture to become more stable. For the lighter nuclei, positron emission is favored and for the heavier nuclei, electron capture is favored.
  • There are certain numbers of protons and neutrons that produce very stable nuclei. These numbers are referred to as magic numbers and are 2, 8, 20, 28, 50, 82, and 126. This behavior is similar to the magic numbers for atoms which are 2, 10, 18, 36, 54, and 86 (noble gas configuration).
(http://www.geocities.com/junebug_sophia/nucStab.htm)

external image nucStability.gif

(http://www.geocities.com/junebug_sophia/nucStab.htm)


The previous chart plots every atomic nuclei based on protons vs. neutrons, where between the blue and the red trend line is where stable nuclei are found.
external image stabilityall.gif
**http://www.hasdeu.bz.edu.ro/softuri/fizica/mariana/Atomica/Table/lessons/11nuclear/stabilityall.gif**

Nuclear Binding Energy:

The energy required to break up a nucleus into component protons and neutrons.

Before one can find the nuclear binding energy, one must first grasp the concept that the mass of a nuclei is always less than the sum of all the particles which make it up, the nucleons. When one calculates that for example simply the mass of the nucleus of He, the calculation is two protons plus two neutrons. The sum of the two protons is found by, 1.007825 x 2 = 2.01565, and the sum of the neutrons is found to be, 1.008665 x 2 = 2.01733. When those sums are added together we find that the nucleus should weigh 4.03298 amu, when the actual weight of the He atom is 4.002602. That is a difference of .030378 amu, this inconsistency is called mass defect.

Einstein was able to find that this lost mass was converted into energy through the famous mass-energy equivalence relationship,

ΔE =(Δm)c2. Now since we know the constant, c, we can find the energy with the change in mass that we previously found. When calculated we find the energy to be 2,734,020,000,000,000 J given off. This is a tremendous amount of energy given off, and that is why nuclei are so hard to take apart because it would require that much energy. This is how one can find the binding energy of the nucleus. To find the binding energy of one nucleon one would simply divide that amount by the number of particles in the nucleus (nucleons).

Using the nuclear binding energy per nuclei one can then compare the stability of the nucleus on a standard level.


A graph of the relative stability of the nucleus vs. the mass number of the nucleus
A graph of the relative stability of the nucleus vs. the mass number of the nucleus




http://library.thinkquest.org/17940/texts/binding_energy/binding_energy.html



Natural Radioactivity:

Nuclei outside the stability belt as well as nuclei with more than 83 protons tend be unstable and emit particles from the nucleus naturally, this is called radioactivity. This makes isotopes gradually decay from one to another, parent isotope to daughter isotope. These isotopes decaying to one another forms what is called a radioactive decay series which would end in a stable isotope. Radioactive decay always follows first order kinetics in terms of rate thus, t = λN. Where t is the time passed, λ is the first order rate constant (like k), and N is the number of radioactive nuclei present. Also from a rate equation we can find that the number of radioactive nuclei at time zero and time N, with the equation external image halflif2.gif. The derivation from the given equation is outlined in your textbooks. This equation will calculate the half life of a given element which can vary anywhere from millions of years, to fractions of seconds. Radioactive decay can be used for a number of things, but is most important in dating ancient and prehistoric things such as rocks and fossils. There a various methods including Carbon-14, Uranium-238, and Potassium-40 dating. They all use the half life of the isotope and the given ratios present in the material to date the given object.


Nuclear Transmutation:

Due to our good friend Rutherford, we can also study nuclear transmutation. Transmutation is the same as natural radioactivity except that it artificial radioactivity formed in a lab. The first such case of transmutation was when Rutherford preformed an experiment in which he shot many alpha particles at a sample of nitrogen. His result was the nitrogen emitted a proton and thus decayed to oxygen-17. The main difference between natural decay and transmutation, is that transmutation requires the collision of two particles. Nitrogen-14(α,p)Oxygen-17 is the shorthand for this transmutation because it starts with nitrogen-14, is bombarded with alpha particles, then gives off a proton, and becomes oxygen-17.
With the advent of the particle accelerator it became possible to create elements with atomic numbers greater than 92, synthetically. These elements are not found in nature and are all radioactive. They are created by particle accelerators bombarding various isotopes with various particles, and creates these transuranium elements that follow:
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Some are still are yet to be synthesized and named.


Nuclear Fission:

Nuclear Fission: A heavy nucleus divides to form smaller nuclei of intermediate mass and one or more neutrons.
Since this process takes a very unstable nuclei and creates much more stable nuclei vast amounts of energy are given off by these types of reactions. One such reaction of this sort was first preformed with Uranium-235. The reaction was bombarding the U-235 with slow neutrons. After catching one of these neutrons the U-235 goes through fission and divides into 30 different elements, the most common fission product is Sr-90 and Xe-143. One can use the binding energies and subtract products from reactants to find that this fission would release 2 x 1013J. The process also emits 2.4 neutrons on average per fission of U-235. With the emissions of more neutrons nuclear chain reactions are then possible. If one was to group many isotopes of U-235 together and then set off a fission reaction by firing a neutron, the aforementioned reaction would emit more neutrons thus setting off another fission reaction and so forth. With all the fission reactions happening massive amounts of energy can be achieved, this is what fuels atomic bombs as well as nuclear reactors.

For lab on this visit http://phet.colorado.edu/simulations/sims.php?sim=Nuclear_Fission and then to check comprehension answer the questions on this lab, http://phet.colorado.edu/admin/get-contribution-file.php?contribution_file_id=1001



Nuclear Fusion:


Rather than adding nucleons to an already established nucleus the process of fusion fuses two nuclei together to form a larger nucleus. This process takes intense heat upwards sometimes of 15 million degrees Celsius. The equation for fusion reactions look like the following:
external image hfus2.gif
Since these fusions take place at such high temperatures no one has yet been able to create a fusion reactor, however it would be optimum compared to fission because of the relatively small amounts of radiation given off. As in fission fusion gives off massive amounts of energy and could be a future solution to the energy crisis. Fusion is also the process which is going on in the sun.
Other Uses for Isotopes:
Not only used for dating and reactions, isotopes have many real world applications. Isotopes allow chemists to determine shapes of molecules, study photosynthesis, and are invaluable in cutting edge medicine. The ability to identify one isotope from another is extremely important and has many applications.



Biological Effects of Radiation:

As is well documented radiation can be extremely harmful to the human body. The unit for radioactivity is the curie and the unit for intensity is the rad. One can use these measures to calculate how much radiation the body can take before it is harmful. The rad value is multiplied by the Relative Biological Effectiveness, to determine the Roentgen Equivalent for Man. Of the penetrating rays are γ, then β, and last α. The average American receives between 133-188 rem per year from various sources and it considered deadly for some to receive in the range of 500 rem. The actual damage comes from free radicals in the body which are molecular fragments with one or more unpaired electrons. High energy radiation will cause cancer, but however cancer can also be fought with specialized radiation treatments. To protect ones self from radiation use sunscreen, don't get too many x-rays, and try not to reside in areas where nuclear weapons have been set off.


Review Questions:

1. How do nuclear reactions differ from ordinary reactions?
2. Complete the following nuclear reactions:
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3. Write equations for the following nuclear reactions:
A) Radon-222 decays by alpha emission.
B) The carbon-14 isotope undergoes beta decay.
4. A radioisotope decays to give an alpha particle and Rn-222. What was the original
isotope?
a) Po-218 b) Th-224 c) Pb-220 d) Ra-226 e) none of these
5.What is the belt of stability?
6. If you ingest a sample containing Iodine-131, how much time is required for the isotope
to fall to 5.0 % of its original activity? The half-life for I-131 is 8.05 days.
7. The half-life of 98Au is 2.7 days. If you begin with 5.6 mg of this gold isotope, what mass
remains after 9.5 days?
8. Define nuclear fission, chain reaction, and critical mass.
9. What are the advantages of a fusion reactor over a fission reactor? What are the practical difficulties in operating a large scale fusion reactor?
10. How does a hydrogen bomb work?


Answers:


1. Nuclear reactions differ from ordinary chemical reactions because rather than combining or breaking apart molecules, nuclear reactions combine and break apart atoms.
2.
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4. d.
5. The belt of stability is a ratio of neutrons to electrons to protons, which can be used to calculate whether an isotope is stable or not.
6. 33.81 days
7. .336 mg
8. Nuclear fission is a type of nuclear reaction where isotopes give off particles, a chain reaction is one fission triggering other fissions do to closely packed particles, and critical mass is the minimum mass required of fissionable material required to generate a chain reaction.
9. Fusion reactors can generate more energy and produce far less radiation, this is hard to sustain however because of the difficulties in harnessing large scale quantities of plasma.
10. The hydrogen bomb uses a fusion reaction on a small scale to set off many fission chain reactions, this is also known as the thermo nuclear bomb.


AND THAT IS NUCLEAR CHEMISTRY IN A NUTSHELL
external image nucleardet2.jpghttp://utenti.lycos.it/paolaharris/Graphics/nucleardet2.jpg