Nuclear Chemistry

Chemical ReactionsNuclear Reactions - Occur when bonds are broken or formed -Occur when the nucleus emits particles or rays -Atoms remain unchanged, just rearranged -Atoms are often converted into atoms of another element - Involve only valence electrons - May involve protons, neutrons, or electrons - Small energy changes may occur - Very large energy changes may occur Chemical Reactions vs. Nuclear Reactions

The Nucleus Remember that the nucleus is comprised of the two nucleons, protons and neutrons. The number of protons is the atomic number. The number of protons and neutrons together is effectively the mass of the atom.

Isotopes Not all atoms of the same element have the same mass because of different numbers of neutrons in those atoms. For example, there are three naturally occurring isotopes of uranium: –Uranium-234 –Uranium-235 –Uranium-238

Radioactivity It is not uncommon for some isotopes of an element to be unstable, or radioactive. We refer to these as radioisotopes. There are several ways a radioactive nucleus can decay into a more stable nucleus.

Types of Radioactive Decay Alpha Decay Loss of an  -particle (a helium nucleus) He 4242 U  Th He

Types of Radioactive Decay Beta Decay Loss of a  -particle (a high energy electron)  0 −1 e 0 −1 or I Xe  + e 0 −1

Types of Radioactive Decay Gamma Emission Loss of a  -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle)  0000

Penetrating Ability

Types of Radioactive Decay Positron Emission Loss of a positron (a particle that has the same mass as but opposite charge than an electron) e 0101 C 11 6  B e 0101

Types of Radioactive Decay Electron Capture Addition of an electron to a proton in the nucleus –As a result, a proton is transformed into a neutron. C e 0 −1  B 11 5

Balancing Nuclear Reactions In the reactants (starting materials – on the left side of an equation) and products (final products – on the right side of an equation) Atomic numbers must balance and Mass numbers must balance Use a particle or isotope to fill in the missing protons and neutrons

Nuclear Reactions Alpha decay example

Nuclear Reactions Beta decay example

Other Types of Nuclear Reactions Positron ( 0 +1  ): a positive electron Electron capture: Electron capture: the capture of an electron 207

Learning Check What radioactive isotope is produced in the following bombardment of boron? 10 B + 4 He ? + 1 n 5 2 0

Radioactive Decay Radioactive elements are unstable. They decay, or change into different elements over time. Here are some facts to remember: The half-life of an element is the time it takes for half of the material you started with to decay. Remember, it doesn’t matter how much you start with. After 1 half-life, half of it will have decayed. Each element has its own half-life Each element decays into a new element C 14 decays into N 14 while U 238 decays into Pb 206 (lead) The half-life of each element is constant. It’s like a clock keeping perfect time. Now let’s see how we can use half-life to determine the age of a rock or other artifact.

The grid below represents a quantity of C 14. Each time you click, one half-life goes by. Try it! C 14 – blue N 14 - red As we begin notice that no time has gone by and that 100% of the material is C 14 Half lives % C 14 %N 14 Ratio of C 14 to N %0%no ratio

The grid below represents a quantity of C 14. Each time you click, one half-life goes by. Try it! C 14 – blue N 14 - red Half lives % C 14 %N 14 Ratio of C 14 to N %0%no ratio 150% 1:1 After 1 half-life (5700 years), 50% of the C 14 has decayed into N 14. The ratio of C 14 to N 14 is 1:1. There are equal amounts of the 2 elements.

The grid below represents a quantity of C 14. Each time you click, one half-life goes by. Try it! C 14 – blue N 14 - red Half lives % C 14 %N 14 Ratio of C 14 to N %0%no ratio 150% 1:1 225%75%1:3 Now 2 half-lives have gone by for a total of 11,400 years. Half of the C 14 that was present at the end of half-life #1 has now decayed to N 14. Notice the C:N ratio. It will be useful later.

The grid below represents a quantity of C 14. Each time you click, one half-life goes by. Try it! C 14 – blue N 14 - red Half lives % C 14 %N 14 Ratio of C 14 to N %0%no ratio 150% 1:1 225%75%1: %87.5%1:7 After 3 half-lives (17,100 years) only 12.5% of the original C 14 remains. For each half-life period half of the material present decays. And again, notice the ratio, 1:7

Practice question If the half-life of an element is 10 days, how much of a 80 gram sample will remain after 50 days?

Practice question 2 If you start with a 200 gram sample, and 25 grams remains after 75 years, what was the half-life of the sample?

Test question may involve graphs like this one. The most common questions are: "What is the half-life of this element?" Just remember that at the end of one half-life, 50% of the element will remain. Find 50% on the vertical axis, Follow the blue line over to the red curve and drop straight down to find the answer: The half-life of this element is 1 million years.

Neutron-Proton Ratios Any element with more than one proton (i.e., anything but hydrogen) will have repulsions between the protons in the nucleus. A strong nuclear force helps keep the nucleus from flying apart. Neutrons play a key role stabilizing the nucleus. Therefore, the ratio of neutrons to protons is an important factor.

Neutron-Proton Ratios For smaller nuclei (Z  20) stable nuclei have a neutron-to-proton ratio close to 1:1.

Neutron-Proton Ratios As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.

Stable Nuclei The shaded region in the figure shows what nuclides would be stable, the so- called belt of stability.

Stable Nuclei Nuclei above this belt have too many neutrons. They tend to decay by emitting beta particles.

Stable Nuclei Nuclei below the belt have too many protons. They tend to become more stable by positron emission or electron capture.

Stable Nuclei There are no stable nuclei with an atomic number greater than 83. These nuclei tend to decay by alpha emission.

Practice Questions: Predict the type of decay of carbon-14 and plutonium-239.