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Nuclear Chemistry Types of Nuclear Radiation There are three primary types of radiation: Alpha (helium nucleus-‐two protons & two neutrons) Beta (an electron) Gamma (wave of energy) See Table O (&N) in the Chemistry Reference Tables.
Summary: Alpha particles are positively charged and have low penetrating power (relatively large mass: 4amu). Beta particles are negatively charged and have medium penetrating power (relatively low mass: 1/2000amu). Gamma rays have no charge and have high penetrating power (& are considered massless).
Why does nuclear radiation happen? Radiation is a means to help stabilize the nucleus. Radioisotopes are isotopes that have an unstable nucleus and emit radiation in order to become more stable. For example, carbon-‐12 is stable, but carbon-‐13 is considered a radioisotope. There are two reasons why a nucleus may be unstable: 1. The neutron/proton ratio (N/Z) is not correct. 2. The nucleus is too large. Any element that has more than 83 protons (Z > 83) is unstable and considered radioactive.
The Band of Stability
Notice that for smaller atoms (Z>20) the nucleus is stable when the neutron proton ratio (N/Z) is approximately 1:1. Larger nuclei need more neutrons to have stability. There are no stable nuclei beyond 83 protons.
How do radioisotopes emit radioactivity? Radioisotopes go through “nuclear decay” in order to stabilize themselves. This is also called nuclear transmutation, where an element is naturally changed into another element. There are various types of this decay. Alpha Decay Beta Decay Positron Decay
Alpha Decay
“Reactants” are on the left; “Products” are on the right. Larger nuclei tend to go through alpha decay because it means it gets to lose mass. Remember that nuclei that have over 83 protons cannot be stable. This means that large nuclei tend to decay multiple times in order to reduce their mass.
Beta Decay
A neutron changes into a proton and an electron is emitted.
Notice that you may write the beta particle two different ways. Beta decay is often used for smaller atoms in order to alter their neutron/proton ratio (N/Z), but can also be utilized by larger nuclei.
Positron Decay (or Beta-‐positive Decay)
A proton transforms into a neutron and releases a positron (a positive electron).
This type of decay helps the nucleus to find a proper N/Z ratio. Summary -‐There are two reasons why nuclei are unstable: 1. N/Z ratio 2. Mass -‐Alpha decay reduces the mass of the atom. -‐Beta and positron decay alter the N/Z ratio.
Types of Nuclear Transmutation 1) Natural Transmutation 2) Artificial Transmutation Natural Transmutation When a single nucleus is unstable and changes (or mutates) into another element. -‐All Natural Transmutations have only ONE REACTANT.
Example: Nuclear Decay.
Reactant à Products
Artificial Transmutation When a particle collides with a nucleus to produce other elements. Artificial transmutation can be accomplished through the use of particle accelerators that strike elements with alpha particles, deuterons, or small nuclei. With this process, some of the protons from the bombarding particles are lodged in the target nucleus, promoting the transmutation into a different element. In a “traditional” nuclear reactor, the target nucleus is struck with neutrons, resulting in fission or breaking of large nuclei.
-‐For this to happen, the original nucleus does not have to be radioactive.
-‐All Artificial Transmutations have TWO REACTANTS.
Reactant + Reactant à Products
Other examples of artificial transmutation include:
Fusion: When two nuclei combine (or fuse) together to make a larger single nucleus.
Fission: When a small particle collides to break down a large
nucleus into multiple smaller nuclei.
Balancing Nuclear Equations
Summary: The Law of Conservation of Matter says that matter cannot be created or destroyed. 1) Balance the Mass Numbers. 2) Balance the Atomic Numbers. 3) Find the element that may be missing using its Atomic Number.
Before balancing, explain if the reactions are Natural or Artificial Transmutation.
Problems: 1) Write the balanced nuclear equation for the decay of 220Fr. Include all mass numbers and atomic numbers. (Don’t know how 220Fr decays? Use Table N.) 2) Write the balanced nuclear equation for potassium-‐42. Include all mass numbers and atomic numbers. 3) Uranium-‐238 decays 14 times before it becomes a stable isotope. Write the series of decay of uranium-‐238: For each step, determine the balanced nuclear equation. 1) alpha 2) beta 3) beta 4) alpha 5) alpha 6) alpha 7) alpha
8) alpha 9) beta 10) beta 11) alpha 12) beta 13) beta 14) alpha
What is the stable isotope at the end of this decay series?
Half-‐life of a Radioisotope Radioisotopes decay at a certain rate (or speed). Half-‐life is the time that it takes for half of the nuclei of a sample to decay or transform into another element. Half-‐life is always the same, regardless of quantity, temperature or other external conditions.
Remember that the original, unstable nucleus (parent) does not disappear, rather it transforms into another element (daughter).
If a radioisotope decays quickly, then it will have a SHORT half-‐life. And the opposite is true: If a radioisotope has a LONG half-‐life, then it means that it decays slowly. Problem #1 Analyze the graph and determine the half-‐life of bismuth.
Problem #2 a. Determine the half-‐life of this radioisotope. b. Using Table N, determine the identity of the radioisotope.
Equation for Half-‐life
Problem #1: The half-‐life of Zn-‐71 is 2.4 minutes. If one had 100.0 g at the beginning, how many grams would be left after 7.2 minutes has elapsed? Problem #2: Os-‐182 has a half-‐life of 21.5 hours. How many grams of a 10.0 gram sample would have decayed after exactly four half-‐lives? Problem #3: After 24.0 days, 2.00 milligrams of an original 128.0 milligram sample remain. What is the half-‐life of the sample?
How do we measure radiation? The Geiger Counter
In order to create electrical current, there must be free-‐flowing ions (solutions of ions or ionized gas) or free flowing electrons (metals).
Units of a Geiger Counter CPM (counts per minute) is a measure of radioactivity, a unit of measurement for a Geiger counter. Technically, “It is the number of atoms in a given quantity of radioactive material that are detected to have decayed in one minute.” 1,200 CPM on the meter (for Cs137) is about 1 mR/hr (milliRad per hour). 120 CPM on the meter (for Cs137) is about 1 uSv/hr (microSievert per hour) Average annual human exposure to radiation (U.S.) 600 milliRem (mRem) 6 milliSievert (mSv) Radiation dose for increase cancer risk of 1 in a 1,000 1,250 milliRem (mRem) 12.5 milliSievert (mSv) Earliest onset of radiation sickness 75,000 milliRem (mRem) 750 milliSievert (mSv) Onset of radiation poisoning 300,000 milliRem (mRem) 3,000 milliSievert (mSv) Expected 50% death from radiation 400,000 milliRem (mRem) 4,000 milliSievert (mSv)
Days to receive chronic dose for increase cancer risk of 1 in a 1,000 432 (at 100 CPM) 86 (at 500 CPM) 28 (at 1,500 CPM) 4 (at 10,000 CPM) Days for earliest onset of radiation sickness 25,937 (at 100 CPM) 5,187 (at 500 CPM) 1,729 (at 1,500 CPM) 259 (at 10,000 CPM) All food sources expose a person to around 40 millirems per year on average. Many foods are naturally radioactive, and bananas are particularly so, due to the radioactive potassium-‐40 they contain. The equivalent dose for 365 bananas (one per day for a year) is 3.6 millirems (36 μSv). Other foods that have above-‐average levels are potatoes, kidney beans, nuts (especially brazil nuts), and sunflower seeds. Ways to limit radiation exposure: 1. Time (limit exposure time) 2. Distance (intensity decreases sharply according to the inverse-‐square-‐law) 3. Shielding (alpha: nearly anything… a sheet of paper will stop it – danger of breathing it) (beta: wood, water, plastic-‐acrylic, aluminum) (gamma: water, concrete, lead)
Nuclear Energy Fission and Fusion utilize the exception of the Law of Conservation of Matter (Later, called the Law of Conservation of Energy, which states that mass or energy cannot be created or destroyed). In Fission and Fusion reactions, the total mass of the products is LESS than the total nuclear mass of the reactants. This means that the small amount of “lost” matter was actually converted into energy. Einstein expressed this relationship as: E=mc2 where E is energy, m is mass, and c is the speed of light (3.00x108 m/s). Because the speed of light is such a large number, it is easy to see that a small amount of mass (a fraction of an amu) could produce an extremely large amount of energy. These small losses of mass are called “mass defect”. Nuclear Reactions vs. Chemical Reactions 1.0 gram of nuclear 1.0 gram of chemical reactant (fission): reactant (combustion): 9.00x1013J 5.56x104 Nuclear reactions give off over a billion times more energy than chemical reactions.
Nuclear Fission Nuclear Fission occurs when a large nucleus collides with a particle and breaks down into two (or more) smaller nuclei and at least two neutrons.
Since the reaction creates more neutrons, this creates a chain reaction:
Nuclear Fission Products of Nuclear Fission:
Nuclear Fission The speed of the chain reaction: The released neutron travels at speeds of about 10 million meters per second. A critical mass of uranium is about the size of a baseball (0.1 meters). The time, t, the neutron would take to cross the sphere is:
t = 0.1 meters
1 x 10 7 meters/second
t =
1 x 10 -‐8 seconds
The complete process of a bomb explosion is about 80 times this number, or about a microsecond.
Nuclear Fission Critical Mass is the amount of material needed to create a self-‐sustaining nuclear fission reaction.
This type of Uranium must be U-‐235, which is a rare isotope with approximately 0.7% abundance. In order to create critical mass, the uranium in the fuel should be from 3.5-‐5.0% U-‐235. This is complex enrichment process includes: 1) Turning the uranium into gas and converting it to uranium hexafluoride. 2) Condensing it and then using a centrifuge to separate the isotope.
Nuclear Fission Reactor
Nuclear Fission: Fuel Rods
Nuclear Fission: Control Rods
Control rods are often made of isotopes of boron and cadmium. These element readily absorb neutrons.
Nuclear Fission: Cooling Towers
Nuclear Fusion: Fusion reactions involve the combining of light nuclei to form heavier ones. Nuclei are positively charged, which means that if they get close to each other, they will repel. So the only way for fusion to occur is to make atoms move extraordinarily fast (plasma). The kinetic energy of the atoms overcomes the electrostatic force. An example of fusion is the sun (or other stars).
The sun reaches 5,600°C on the surface and up to 15,000,000°C in its core. The minimum temperature for nuclear fusion is approximately 13,000,000°C. This is the temperature where hydrogen will be plasma. This is the sun’s process of fusing hydrogen:
