NUCLEAR CHEMISTRY

Section 22-1: The Nucleus Objectives Explain what nucleons are. Explain what a nuclide is, and describe the different ways it can be written. Define nuclear binding energy. Explain the relationship between nucleon number and stability of nuclei.

The Nucleus The nucleus is composed of nucleons Protons Neutrons A nucleus is characterized by two numbers mass number (A; total # of nucleons) atomic number (Z; number of protons) ZAE

1327Al total number of nucleons is 27 total number of protons is 13 the number of neutrons is 14 in nuclear chemistry, an atom is referred to as a nuclide.

Subatomic Particles one atomic mass unit (u) is defined as 1/12th the mass of a carbon-12 atom

Mass of 24He The actual mass is 4.00260 amu. Why the difference in mass? mass defect

Einstein’s Equation Energy and mass can be interconverted E = mc2 E- energy, m-mass, c-speed of light When protons & neutrons are packed together to form a nucleus, some of the mass is converted to energy and released. Mass is lost

Nuclear Binding Energy Nuclear Binding Energy – the energy released when a nucleus is formed from protons and neutrons. Can also be thought of as the amount of energy required to break apart the nucleus. of an existing atom. The higher the binding energy, the more tightly the nucleus is held together.

Binding Energy Curve graph peaks at A=56 the higher the BE, the more stable the nucleus mass number of 56 is maximum possible stability

How Many Neutrons? Stable nuclides have certain characteristics The number of neutrons in a nucleus can vary as we have seen Range limited by the degree of instability created by: having too many neutrons too few neutrons Stable nuclei do not decay spontaneously Unstable nuclei have a certain probability to decay

Nuclear Stability Facts 265 stable nuclides For light elements (Z  20), Z:N ratio is ~1 Example: helium-4 (2 neutrons, 2 protons) Z:N ratio increases toward 1.5 for heavy elements Example: lead-206 (124 neutrons, 82 protons) For Z > 83 (bismuth), all isotopes are radioactive This is due to repulsive forces of the protons

Nuclear Stability Facts Stable nuclei tend to have an even number of nucleons. Out of 265 stable nuclides, 159 have even numbers of both protons and neutrons. Only 4 nuclides have odd numbers of both.

The most stable nuclides are those having: 2, 8, 20, 28, 50, 82, 126 Protons, neutrons or total nucleons Examples: Sn (Z=50) has 10 isotopes; In (Z=49) & Sb (Z=51) have only 2 isotopes Pb-208 has a double magic number (126n, 82p) & is very stable “Magic numbers” of protons or neutrons which are unusually stable

Nuclear Reactions Nuclear Reaction – a reaction that affects the nucleus of an atom. Unstable nuclei undergo spontaneous changes that change their number of protons and neutrons. They also give off a large amount of energy and increase their stability in the process.

Nuclear Reactions In a nuclear reaction, the total of the atomic numbers and the total of the mass numbers must be equal on both sides of the equation. Example: 49Be + 24He 612C + 01n

49Be + 24He 612C + 01n Note that when the atomic number changes, the identity of the element changes. Transmutation – a change in the identity of a nucleus (element) as a result of a change in the number of its protons.

Sample Problem Identify the product that balances the following nuclear reaction: 84212Po ? + 24He

Sample Problem Identify the product that balances the following nuclear reaction: 84212Po 82208Pb + 24He

Classwork Problems 1-3, page 704

Homework Page 724 Problems: 31, 33 and 40

Section 22-2 Radioactive Decay

Radioactivity Objectives Define the terms radioactive decay and nuclear radiation. Describe the different types of radioactive decay. Define the term half-life, and how it relates to stability.

Radioactivity The spontaneous decay of an unstable nucleus into a more stable nucleus. Energy is released. Certain isotopes are just not stable and will spontaneously decay.

Types of Radioactive Decay All elements with 84 or more protons (Polonium) are unstable, and will undergo radioactive decay. Naturally occurring radioactive isotopes decay in four primary ways: Alpha particle emission Beta particle emission Gamma radiation emission Positron emission

Alpha Emission Alpha particle (a) – is two protons and two neutrons bound together and emitted from the nucleus. They are helium nuclei with a charge of +2. It has no electrons. Represented by the symbol: 24He or 24a Restricted to heavy elements: Ex. uranium

Alpha Emission Process which is effective to lose a lot of mass form the element. Example: 84210Po 82206Pb + 24a The atomic number decreases by two (a new element) and the mass number decreases by four.

Alpha Emission 92235U 90231Th + 24 (or 24He) Quick way for a large atom to lose a lot of nucleons

Beta Emission Beta particle – is essentially an electron that’s emitted from the nucleus: -10b In the nucleus, a neutron is converted (decayed) into a proton and an electron. 01n 11p + -10b The electron is emitted as a beta particle. Represented by the symbol: -10e or -10b A good way to decrease the number of neutrons.

Beta Emission By decreasing the number of neutrons you improve the neutron/proton ratio. Example: 53131I 54131Xe + -10b The mass number stays the same in going from I-131 to Xe-131 but the atomic number increases by 1. Loss of neutron!

Beta Emission 1940K 2040Ca + -10b Identity of atom changes

Gamma Emission Gamma rays – are high energy electromagnetic waves emitted from the nucleus. There is no mass change with gamma emission, only radiation. Usually occurs immediately following other types of decay. Not shown in a balanced nuclear reaction.

Gamma Emission An example is cobalt-60 (Co-60) which gives off a large amount of gamma radiation. Co-60 used in the radiation treatment of cancer.

Electromagnetic Radiation Electromagnetic radiation is a form of energy that can pass through empty space It is not just a particle, and it is not just a wave. It may be both.

Electromagnetic Radiation Gamma rays are similar to x-rays – high energy, short wavelength radiation.

Positron Emission Positron particle – is essentially an electron that has a positive charge. A proton can be converted into a neutron by emitting a positron 11p 01n + +10b A useful way to decrease the number of protons. Represented by the symbol: +10e or +10b

Positron Emission Example: 1938K 1838Ar + +10b Notice that the atomic number decreases by one but the mass number stays the same. Can be viewed as the opposite of beta emission.

Half-Life Half-life - the amount of time it takes for one-half of a radioactive sample to decay is called the half-life of the isotope It is given the symbol: t1/2 No two radioactive isotopes decay at the same rate

Half-Life Useful application of half-life is radioactive dating. Used to determine the age of things. Carbon-14 (t1/2 – 5715 years) dating can be used to determine the age of something that was once alive. Examples include animal and plant species. Cannot be used to determine the age of rocks.

Half-Life Radium-226 has a half-life of 1599 years. Half of a given amount of radium-226 decays in 1599 years. In another 1599 years, half of the remaining radium-226 will decay. This will continue until there is a negligible amount of radium-226 remaining.

Half-Life Decay of radium-226

Half-Life Each radioactive element has its own half-life. More stable elements decay slowly and have longer half-lives. Less stable elements decay quicker and have shorter half-lives

Radioactive Decay Rates

The time required for half of a sample to decay Half-Life The time required for half of a sample to decay

Half-Life

Half-Life Problem: Phosphorus-32 has a half-life of 14.3 days. How many milligrams of phosphorus-32 remain after 57.2 days if you start with 4.0 mg of the isotope. First determine the number of half-lives that have elapsed!

Half-Life Answer: First determine the number of half-lives that have elapsed! 57.2 days / 14.3 days = 4 half lives 4.0 mg 2.0 1.0 0.5 0.25 mg 1 2 3 4

Classwork Review HW Page 709 Problems 1, 3, 4 and 6 Page 724

Decay Series Some nuclides (particularly those Z>83) cannot attain a stable, nonradioactive nucleus by a single emission. The product of such an emission is itself radioactive and will undergo a further decay process. Heavy nuclei may undergo a whole decay series of nuclear disintegrations before reaching a nonradioactive product.

Trying To Reach Nuclear Stability Decay Series – a series of radioactive nuclides produced by successive radioactive decay until a stable nuclide is reached. The heaviest nuclide of each series is called the parent nuclide. The nuclides produced are called daughter nuclides.

The Four Known Decay Series

Artificial Transmutations Transmutation – a change in the identity of a nucleus (element) as a result of a change in the number of its protons. Artificial radioactive nuclides are radioactive nuclides not found naturally on Earth. Artificial Transmutation – bombardment of stable nuclei with charged and uncharged particles (protons, alpha particles and neutrons)

Artificial Transmutations Great quantities of energy are required to bombard nuclei with these particles. These reactions are usually done by accelerating the particles in a high magnetic or electrical field. The instrument used is a particle accelerator.

Artificial Transmutations Used to produce all the elements in the periodic table with more than 92 protons in their nucleus. Also used to produce technetium (43 protons), astatine (85), francium (87) and promethium (61).

Examples of elements created by artificial transmutation

Homework Page 724 Problems 36 - 39

Section 22-4 Nuclear Fission and Nuclear Fusion

Objectives Define the terms nuclear fission, chain reaction and nuclear fusion. Explain how a fission reaction is used to generate power. Discuss fusion reactions.

Nuclear Fission Nuclear Fission – a very heavy nucleus splits into more stable nuclei of intermediate mass. This process releases an enormous amount of energy. Can occur spontaneously or when nuclei are bombarded with particles.

Nuclear Fission Example: Uranium-235 When uranium-235 is bombarded with neutrons, the uranium nuclei becomes unstable. The nucleus splits into medium-mass parts with the emission of more neutrons. The mass of the products is less than the mass of the reactants. The missing mass is converted to energy.

Example: Uranium-235 92235U + 01n 56142Ba + 3691Kr + 3 01n The product are barium, krypton and 3 more neutrons. The reaction is balanced.

Nuclear Fission When fission of an atom bombarded by neutrons produces more neutrons, a chain reaction can occur. Chain Reaction – a reaction in which material that starts the reaction is also one of the products and can start another reaction. In the last slide neutrons are generated from the initial reaction and can start a second reaction.

The chain reaction continues until all of the uranium-235 atoms have split or until the neutrons fail to strike uranium-235 nuclei. Critical Mass - the minimum amount of nuclide that provides the number of neutrons needed to sustain a chain reaction. Atomic Bombs – uncontrolled chain reactions. Nuclear Reactors – controlled chain reactions.

Nuclear Fission Only two fissionable isotopes are used during nuclear reactions. Uranium-235 and plutonium-239 Uranium-238 (the more abundant isotope) only produces 1 neutron. This will not sustain a chain reaction. 92238U + 01n 56145Ba + 3693Kr + 1 01n

Atomic Bombs In an atomic bomb, two pieces of a fissionable isotope are kept apart. Each piece by itself is not going to explode. When time comes for the bomb to explode, conventional explosives force the two pieces together to cause a critical mass. The resulting chain reaction is uncontrolled, releasing a tremendous amount of energy.

Nuclear Power Plants The secret to controlling a chain reaction is to control the neutrons. If the neutrons can be controlled, then the energy produced can be controlled. That is what takes place with nuclear power plants.

Nuclear Power Plants In many respects, a nuclear power plant is similar to a conventional power plant (coal, oil, natural gas). In these power plants the fuel is burned and the heat produced is used to boil water, which in turn is used to make steam. The steam is then used to turn a turbine that’s attached to a generator that produces electricity.

Nuclear Power Plants In a nuclear power plant the heat is produced through a fission reaction. The fissionable isotope (uranium-235) is contained in fuel rods in the reactor core. Control rods made of boron or cadmium (not fissionable) are also in the reactor core.

Nuclear Power Plants The control rods act like neutron sponges to control the rate of radioactive decay of the uranium-235. Operators can stop a chain reaction by pushing all the control rods into the reactor core, where they absorb all the neutrons. Operators can pull out the control rods slowly to produce the desired amount of heat.

Nuclear Power Plants Water is circulated through the reactor core, and the heat generated by the fission reaction is absorbed The water flows to a steam generator where steam is produced. The steam then is used to turn the turbine that generates electricity. The water is recycled in a closed system.

Nuclear Reactor

Nuclear Fusion Nuclear Fusion – light-mass nuclei combine to form a heavier, more stable nucleus. Nuclear fusion is essentially the opposite of nuclear fission. Nuclear fusion releases more energy than nuclear fission.

Nuclear Fusion Nuclear fusion reactions are what power the sun for light and heat. 4 11H 24He + 2 10b

Nuclear Fusion Uncontrolled fusion reactions of hydrogen are the source of energy for the hydrogen bomb. A fission reaction is used to provide the heat and pressure necessary for the fusion reaction. A hydrogen bomb is about 1,000 times more powerful than an atomic bomb.

Nuclear Fusion Nuclear fusion: the goal of scientists has been to control a fusion reaction. Fusion reactions can provide an unlimited supply of energy that has no nuclear waste. The product from a fusion reaction is helium, already found in our atmosphere.

Nuclear Fusion Three major obstacles need to be solved: Temperature - The estimated temperature required to start the reaction is 40,000,000K. Time – nuclei must be held together long enough. Containment – At the required temperature all known material would vaporize.

Classwork Distinguish between nuclear fission and fusion Define chain reaction Explain how fusion can be a good source of energy

Review Classwork Page 724 Problems: 36 - 39

End of Chapter

Radiation Energetics Alpha Particles Beta Particles relatively heavy and doubly charged lose energy quickly in matter Beta Particles much smaller and singly charged interact more slowly with matter Gamma Rays & X-rays high energy more lengthy interaction with matter

Hazards of Radiation Types Alpha Emissions easily shielded considered hazardous if alpha emitting material is ingested or inhaled Beta Emissions shielded by thin layers of material considered hazardous if a beta emitter is ingested or inhaled Gamma Emissions need dense material for shielding considered hazardous when external to the body

The Radon Story

Radon-222 Originates from U-238 which occurs naturally in most types of granite Radon-222 has a half-life of 3.825 days It decays via alpha emissions This isotope is a particular problem because it is a gas which can leave the surrounding rock and enter buildings with the atmospheric air