Chapter 21 Nuclear Chemistry

Chapter 21Nuclear Chemistry 21.1 The Nucleus

Nuclear Notation • During a nuclear reaction, an atom can gain or lose protons & neutrons so proper notation must be used to keep track. • Since altering the number of protons changes the identity of an atom, these reactions actually convert atoms into new atoms, not just new substances. • Isotopes must be indicated, as some are radioactive and can decay further. • A nucleus is often referred to as a nuclide when it is shown in nuclear or isotope notation.

Nuclear Notation • The mass number is shown as a superscript, and the atomic number is shown as a subscript below the mass number. • Remember, mass number = protons + neutrons

Nuclear Reactions • Nuclear reactions produce a very large amount of energy, as mass particles are converted into energy. • Einstein was the first scientist to discover the relationship between mass and potential energy: Where E = energy m = mass c = speed of light, 3.0 x 108 m/s E = mc2

Mass Defect • Mass defect is caused by a conversion of mass into energy when the nucleus is formed. • Mass defect is defined as the difference between the mass of an atom & the sum of the masses of its subatomic particles. • Since energy is released when the nucleus is formed, thus mass lost, that energy is called the nuclear binding energy.

Mass Defect • Nuclear reactions are the sole method of the loss of mass in a reaction. Nuclear Binding Energy E = mc2 c = speed of light (3.00 x 108 m/s)

Calculating Mass Defect • The measured mass of a Li-7 atom is 7.01600 amu. 3 protons x 1.007276 amu = 3.021828 amu 4 neutrons x 1.008665 amu = 4.03466 amu 3 electrons x 0.0005486 amu = 0.0016458 amu 7.05813 amu 7.05813 amu - 7.01600 amu = 0.04213 amu The mass of a proton The mass of a neutron The mass of an electron Calculated Mass Measured Mass Mass Defect

Calculating Nuclear Binding Energy • Convert the mass defect into energy using Einstein’s equation, E = mc2 First convert amu to kg to coincide with the units for energy: kg·m2/s2 (= J) 0.04213 amu x 1.6605 x10-27 kg = 6.99569 x 10-29 kg 1 amu E = (6.99569 x 10-29 kg) (3.0 x 108 m/s)2 = 6.29612 x 10-12 J Nuclear Binding Energy

Binding Energy per Nucleon • Binding energy per nucleon is the binding energy divided by the number of nucleons, or protons & neutrons. • The higher this value is, the more tightly the nucleons are held together. • A higher binding energy per nucleon indicates a more stable atom.

Nuclear Stability He-4 2 protons 2 neutrons N • For small atoms, the most stable neutron-proton ratio is 1:1 • As atomic number increases, this ratio change to 1.5:1 • Protons repel each other in the long range through electrostatic repulsion. • In the short range, protons attract each other through nuclear force. P P N Pb-206 124 neutrons 82 protons

Nuclear Stability • As the nucleus gets larger, the repulsive forces increase and more neutrons are needed to stabilize the nucleus. • After atomic number 83, the repulsive force is so great that there are no more stable nuclei. • Even numbers of nucleons provide the most stable nuclei, this indicates that nucleons are most stable when paired, like electrons.

Nuclear Shell Model • Nucleons exist in different energy levels, like shells, in the nucleus. • Completed nuclear energy levels are represented by the following numbers of nucleons: 2,8,20,28,50,82 & 126 • These numbers are known as magic numbers.

Nuclear Reactions • Any reaction that affects the nucleus of an atom is called a nuclear reaction. • The total number of nucleons must be the same on both sides of the equation. • When a nuclear reaction changes the identity of an atom by altering its atomic number, that is called transmutation. • See sample & practice problems on pg. 684

Chapter 21Nuclear Chemistry 21.2 Radioactive Decay

Discovering Radioactivity • Henri Becquerel did experiments with a phosphorescent uranium compound. He exposed it to sunlight then used photograph paper to capture the emitting energy. • The uranium would be placed onto the paper, then developed to show the area where the image was placed. • He accidentally discovered that these compounds spontaneously give off energy, without prior exposure to sunlight!

Phosphorescence • Some substances will appear to “glow” after being exposed to sunlight. • These substances absorb the energy from sunlight and the electrons move to higher energy states. • As the electrons return to their ground states, the emit the energy as visible light. • Some of these substances may also be radioactive…

Phosphorescent substances will only emit energy after exposure to an energy source. This involves only a change in electrons. Radioactive substances can emit energy without prior exposure to an energy source. This involves a change in the nucleus! Phosphorescence vs. Radioactivity

Radioactivity • A spontaneous emission of radiation by an unstable nucleus is called radioactivity. • Marie & Pierre Curie further investigated Becquerel’s observations and determined that for the first time, reactions involving a change in the nucleus were being correctly observed. • All scientists were awarded the Nobel Prize for Physics in 1903 for this discovery.

Radioactive Decay • Decay is the emitting of radiation by radioisotopes. These nuclei are not stable. • There are too many protons in the nucleus which causes excessive repulsion. • There are 5 different kinds of radiation: alpha () beta () positron emission electron capture gamma ()

Alpha Decay • Alpha particles consist of helium nuclei, 2 protons & 2 neutrons. • They are considered “large” particles in comparison to other forms of radiation. • These particles collide often, do not pass through many substances & can be blocked by thin material. • Whenever a radioisotope decays, or loses, 2 protons & 2 neutrons it is called alpha decay.

Chemical Equations depicting Alpha Decay • Check to see that the law of conservation of matter is upheld. The mass numbers & atomic numbers should add up to be equal on both sides of the equation

Beta Decay • Beta particles are smaller & lighter so they move faster & pass through more substances than alpha particles. • They are high energy electrons, with a -1 charge. • They can be stopped by stacked sheets of metal, heavy clothing or blocks of wood.

Chemical Equations depicting Beta Decay • The emitted electron comes from the conversion of a neutron into a proton & an electron. • Since only 1 electron is emitted, the mass number will stay the same but the atomic number will increase by one. This is called transmutation.

Positron Emission • Atoms with too many protons, an unstable nucleus, can convert a proton into a neutron and emit a positron. • A positron is a positively charged particle that has the equivalent mass of an electron.

Positron Emission Equations • Atomic number decreases by one, mass number stays the same. A Neutrino….

Electron Capture • If too many protons are present, another decay may occur where an inner shell electron is captured by the nucleus and combined with a proton to form a neutron; called electron capture.

Electron Capture Equations • Similar to positron emission, the atomic number decreases by one and mass number stays the same.

Gamma Decay • High energy electromagnetic radiation without mass or charge. • Since it is able to pass through most substances, gamma radiation can cause damage to living cells. • It is often observed in combination with other types of decay.

Predicting Decay • Alpha decay predominates in isotopes with Z > 83. For isotopes with Z < 83, decay will occur if it is outside the belt of stability. • Isotopes that are neutron-rich will undergo beta decay. • Isotopes that are neutron-poor will decay by electron capture or positron decay. • Lighter isotopes (Z < 50) are more likely to decay by positron emission. • Heavier elements are more likely to decay by electron capture.

Summary • If you analyze a nuclear reaction & observe the products, you can determine the type of reaction that took place: 1- If both the mass number & atomic number decrease, alpha decay occurred. 2- If only the atomic number increases, beta decay has occurred. 3- If neither mass number or atomic number change, gamma decay has occurred. 4- If atomic number decreases and mass number stays the same, it is positron emission if a positron is a product, and electron capture is an electron is a reactant. • Often, these kinds of decay occur in combinations and chain reactions.

Decay Series • A decay series of a chain of decay reactions. • These proceed until stable nuclei are formed. • The heaviest nuclide formed is the parent nuclide. • The other nuclides produced are called daughter nuclides.

Artificial Transmutations • Bombardment of stable nuclei with particles to artificially create larger atoms. Fermi International Accelerator Laboratory (underground particle accelerators)

Half-Life • The rates of nuclear decay are constant and spontaneous. They cannot be altered. • Half-Life is half of the amount of time it takes for a given amount of a radioactive material to undergo decay and result in stable nuclei. • By studying the state of decay, you can predict the age of a fossil or other artifact if you know the half-life. • carbon-14, uranium-238, rubidium-87 & potassium-40 are commonly used for dating objects.

Half Life • After 1 half-life, 50% of the sample remains. • After 2 half-lives, 25% of the sample remains.

Chapter 21Nuclear Chemistry 21.3 Nuclear Radiation

The decay of an atom that results in the emission of particles from the nucleus = “nuclear reaction” What is Radioactivity? 3 Types: 1. alpha, α, emission Increasing Strength Decay naturally occurs in elements above 83 2. beta, β, emission 3. gamma, γ, emission Water Pipe Leakage, Radiation Therapy, Sterilizing Medical Equipment or Packaged Food, Medical Tracer, Smoke Detectors Paper, Plastic or Steel Sheeting

Natural Sources of Radiation • Radiation from outer space • cosmic rays from the Sun • Radioactivity from naturally occurring radioisotopes in rocks at the surface • traces of radioisotopes of uranium in granite rocks • the radioactive gas Radon is formed in the process, and can build up to harmful levels in cellars • Radioactivity from naturally occurring radioisotopes deep in the Earth's core, the energy released keeps the core very hot and heats the magma in the Earth's mantle

Smoke Detectors • Alpha particles emitted from source ionize the air and provide the charge necessary to conduct current through the air. • Charges stick to the heavy smoke particles and the current drops, causing the alarm to buzz.

Detecting Radiation • Radiation cannot be seen, heard, smelled, touched or tasted. • Geiger counters are used to detect radiation by detecting charged particles that cause a clicking sound to register on the meter. • A scintillation counter measures radiation by detecting light. Photographic paper will also detect close proximity radiation.

Carbon-14 Dating • One out of every million carbon atoms is carbon-14, a radioactive isotope. Since all living things contain carbon, the amount of C-14 remains relatively stable over a lifetime. After death, no more carbon is taken in, so the amount of C-14 gradually decreases as the C-14 decays. • The half-life of carbon is relatively short, 5,730 years.

Other Dating Methods • Since carbon has a short half-life, other methods of dating are used to determine the age of objects older than 60,000 years. • Potassium-40 t1/2 = 1.25 billion years • Uranium-238 t1/2 = 4.5 billion years • Rubidium-87 t1/2 = 48 billion years

Radioisotopes as Medical Tools Radioisotopes are used as tracers to track the movement of chemicals in the body. They are also used in scanning devices to visualize organs & glands.

Environmental Tracers • Pesticides can be altered to contain a radioisotope. When they are sprayed onto a field, they can be traced by detecting the radioisotope. • Runoff into water sources, or movement by animals can be tracked to see the effect of the initial spraying.

Food Irradiation Applications • Low dose (up to 1 kGy) Inhibition of: sprouting potatoes, onions, garlic, ginger, yam Insect and parasite disinfestation: cereals, fresh fruit, dried foods Delay ripening: fresh fruit, vegetables • Medium dose (1-10 kGy) Extend shelf life: fish, strawberries, mushrooms Halt spoilage, kill pathogens: seafood, poultry, meat • High dose (10-50 Gy) Industrial sterilization: meat, poultry, seafood, prepared foods Decontamination: Spices, etc.

Chapter 22Nuclear Chemistry 22.4 Nuclear Fission & Fusion

Ba-141 N Nuclear Fission U-235 + N Kr-92 N • Fission reactions can release more energy than a nuclear decay reaction. • When additional neutrons are introduced to an atom, it disrupts the ratio of neutrons to protons which can result in an unstable nucleus. • Fission is the splitting of an unstable nucleus into 2 separate nuclei. • This was first observed & explained in 1938 by German & Austrian physicists.

Fissionable Isotopes • U-235 is fissionable, but U-238 is not. • Pu-239 is fissionable • Th-232 is fissionable only with a fast moving neutron. • Slow-moving neutrons are used for controlled fission reactions in power plants.

Chapter 21 Nuclear Chemistry