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The Nucleus • Remember that the nucleus is comprised of 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 due to different numbers of neutrons in those atoms. • 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. • These unstable isotopes are called radioisotopes • Radioactive isotopes undergo changes in the nucleus to gain a more stable arrangement. • When the nuclei undergo these changes they emit rays and particles this process is called radioactivity. • The penetrating rays and particles emitted by a radioactive source are called radiation.
Nuclear Reactions • Nuclear reactions, which account for radioactivity, differ from chemical reactions. • Chemical Reactions • Involve the gain, loss and sharing of electrons • Involve relatively small changes in energy • Nuclear Reactions • Involve changes in the nucleus • Give off very large amounts of energy • Are not effected by temperature, pressure, catalysts, presence of other elements • Nuclear reactions cannot be sped up, slowed down or turned off
Types of Radiation (Radioactive Decay) • Alpha radiation • Can penetrate up to 0.5mm • Can be blocked by a piece of paper & clothing • Beta radiation • Can penetrate up to 4 mm • Can be blocked by metal foil • Gamma radiation • Can penetrate paper, wood and the body easily • Can be blocked by several meters of concrete or several centimeters of lead • There are other types of radiation but we will not be studying these in this chapter.
Types of Radiation (Radioactive Decay) • Radio waves, microwaves, visible light, ultraviolet light, and X-rays are all forms of radiation like gamma radiation. • How does a gamma ray differ from these other types of electromagnetic radiation? • The other types of radiation are not directly produced by the • Radioactive decay of a nucleus.
Alpha Radiation: 238 92 234 90 4 2 4 2 He U Th He + Loss of an -particle (a helium nucleus) Uranium -238 Thorium -234 Alpha particle Remember a helium nucleus has 2 protons and 2 neutrons and positive charge.
Beta Radiation: 0 −1 0 −1 0 −1 14 7 14 6 e e C N or Loss of a -particle (a high energy electron) + Carbon-14 radioactive Nitrogen-14 stable Beta Particle In this nuclear reaction a neutron is converted into a proton and an electron is released which has a negative charge.
Gamma Emission: 226 88 230 90 0 0 0 0 4 2 Ra He Th + + Loss of a gamma-ray (): high-energy electromagnetic radiation that almost always accompanies the loss of a nuclear particle) Alpha particle Gamma Ray Thorium -230 Radon -226 Gamma rays have no charge or mass – this is why they can penetrate matter so easily
Positron Emission: 11 6 11 5 0 1 0 1 e C B e + Loss of a positron (a particle that has the same mass as but opposite charge than an electron) Carbon-11 Boron-11 positron In this nuclear reaction a proton is converted into a neutron and a positron is released which has a positive charge.
Radiation Summary • Alpha decay • ↓ # of protons by 2 • ↓ # of neutrons by 2 • Beta decay • ↑ # of protons and ↓ # of neutrons • Gamma Rays • Pure energy released no nuclear particles gained/lost • Positron emission • ↑ # of neutrons and ↓ # of protons
Review Section 25.1 • Q: How does an unstable nucleus release energy? A: An unstable nucleus releases energy by emitting radiation during radioactive decay • Q: What are the 4 main types of radiation? A: Alpha, Beta, Gamma & Positron Emission
Review Section 25.1 • Q: What part of an atom undergoes change during radioactive decay? A: The nucleus • Q: How is the atomic number of a nucleus changed by alpha decay? By beta decay? By gamma decay? A: In alpha decay the atomic number decreases by 2, in beta decay the atomic number increases by 1, in gamma radiation there is no change in the atomic number.
Review Section 25.1 • Q: Between alpha, beta & gamma radiation which is the most penetrating? A: Gamma radiation
Section 2 Nuclear Stability & Decay • A radioisotope undergoes changes as it emits radiation • Radioisotopes have unstable nuclei • The stability of a nucleus depends on the ratio of protons and neutrons • Atomic Number < 20 the most stable ratio is 1:1 • Atomic Number > 20 the most stable arrangements have more neutrons than protons. • But too many or too few neutrons can create an unstable nucleus
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.
Neutron-Proton Ratios • 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.
Half Life • Half-life, t1/2, is the time required for half the atoms of a radioactive nuclide to decay. • Each radioactive nuclide has its own half-life. • More-stable nuclides decay slowly and have longer half-lives. • Animation
Half Life Calculations Sample 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? Given:original mass of phosphorus-32 = 4.0 mg half-life of phosphorus-32 = 14.3 days time elapsed = 57.2 days Unknown:mass of phosphorus-32 remaining after 57.2 days
Half Life Calculations • Solution
Radioactive Series • Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation. • They undergo a series of decays until they form a stable isotope (often a isotope of lead). • Animation
Transmutation • The conversion of an atom of one element to an atom of another element is called transmutation. • Transmutation can occur by radioactive decay. Transmutation can also occur when particles bombard the nucleus of an atom.
Transmutation • The elements in the periodic table with atomic numbers above 92, the atomic number of uranium, are called the transuranium elements. • All transuranium elements undergo transmutation. • None of the transuranium elements occur in nature, and all of them are radioactive.
Transmutation Reactions • Transuranium elements are synthesized in nuclear reactors and nuclear accelerators.
Nuclear Transformations Nuclear transformations can be induced by accelerating a particle and colliding it with the nuclide.
Particle Accelerators These particle accelerators are enormous, having circular tracks with radii that are miles long.
Energy in Nuclear Reactions • There is a tremendous amount of energy stored in nuclei. • Einstein’s famous equation, E = mc2, relates directly to the calculation of this energy.
Review Section 25.2 • Q: What determines the type of decay a radioisotope will undergo? A: The neutron-to-proton ratio • Q: How much of a sample of radioisotope remains after 1 half life? After 2 half lives? A: After 1 half life 50% of the sample remains. After 2 half lives 25% remains.
Review Section 25.2 • Q: What are 2 ways transmutation can occur? A: Radioactive decay & particle bombardment of a nucleus • Q: A radioisotope has a half life of 4 days. How much of a 20.0 gram sample will be left at the end of 4 days? At the end of 8 days? A: After 4 days: 10.0 grams remain; after 8 days: 5 grams remain.
Review Section 25.2 • Q: The mass of cobalt-60 in a sample is found to have been decreased from 0.800 grams to 0.200 grams in a period of 10.5 years. Calculate the half-life of cobalt-60? A: 5.25 years
Nuclear Fission • How does one tap all that energy? • Nuclear fission is the type of reaction carried out in nuclear reactors. • When the nuclei of certain isotopes are bombarded with neutrons, they undergo fission, the splitting of a nucleus into smaller fragments. • In a chain reaction, some of the neutrons produced react with other fissionable atoms, producing more neutrons which react with still more fissionable atoms.
25.3 Nuclear Fission • Nuclear Fission
Nuclear Fission • Bombardment of the radioactive nuclide with a neutron starts the process. • Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.
Nuclear Fission This process continues in what we call a nuclear chain reaction.
Nuclear Fission If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out.
Nuclear Fission Therefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained: Critical Mass.
Nuclear Reactors In nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator.
Nuclear Reactors • The reaction is kept in check by the use of control rods. • These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass.
Nuclear Fusion • Fusion occurs when nuclei combine to produce a nucleus of greater mass. In solar fusion, hydrogen nuclei (protons) fuse to make helium nuclei and two positrons.
Nuclear Fusion • Fusion would be a superior method of generating power. • The starting materials are inexpensive and readily available • The products of the reaction are not radioactive. • The bad news is that in order to achieve fusion, the material must be in the plasma state at several million Kelvin's.
Nuclear Fusion • Tokamak apparati like the one below show promise for carrying out these reactions. • They use magnetic fields to heat the material.