NUCLEAR STABILITY Almost all atoms we’ve encountered have stable nuclei – Not radioactive Radioactive atoms are relatively rare, which is fortunate since radiation can be harmful Radioactive atoms have nuclei that are disintegrating So why are some nuclei stable and others unstable? The fundamental force that holds the nucleus together is called the Strong Nuclear Force. ̵ If the force is strong enough the nucleus will be stable One reason the force might not be strong enough is the number of protons and neutrons in the nucleus. – Not all combinations of protons and neutrons are stable. – There must be a “magic” number of each particle to ensure stability The first 20 stable nuclei follow a distinct pattern. – For elements with atomic numbers between 1 and 20 stable nuclei have almost equal numbers of protons and neutrons. – Beyond 20 protons, nuclei need increasingly more neutrons than protons to be stable. Nuclei are unstable not only if they contain too few neutrons, but also if they contain too many. Radioactive elements/isotopes are responsible for producing what we think of as radiation. There are three different forms of nuclear radiation – Alpha Particles emitted – Beta from nucleus – Gamma AKA: ionizing radiation Spontaneous emission of radiation from an atom is known as radioactivity. Alpha Particles Alpha radiation consists of a stream of high-energy alpha particles Consists of 2 protons and 2 neutrons and is identical to a helium-4 nucleus Can be represented by the symbol Alpha Particles Alpha particles do not have much penetrating power. They are able to travel only a few centimeters through air and are easily stopped by paper or clothing Not normally harmful to humans − Can be very dangerous if source is ingested Beta Particles Beta radiation consists of a stream of high-speed electrons A neutron changes into a proton and an electron – The proton remains in the nucleus and the electron is propelled out of the nucleus Beta radiation is represented by the symbol Beta Particles The mass number is zero 100 times more penetrating then alpha radiation Can damage the skin and tissues − Ionizing radiation can cause damage to DNA, which might lead to… • Apoptosis (cell death) • Mutation Gamma Rays A Gamma ray is highly energetic light, similar to x-rays Does not consist of particles Gamma radiation accompanies alpha and beta radiation Much more penetrating than either of a or b It is able to penetrate deeply into solid material, including body tissue Symbolized by: Properties of Some Radiations Alpha Beta Gamma Property Radiation Radiation Radiation Comp- Alpha Beta High-energy osition Particle Particle photon 4 0 Symbol a, 2 He b, e -1 Charge 2+ 1- 0 Mass (amu) 4 1/1837 0 Properties of Some Radiations Alpha Beta Gamma Property Radiation Radiation Radiation Common Radium- Carbon-14 source 226 Cobalt-60 Appox. 0.05 to 1 energy 5 MeV MeV 1MeV Low Moderate Very high Power (0.05mm (4mm body (penetrates body body tissue) tissue) easily) Shield- Paper, Lead, Metal foil ing clothing concrete Radioactive Decay When an atom emits one of these kinds of radiation, it is said to be decaying. An atom may undergo an alpha or beta decay. The radiation is called decay because the nucleus decomposes to form a new nucleus, called transmutation The best way to understand a decay is with a nuclear equation Alpha Decay Equations An alpha particle is a particle composed of 2 protons and 2 neutrons. With each expulsion of an alpha particle from the atom’s nucleus the atom loses 4 units of mass & 2 protons (+2 charged particle) Any change in #’s of protons changes the type of atom, this is transmutation. Beta Decay Equations If you remember a b is an e- that is expelled from an atom This electron is the result of one of the atom’s neutrons decomposing into a proton and an electron. This results in the atom having one more proton which causes it to transmutate into a different atom. Other Nuclear Decay Particles… There are 2 other types of radioactive decays observed. – The antimatter equivalent to a beta 0 particle is called a positron ( +1e ) 44 22Ti 44 21 Sc 0 1 e – The absorption of an electron by a nucleus, a.k.a. electron capture 44 22 Ti e 0 1 44 21 Sc Half-Life Every radioactive isotope has a unique rate of decay or half-life – 1 half life is the time required for ½ of nuclei of a radioactive sample to decay – After 1 half-life, ½ of the original nuclei have undergone transmutation Half lives may be as as short as a fraction of a second or as long as billions of years Half-Life – Most artificially produced radioactive isotopes have very short half-lives – Rapidly decaying isotopes do not pose long term bio-radiation hazards to med patients A simple half-life equation: n 1 amount left Original amount 2 n thenumber of half - lives Half-Lifes & Radiation Radiation Isotope Half-life emitted Carbon-14 5730 years b Potassium-40 1.25x109 b, Radon-222 3.8 days a Radium-226 1600 years a, Thorium-230 75,400yrs a, Thorium-234 24.1 days b, Uranium-235 7.0x108yrs a, Uranium-238 4.46x109yrs a Half-Life One radioactive isotope that has a long half-life is U-238, which decays through a complex series of radio- active intermediates to the eventual stable isotope of Pb-206 – It’s possible to use this method to date rocks nearly as old as the solar system Other Nuclear Reactions Radioactive decay is only one of several types of nuclear rxns. Two other rxns that involves radio- active particles are fission & fusion –These rxns deal with the interaction of nuclear particles not a decay of a nucleus Nuclear Fission In a nuclear fission reaction, a large nucleus is split into two smaller nuclei of approximately equal mass Fission rxns are used to provide what is commonly called nuclear power. -carefully controlled fission rxns In a nuclear reactor, the fission of 4.5g of U-235 will satisfy a person’s energy needs for 1 year. Fission reactors are used for a clean & efficient source of power. Nuclear Fission One fission rxn produces enough neutrons to start 3 more fission rxns, which in turn produces the neutrons needed to start 3 more rxns, and so on, in a series called a nuclear chain reaction. Nuclear Fusion In a nuclear fusion rxn, 2 small nuclei join to form a larger nucleus. Like a fission rxn, a fusion reaction converts some of the mass of the original nuclei into energy- a great deal of energy –According to the eqn: E=mc2 Nuclear Fusion Fusion rxns are hard to initiate and to control So far it takes a tremendous amount of heat to start Cold fusion is a natural research opportunity, –The goal is to harness the power of the sun Using Radiation Although radiation can be harmful & should always be handled with care; it can be used safely & is important in many procedures – Radioisotopes called tracers are used to study chem rxns and molecular structures – Also they can be used to study the inner workings of the body Using Radiation One of the reactants is labeled with a radioisotope and added to the rxn mixture After the rxn is complete, the radiation of the product is measured to determine the uptake of the tracer – Using this technique much can be learned about the reaction mechanism Using Radiation Tracers are used in agricultural research – The tracer is introduced to the substance being tested – Plants are treated with the radioactive labeled substance – Tracer measured to determine the location of the substance – Often the tracer is also monitored in animals that consume the plants, in water, and in soil. Using Radiation Radioisotopes can even be used to diagnose diseases - I-131, is used to detect thyroid problems - Tc-99 is used to detect brain tumors & liver disorders - P-32 is used to detect skin cancer Tl-207 scan of the heart Using Radiation Radiation has become a routine part of the treatment of some cancer - The fast-growing cancer cells are more susceptible to damage by high- energy radiation killing the cancer cells - Unfortunately if it isn’t used localized to the cancer cells it can kill healthy cells as well.
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