Nuclear Chemistry PDF (Week 8, Lesson 2)

WEEK EIGHT; LESSON TWO NUCLEAR CHEMISTRY  LESSON OBJECTIVES By the end of these lessons, students should be able to: Explain the types of radiations and their properties Understand radioactive disintegration Balance nuclear reactions Explain half-life and radioac...

WEEK EIGHT; LESSON TWO NUCLEAR CHEMISTRY  LESSON OBJECTIVES By the end of these lessons, students should be able to: Explain the types of radiations and their properties Understand radioactive disintegration Balance nuclear reactions Explain half-life and radioactive carbon dating Understand detection and applications of radioactivity.  Types of Radiation 1. Alpha particles. Alpha particles are massive and highly charged, which means that they interact with matter most strongly of the three common emissions. As a result, they penetrate so little that a piece of paper, light clothing, or the outer layer of skin can stop a radiation from an external source. However, if ingested, an α emitter can cause grave localized internal damage through extensive ionization. 2. Beta particles and positrons. Beta particles (β-) and positrons (β+) have less charge and much less mass than a particles, so they interact less strongly with matter. Even though a given particle has less chance of causing ionization, a β- (or β+) emitter is a more destructive external source because the particles penetrate deeper. Specialized heavy clothing or a thick (0.5 cm) piece of metal is required to stop these particles. 3. Gamma rays. Neutral, massless g rays interact least with matter and, thus, penetrate most. A block of lead several inches thick is needed to stop them. Therefore, an external γ ray source is the most dangerous because the energy can ionize many layers of living tissue.  Types of Radiations When a nuclide of one element decays, it emits radiation and usually changes into a nuclide of a different element. The three natural types of radioactive emission are Alpha particles (symbolized α, 42𝛼 42𝐻𝑒 2+ ) are identical to helium-4 nuclei. Beta particles (symbolized β, β− , −10β, −10e) are high-speed electrons. (The emission of electrons from the nucleus may seem strange, but as you’ll see shortly, they result from a nuclear reaction.) Gamma rays (symbolized 𝛾, 00𝛾) are very high-energy photons. Figure 23.1. How the three types of radiation active emissions behave in an electric field  Modes of Radioactive Decay; Balancing Nuclear Equations Figure 23.1 illustrates the behavior of these emissions in an electric field: the positively charged α particles curve to a small extent toward the negative plate, the negatively charged β particles curve to a greater extent toward the positive plate (because they have lower mass), and the uncharged 𝛾 rays are not affected by the electric field. When a nuclide decays, it forms a nuclide of lower energy, and the excess energy is carried off by the emitted radiation and the recoiling nucleus. The decaying, or reactant, nuclide is called the parent; the product nuclide is called the daughter. Nuclides can decay in several ways  Modes of Radioactive Decay; Balancing Nuclear Equations Alpha (a) decay involves the loss of an α particle from a nucleus. For each a particle emitted by the parent, A decreases by 4 and Z decreases by 2 in the daughter. Every element beyond bismuth (Bi; Z=83) is radioactive and exhibits a decay, which is the most common means for a heavy, unstable nucleus to become more stable. For example, radium undergoes a decay to yield radon (Rn; Z= 86): 226 88Ra → 222 86 Rn + 4 2α Note that the A value for Ra equals the sum of the A values for Rn and α (226 = 222 + 4), and that the Z value for Ra equals the sum of the Z values for Rn and α (88 = 86 + 2).  Modes of Radioactive Decay; Balancing Nuclear Equations Beta (b) decay is a more general class of radioactive decay that includes three modes: β− , β∓ and electron capture. β− decay (or negatron emission) occurs through the ejection of a β − particle from the nucleus. This change does not involve expulsion of a β− particle that was in the − nucleus; rather, a neutron is converted into a proton, which remains in the nucleus, and a β particle, which is expelled immediately: 1 1 0 0n → 1p + −1β As always, the totals of the A and the Z values− for reactant and products are equal. Radioactive nickel-63 becomes stable copper-63 through β decay: 63 63 0 28Ni → 29Cu + −1β Another example is the β− decay of carbon-14, used in radiocarbon dating: 14 14 0 6C → 7N + −1β Note that β− decay results in a product nuclide with the same A but with Z one higher (one more proton) than in the reactant nuclide. In other words, an atom of the element with the next higher atomic number is formed.  Modes of Radioactive Decay; Balancing Nuclear Equations Positron (β+ ) emission is the emission of a β+ particle from the nucleus. A key idea of modern physics is that most fundamental particles have corresponding antiparticles with the same mass but opposite charge. The positron is the antiparticle of the electron. Positron emission occurs through a process in which a proton in the nucleus is converted into a neutron, and a positron is expelled: 1 1 0 1 𝑝 → 0 𝑛 + 1𝛽 In terms − of the effect on A and Z, positron emission has the opposite effect of β decay: the daughter has the same A but Z is one lower (one fewer proton) than the parent. Thus, an atom of the element with the next lower atomic number forms. Carbon-11, + a synthetic radioisotope, decays to a stable boron isotope through b β emission: 11 11 0 6 C → 5 B + 1β  Modes of Radioactive Decay; Balancing Nuclear Equations Electron (e-) capture (EC) occurs when the nucleus interacts with an electron in a low atomic energy level. The net effect is that a proton is transformed into a neutron: 1 0 1 1 𝑝 + −1 𝑒 + 0𝑛 (We use the symbol “e” to distinguish an orbital electron from a beta particle,β) The orbital vacancy is quickly filled by an electron that moves down from a higher energy level, and that process continues through still higher energy levels, with x-ray photons and neutrinos carrying off the energy difference in each step. Radioactive iron forms stable manganese through electron capture: 55 0 55 0 26 Fe + −1 e → 25 Mn + −1β (x − rays and neutrinos) Even though the processes are different, electron capture has the same net effect as positron emission: Z lower by 1, A unchanged.  Modes of Radioactive Decay; Balancing Nuclear Equations 3. Gamma (g) emission involves the radiation of high-energy g photons (also called γ rays) from an excited nucleus. Just as an atom in an excited electronic state reduces its energy by emitting photons, usually in the UV and visible ranges, a nucleus in an excited state lowers its energy by emitting γ photons, which are of much higher energy (much shorter wavelength) than UV photons. Many nuclear processes leave the nucleus in an excited state, so γ emission accompanies many other (mostly β) types of decay. Several g photons of different energies can be emitted from an excited nucleus as it returns to the ground state: 215 211 4 84 Po → 82 Pb + 2α (several γ emitted) Gamma emission often accompanies β− decay: 99 99 0 43 Tc → 44 Ru + −1β (several γ emitted)  Modes of Radioactive Decay; Balancing Nuclear Equations Because g rays have no mass or charge, γ emission does not change A or Z. Two gamma rays are emitted when a particle and an antiparticle annihilate each other. In the medical technique known as positron- emission tomography a positron and an electron annihilate each other (with all A and Z values shown): 0 0 0 1 β + −1 e → 2 0γ  Half-Life All radioactive nuclei decay via first-order kinetics, so the rate of decay in a particular sample is directly proportional to the number of nuclei present: Rate = kN where N is the number of radioactive nuclei and k is the rate constant. Different radioactive nuclides decay into their daughter nuclides with different rate constants. Some nuclides decay quickly (large rate constant) while others decay slowly (small rate constant). The time it takes for one-half of the parent nuclides in a radioactive sample to decay to the daughter nuclides is the half-life, ln 2 T1Τ2 = k 0.693 T1ൗ = 2 k  Radiocarbon Dating: Using Radioactivity to Measure the Age of Fossils and Artifacts Archeologists, geologists, anthropologists, and other scientists use radiocarbon dating, a technique devised in 1949 by Willard Libby at the University of Chicago, to estimate the ages of fossils and artifacts This signature results from the presence of carbon-14 (which is radioactive) in the environment. Carbon-14 is constantly formed in the upper atmosphere by the neutron bombardment of nitrogen: 14 14 0 6 C → 7 N + −1β 1.High-energy cosmic rays, consisting mainly of protons, enter the atmosphere from outer space and initiate a cascade of nuclear reactions, some of which produce neutrons that bombard ordinary N-14 atoms to form C-12: 14 1 14 1 7 N + 0 n → 6 C + 1p  Radiocarbon Dating: Using Radioactivity to Measure the Age of Fossils and Artifacts Through the competing processes of this formation and radioactive decay, the amount of C-14 in the atmosphere has remained nearly constant. 2. The C-14 atoms combine with O2, diffuse throughout the lower atmosphere, and enter the total carbon pool as gaseous 14CO2 and aqueous H14CO3-. They mix with ordinary 12CO2 and H12CO3-, reaching a constant 12C/14C ratio of about 1012/1. 3. CO2 is taken up by plants during photosynthesis, and then taken up and excreted by animals that eat the plants. Thus, the 12C/14C ratio of a living organism is the same as the ratio in the environment. 4. When an organism dies, it no longer absorbs or releases CO2, so the 12C/14C ratio steadily increases because the amount of 14C decreases as it decays: 14 14 0 6 C → 7 N + −1β The difference between the 12C/14C ratio in a dead organism and the ratio in living organisms reflects the time elapsed since the organism died.  Detection of Radioactivity The particles emitted by radioactive nuclei have a lot of energy and can therefore be readily detected. In a radiation detector, the particles are detected through their interactions with atoms or molecules. The simplest radiation detectors are pieces of photographic fi lm that become exposed when radiation passes through them. Film badge dosimeters-which consist of photographic film held in a small case that is pinned to clothing-are issued to most people working with or near radioactive substances). These badges are collected and processed (or developed) regularly as a way to monitor a person's exposure. The more exposed the film has become in a given period of time, the more radioactivity the person has been exposed to during that time.  Detection of Radioactivity Radioactivity can be instantly detected with devices such as a Geiger- Miiller counter. In this instrument (commonly referred to simply as a Geiger counter), particles emitted by radioactive nuclei pass through an argon-filled chamber. The energetic particles create a trail of ionized argon atoms. High voltage applied between a wire within the chamber and the chamber itself causes these newly formed ions to produce an electrical signal that can be displayed on a meter or turned into an audible click. Each click corresponds to a radioactive particle passing through the argon gas chamber. This clicking is the stereotypical sound most people associate with a radiation detector.  Detection of Radioactivity A second type of device commonly used to detect radiation instantly is a scintillation counter. In a scintillation counter, the radioactive emissions pass through a material (such as NaI or CsI) that emits ultraviolet or visible light in response to excitation by energetic particles. The radioactivity excites the atoms to a higher energy state. The atoms release this energy as light, which is then detected and turned into an electrical signal that can be read on a meter. Scintillation counters usually give radioactivity in units of counts per minute, which is directly related to the number of nuclear disintegrations per minute that are detected.  Assignment Write comprehensive note on Detection and Applications of Radioactivity.