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Michael Baker

Nuclear Chemistry Chapter 18 Synthesis of the Elements……the thoughts of some…. In the beginning, there was no beginning. Time had no context. Matter had yet to form. And then a singularity of infinitely dense energy began to expand. After one second, the temperature had cooled to about 10 billion K, at which point protons, neutrons and electrons coalesced into being. In the next three minutes, most of the matter of the universe was formed, as H-1, H-2, He-3, He-4, Be-7 and Li-7. At this point, the temperatures had cooled and nuclear fusion was no longer possible. As the mass of this primordial universe expanded, it did so unevenly, and gravitational processes began to form densely packed masses, which we know as stars. The density of these stars was such that nuclear fusion was once again possible. As an example, the hydrogen gas at the core of our own sun burns at a temperature of 15 million Celsius and a pressure 200 billion times greater than earth’s atmosphere. The density of gaseous hydrogen at the sun is greater than the density of uranium on earth. Because hydrogen made up the huge majority of all the matter, these stars were initially powered by hydrogen fusion reactions. 1 H + H → H + e + energy 1 1 1 1 11H + H 1 He + e1ergy 32He + He2→ He + 22H + energy1 As the hydrogen fuel became depleted, helium fusion became significant, producing larger atoms. And so elements were formed. 2He + He 2 Be + en4rgy 42He + Be4→ 126C + energy Why is energy released in these reactions? Einstein’s famous equation E = mc relates matter and energy. Nuclear reactions involve energies millions of times greater that those released in chemical reactions, and allow predicted change in mass to be experimentally determined. As the size of the new atoms continued to increase, further helium fusion reactions became less likely, because the repulsive forces that must be overcome to bring the two nuclei together became much greater. The mass of the star limits the density which gravitational attraction can create. Many stars were unable to generate elements larger than carbon – 12. The limit to this synthesis pattern in stars is Fe – 56, for beyond this all the fusion reactions are endothermic. Exceptionally massive stars can generate heavy elements so quickly that the increasing gravitational attraction accelerates out of control, resulting in a cataclysmic explosion known as a supernova. The huge energies involved are sufficient to drive the endothermic fusion reactions, and all elements heavier than iron were generated this way, and dispersed throughout the universe. 18.1 Nuclear Stability and Radioactive Decay Many nuclei are radioactive: that is they decompose, forming another nucleus and producing one or more particles. An example is carbon – 14, which decays as follows: 16C → 17N + 0-1 0 where -1 represents an electron, which is called a beta particle, or ∃particle, in nuclear terminology. The stability of the nucleus must be considered from both kinetic and thermodynamic points of view. Nuclei that are kinetically unstable decompose into more stable nuclei by radioactive decay. The thermodynamic stability of the nucleus involves the comparison of the energy of the nucleus to that of its component nucleons. When a system gains or loses energy, it also gains or loses a q2antity of mass, given by the relationship E = mc . For a nuclide to be radioactive and decay spontaneously, the mass of the products of decay must be less than the mass of the reactants. The mass of the nucleus of an atom is always less than the sum of the masses of the nucleons that compose the nucleus. The difference between observed mass of a nucleus and the sum of the masses of the nucleons is called the mass defect. The energy equivalent of the mass defect is referred to as the nuclear binding energy of the nucleus, the energy required to break a nucleus apart into its nucleons. The nuclear strong force is the attractive force between nucleons. It is very strong but very short-range. A nucleus is stable if it can’t be transmuted into a nucleus of another element without adding energy from outside the nucleus. Nucleons in a nucleus are paired and exist in different energy levels, similar to electron orbitals. Nuclear reactions are different from chemical reactions in six ways. Nuclear Reactions Chemical Reactions 1. Involve protons and neutrons inside nucleus Involve electrons outside nucleus 2. Elements are transmuted into other elements Both reactants and products contain the same number of each kind of atom. 3. Isotopes react differently. Isotopes react similarly. 4. Independent of state of chemical combination Depend on state of chemical combination. 8 9 3 5. Energy changes of order of 10 – 10 kJ/mol Energies of order or 10 – 10 kJ/mol. 6. Mass changes are detectable. Mass products = mass reactants. Isotopes: atoms of an element that have the same number of protons but a different number of neutrons. One way of representing isotopes is to place the mass number of the isotope after the name or symbol of the element. Two common isotopes of chlorine are represented as chlorine – 35 and chlorine – 37 ( or Cl-35 and Cl – 37) . In 1896 Antoine-Henri Becquerel placed uranium salts on several glass plates coated with light-sensitive material. Even though the plates had previously been wrapped in black paper, the light-sensitive material darkened. Since no light could have penetrated the paper, Becquerel deduced that uranium must have emitted rays capable of penetrating the paper and darkening the light-sensitive surface of the plates. Becquerel called the production of these kinds of rays radioactivity. In later experiments, Ernest Rutherford showed that two different types of rays were produced by radioactive materials. One type was easily stopped by any solid material, such as paper. Rutherford called these alpha -α - rays. The other rays, which were one hundred times more penetrating than alpha rays, he called beta - β - rays. Subsequently a third type of ray called a gamma -( γ- ray was identified by Paul Villard. Gamma rays proved to be even more penetrating than beta ray. Homework: Read Sections 18.1, 18.3, 18.4, 18.6 and 18.7 of Text Radioactivity: Particles and Radiation You must remember these symbols. They will not be provided for you. Particles are matter (they have mass and occupy space) while radiation is electromagnetic radiation (energy). Particles Particle Greek Symbol Chemical Symbol (if there is one) Alpha particle α 4 He 2 (Helium nucleus) 0 Beta particle β -1 (electron) Positron 0 (positive electrons) 1e (Positrons are a form of anti-matter.) Neutron 10n Proton 11 or H1 (Hydrogen nucleus) 2 2 Deuterium 1H or D1 (heavy hydrogen) Remember: 1 p + 0 e → n1 or 1 n → p + 0 e 1 -1 0 0 1 -1 (a proton combines with an electron → a neutron)(a neutron decomposes → a proton + an electron) Radiation Type of Radiation Greek Symbol Chemical symbol (if there is one) 0 Gamma Radiation γ 0 (High energy electromagnetic radiation) x – rays (High energy electromagnetic x – rays radiation that is not quite as high energy as γ) Radioactive Decay Processes Alpha Decay Nuclear reactions, like chemical reactions, are described by equations. Let’s consider the alpha decay of uranium-238. 238 234 4 92U → 90Th + 2He Nuclear equations show the changes that take place in the nucleus. The reactants are written to the left of the arrow and the products to the right. Nuclei are represented by nuclide symbols; the chemical form of the reactants and products is not shown because a nuclide undergoes the same nuclear reaction no matter what its state of chemical combination (element, compound or dissolved ion). The reaction above show Uranium-238 decaying to give thorium-234 and an alpha particle (helium nucleus). A radioactive decay process characterized by the expulsion from the nucleus of a He particle is+ alpha decay. This is commonly accompanied by the emission of energy as ( rays. Gamma rays are an extremely energetic component of the electromagnetic spectrum, even more penetrating than X-rays. The term nucleon is used to refer to both protons and neutrons in the nucleus. The equation for a nuclear reaction must show the same number of nucleons and the same nuclear charge on both sides. The sum of the mass numbers of the products must equal the sum of the mass numbers of the reactants; the sum of the nuclear charges of the products must equal the sum of the nuclear charges of the reactants. Beta decay Beta rays are electrons; electrons have a mass number of zero (because the mass of an electron is only 1/1850 of the mass of a proton or neutron) and a charge of –1. The equation for the decay of thorium – 234 can be written either 2390Th  2391Pa + °-1+ γ or 2390Th → 2391Pa + β- During beta decay, a neutron in the nucleus splits to form a proton and an electron. The electron is ejected from the nucleus as a β particle and the proton remains in the nucleus. As a result, the atomic number increases and the mass number remains the same. So 234 Thorium has been transmuted to 234 Protactinium. 14 C dating invol
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