Chemistry: The Central Science Chapter 21: Nuclear Chemistry A nuclear reaction involves changes in the nucleus of an atom Nuclear chemistry the study of nuclear reactions, with an emphasis in their uses in chemistry and their effects on biological systems 21.1: Radioactivity Two types of subatomic particle: proton and neutron o The particles are referred to as nucleons Mass numbers the total number of nucleons in the nucleus o Atoms with the same atomic number but different mass numbers are known as isotopes Isotopes are labeled using chemical symbols such as,, and The superscript is the mass number The subscript is the atomic number Different isotopes have different natural abundances Nuclide a nucleus with a specified number of protons and neutrons o Nuclei that are radioactive are called radionuclides Atoms containing radionuclides are called radioisotopes Nuclear Equations o Radionuclides are unstable and spontaneously emit particles and electromagnetic radiation Emission of radiation is one of the ways in which an unstable nucleus is transformed into a more stable one with less energy The emitted radiation I the carrier of the excess energy E.g. Uranium-238 Uranium-238 undergoes nuclear reaction in which helium-4 nuclei are spontaneously emitted o The helium-4 particles are known as alpha (α) particles A stream of these particles is called alpha radiation Nuclear equation: o Uranium-238 splits into an α particle and thorium-234 nucleon
o When a nucleus decomposes in such way as the uranium-238, it is said to have decayed or to have undergone radioactive decay Because an alpha particle is involved in the decay of uranium-238, the process is also called alpha decay o Mass and atomic numbers must be balanced in all nuclear equations Radioactive properties of the nucleus are independent of the chemical state of an atom In writing nuclear equations, we are not concerned with the chemical form Types of Radioactive Decay o Three most common kinds of radioactive decay: alpha (α), beta (β), and gamma (γ) radiation Alpha radiation consists of a stream of helium-4 nuclei Beta radiation consists of streams of beta (β) particles, which are highspeed electrons emitted by an unstable nucleus Represent by or o Subscript -1 represents the negative charge of the particle Beta emission is equivalent to the conversion of a neutron to a proton, thereby increasing the atomic number by 1: o The electron comes into being only when the nucleus undergoes a nuclear reaction Gamma radiation (or gamma rays) consists of high-energy photons It changes neither the atomic number nor the mass number of a nucleus Represented as or merely γ Generally, gamma rays are not shown when writing nuclear equations o Two other types of radioactive decay are positron emission and electron capture A positron is a particle that has the same mass as an electron, but an opposite charge Represent by The emission of positron has the effect of converting a proton to a neutron, thereby decreasing the atomic number of the nucleus by 1: o
21.2: Patterns of Nuclear Stability Electron capture is the capture by the nucleus of an electron from the electron cloud surrounding the nucleus Electron capture, like positron emission, has the effect of converting a proton to a neutron o Neutron-to-Proton Ratio o At close distances, a strong force of attraction, called the nuclear force, exists between nucleons Neutrons are intimately involved in this attractive force o The neutron-to-proton ratios of stable nuclei increase with increasing atomic number o The color band in the figure below is the area within which all stable nuclei are found This area is known as the belt of stability
The belt of stability ends at element 83 All nuclei with 84 or more protons are radioactive o The type of radioactive decay that a particular radionuclide undergoes depends largely on how its neutron-to-proton ratio compares to those of nearby nuclei within the belt of stability Nuclei above the belt of stability these neutron-rich nuclei can lower their ratio and move toward the belt of stability by emitting beta particle Nuclei below the belt of stability these proton-rich nuclei can increase their ratio by either positron emission or electron capture Nuclei with atomic number 84 these heavy nuclei tend to undergo alpha emission Emission of alpha particle decreases both the number of neutrons and protons by two, moving the nucleus diagonally toward the belt of stability Radioactive Series o A series of nuclear reactions that begins with an unstable nucleus and terminates with a stable one is known as radioactive series, or a nuclear disintegration series Three of such series occur in nature Begins with uranium-238, ends with lead-206 Begins with uranium-235, ends with lead-207 Begins with thorium-232 and ends with lead-208 Further Observations o Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, 126 neutrons are generally more stable than nuclei that do not contain these numbers of nucleons These numbers of protons and neutrons are called magic numbers o Nuclei with even numbers of both protons and neutrons are generally more stable than those with odd numbers of nucleons 21.3: Nuclear Transmutations A nucleus can change identity if it is struck by a neutron or by another nucleus o Nuclear reactions that are induced in this way are known as nuclear transmutation Nuclear transmutations are sometimes represented by listing, in order, the target nucleus, bombarding particle, the ejected particle, and the product nucleus
o The alpha particle, proton, and neutron are abbreviated as α, p, and n respectively Acceleration Charged Particles o Charged particles, such as alpha particles, must be moving very fast to overcome the electro static repulsion between them and the target nucleus The use of particle accelerators, bearing such names as cyclotron and synchrotron, help accelerate the nucleus The projectile particles are introduced to a vacuum chamber within the cyclotron The particles are then accelerated by making the dees (the hollow D-shaped electrodes) alternately positively and negatively charged Using Neutrons o Most synthetic isotopes used in medicine and scientific research are made using neutrons as projectiles Neutrons are neutral and are not repelled by the nucleus Transuranium Elements o Artificial transmutations have been used to produce the elements with atomic number above 92 These elements are known as the transuranium elements because they occur immediately following uranium in the periodic table 21.4: Rates of Radioactive Decay Decay of radioisotopes can be very fast or very slow Radioactive decay is a first-order kinetic process o First-order process has a characteristic half-life, which is the time required for half of any given quantity of a substance to react Radiometric Dating o Because the half-life of any particular nuclide is constant, the half-life can serve as a nuclear clock to determine the ages of different objects The method of dating objects based on their isotopes and isotope abundances is called radiometric dating o E.g. Carbon-14 has been used to determine the age of organic materials The procedure is based on the formation of carbon-14 by capture of solar neutrons in the upper atmosphere The carbon-14 is incorporated into carbon dioxide then into more complex carbon-containing molecule through photosynthesis
When animals eat the plants, the carbon-14 moolecules becomes incorporated within them o Because a living plant or animal has a constant intake of carbon compounds, it is able to maintain a ratio of carbon-14 to carbon-12 Once the organism dies, it no longer ingests carbon compound o The ratio of carbon-14 to carbon-12 decreases as a result By measuring this ratio and comparing it to that of the atmosphere, we can estimate the age of an object Calculations Based on Half-life o The first-order rate constant,, is called the decay constant o The rate at which a sample decays is called its activity, and it is often expressed as the number of disintegration observed per unit time The Becquerel (Bq) is the SI unit of expressing the rate at which nuclear disintegrations are occurring A becquerel is defined as one nuclear disintegration per second An older, but still widely used, unit of activity is the curie (Ci) A curie is defined as 3.7 10 10 disintegrations per second o As a radioactive sample decays, the amount of radiation emanating from the sample decays as well o A first-order rate law can be transformed into the following equation is the time interval of decay is the decay constant is the number of nuclei at time zero is the number remaining after the time interval o From the equation above, we can obtain the relationship between the decay constant,, and half-life, 21.6: Energy Changes in Nuclear Reactions Energies associated with nuclear reactions can be considered with the aid of Einstein s famous equation relating mass and energy o stands of energy
stands for mass stands for speed of light, 2.9979 10 8 m/s This equation states that the mass and energy of an object are proportional If a system loses mass, it loses energy If a system gains mass, it gains energy The mass changes in chemical reactions are too small to detect o Thus, it is possible to treat chemical reactions as though mass is conserved The mass changes and the associated energy changes in nuclear reactions are much greater than those in chemical reactions o E.g. The nuclei in this reaction have the following masses:, 238.003 amu;, 233.9942 amu; and, 4.0015 amu The mass change, Δm, is the total mass of the products minus the total mass of the reactants The energy change per mole associated with this reaction can be calculated using Einstein s equations: is converted to kilograms to obtain in joules Nuclear Binding Energies o Masses of nuclei are always less than the masses of the individual nucleons of which they are composed The mass difference between a nucleus and its constituent nucleons is called the mass defect o The energy required to separate a nucleus into its individual nucleons is called the nuclear binding energy The larger the binding energy, the more stable is the nucleus toward decomposition
o The binding energies per nucleon can be used to compare the stabilities of different combinations of nucleons The binding energy per nucleon at first increases in magnitude as the mass o= number increases, reach about 1.4 10-12 J for nuclei whose mass numbers are in the vicinity of iron-56 It then decreases slowly to about 1.2 10-12 J for very heavy nuclei o Fission the process in which heavy nuclei gain stability and therefore give off energy if they are fragmented into two mid-sized nuclei o Fusion The process in which very light nuclei are combined or fused together to give more massive nuclei 21.7: Nuclear Power: Fission Uranium-235, uranium-233, and plutonium-239 undergoes fission when struck by a slow-moving neutron o A heavy nucleus can split in many different ways On average, 2.4 neutrons are produced by every fission of uranium-235 o If one fission produces two neutrons, these two neutrons can cause two additional fissions and so forth The number of fissions and the energy release quickly escalate and the result is a violent explosion Reactions that multiply in this fashion are called chain reactions For a fission chain reaction to occur, the sample of fissionable material must have a certain minimum mass o The amount of fissionable material large enough to maintain the chain reaction with a constant rate of fission is called the critical mass
o A mass in excess of a critical mass is referred to as a supercritical mass Nuclear Reactors o Nuclear fission produces the energy generated by nuclear power plants o Rods composed of materials such as cadmium or boron control the fission process by absorbing neutrons These rods regulate the flux of neutrons to keep the reaction chain self-sustaining, while preventing the reactor core from overheating o Steam is used to drive a turbine connected to an electrical generator The steam must be condensed 21.8: Nuclear Power: Fusion Fusion reactions are known as thermonuclear reactions
o Fusion reaction requires high energies to overcome the repulsion between nuclei An apparatus called tokamak uses the magnetic fields to contain and to heat the reaction o Temperatures of over 100,000,000 K have been achieved in a tokamak