Chemistry, The Central Science, 10th edition Theodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. Bursten Chapter 21Nuclear Chemistry John D. Bookstaver St. Charles Community College St. Peters, MO 2006, Prentice Hall, Inc.
January 28 • Nuclear chemistry • HW • 1,11, 13, 29, 33, 35, 41 for tomorrow
DO NOWREACTION OF THE DAY • Hydrochloric acid is added to a solution of lithium sulfide. • Write the molecular equation and then the net ionic equation
The Nucleus • Remember that the nucleus is comprised of the two nucleons, 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 nuclides of an element to be unstable, or radioactive. • We refer to these as radionuclides. • There are several ways radionuclides can decay into a different nuclide.
Nuclear Reactions • The chemical properties of the nucleus are independent of the state of chemical combination of the atom. • In writing nuclear equations we are not concerned with the chemical form of the atom in which the nucleus resides. • It makes no difference if the atom is as an element or a compound. • Mass and charges MUST BE BALANCED!!!
238 92 234 90 4 2 4 2 He U Th He + Alpha Decay: Loss of an -particle (a helium nucleus)
Alpha Decay • Mass changes by 4 • The remaining fragment has 2 less protons • Alpha radiation is the less penetrating of all the nuclear radiation (it is the most massive one!)
131 53 131 54 0 −1 0 −1 0 −1 e I Xe e + or Beta Decay: Loss of a -particle (a high energy electron)
Beta Decay • Involves the conversion of a neutron in the nucleus into a proton and an electron. • Beta radiation has high energies, can travel up to 300 cm in air. • Can penetrate the skin
Beta decay • Write the reaction of decay for C-14
0 0 Gamma Emission: Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle)
11 6 11 5 0 1 0 1 e + C B e + Positron Emission: Loss of a positron ( particle with same mass, but opposite charge than an electron)
Positron emission • Involves the conversion of a proton to a neutron emitting a positron. • The atomic number decreases by one, mass number remains the same.
0 −1 1 1 1 0 p e n + Electron Capture (K-Capture) Capture by the nucleus of an electron from the electron cloud surrounding the nucleus. • As a result, a proton is transformed into a neutron.
81 37 81 36 Rb Kr Electron capture • Rb-81 • Note that the electron goes in the side of the reactants. Electron gets consumed.
Patterns of nuclear Stability • Any element with more than one proton ( all 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. • The ratio of neutrons to protons is key to determine the stability of a nucleus .
Neutron-Proton Ratios As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.
Neutron-Proton Ratios For smaller nuclei (Z 20) stable nuclei have a neutron-to-proton ratio close to 1:1.
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. ( neutron becomes proton )
Above the belt of stabilityBeta particle emission • Too many neutrons. The nucleus emits Beta particles, decreasing the neutrons and increasing the number of protons.
Stable Nuclei • Nuclei below the belt have too many protons. • They tend to become more stable by positron emission or electron capture (both lower the number of protons)
Stable Nuclei • Elements with low atomic number are stable if # proton = # neutrons • There are no stable nuclei with an atomic number greater than 83. • These nuclei tend to decay by alpha emission.
Below the stability beltIncrease the number of neutrons (by decreasing # protons) • Positron emission more common in lighter nuclei. • Electron capture common for heavier nuclei.
Radioactive Series • Large radioactive nuclei cannot stabilize by undergoing only one nuclear transformation. • They undergo a series of decays until they form a stable nuclide (often a nuclide of lead).
Predicting modes of nuclear decay C-14 Xe-118 Pu-239 In-120
beta decay • Positron emission or electron capture • Alpha decay (too heavy, loses mass) • Beta decay (ratio too low, gains protons)
MAGIC NUMBERS2, 8, 20, 28, 50, or 82 Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons tend to be more stable than nuclides with a different number of nucleons.
Some Trends Nuclei with an even number of protons and neutrons tend to be more stable than nuclides that have odd numbers of these nucleons.
Shell model of the nucleus • Nucleons are described a residing in shells like the shells for electrons. • The numbers 2,8,18,36,54,86 correspond to closed shells in nuclei. • Evidence suggests that pair of protons and pairs of neutrons have special stability
Transmutations • To change one element into another. • Only possible in nuclear reactions never in a chemical reaction. • In order to modify the nucleus huge amount of energy are involved. • These reactions are carried in particle accelerators or in nuclear reactors
Nuclear transmutations • Alpha particles have to move very fast to overcame electrostatic repulsions between them and the nucleus. • Particle accelerators or smashers are used. They use magnetic fields to accelerate the particles.
Particle Accelerators(only for charged particles!) These particle accelerators are enormous, having circular tracks with radii that are miles long.
Cyclotron Nuclear transformations can be induced by accelerating a particle and colliding it with the nuclide.
Neutrons • Can not be accelerated. They do not need it either (no charge!). • Neutrons are products of natural decay, natural radioactive materials or are expelled of an artificial transmutation. • Some neutron capture reactions are carried out in nuclear reactors where nuclei can be bombarded with neutrons.
Representing artificial nuclear transmutations • 14N + 4He 7O + 1H Target nucleus ( bombarding particle, ejected particle ) product nucleus • 14N (a, p) 17O • Write the balanced nuclear equations summarized as followed: • 16 O ( p, a) N • 27Al (n, a)24 Na
Measuring Radioactivity • One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample. • The ionizing radiation creates ions, which conduct a current that is detected by the instrument.
Mass defect • The mass of the nucleus is always smaller than the masses of the individual particles added up. • The difference is the mass defect. • That small amount translate to huge amounts of energy E = (m) c2 • That energy is the Binding energy of the nucleus, and is the energy needed to separate the nucleus.
Energy in Nuclear Reactions For example, the mass change for the decay of 1 mol of uranium-238 is −0.0046 g. The change in energy, E, is then E = (m) c2 E= (−4.6 10−6 kg)(3.00 108 m/s)2 E= −4.1 1011 J This amount is 50,000 times greater than the combustion of 1 mol of CH4