Presentation on theme: "Chapter 21 Nuclear Chemistry"— Presentation transcript:
1Chapter 21 Nuclear Chemistry Chemistry, The Central Science, 10th editionTheodore L. Brown; H. Eugene LeMay, Jr.; and Bruce E. BurstenChapter 21 Nuclear ChemistryJohn D. BookstaverSt. Charles Community CollegeSt. Peters, MO 2006, Prentice Hall, Inc.
3The NucleusRemember 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.
4IsotopesNot 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-234Uranium-235Uranium-238
6RadioactivityIt 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.
13Nuclear ReactionsThe 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!!!
14He U Th He Alpha Decay: Loss of an -particle (a helium nucleus) + 4 2 23892Th23490He42+
15Alpha Decay Mass changes by 4 The remaining fragment has 2 less protonsAlpha radiation is the less penetrating of all the nuclear radiation (it is the most massive one!)
16 e I Xe e Beta Decay: Loss of a -particle (a high energy electron) + −1eorI13153Xe54+e−1
17Beta DecayInvolves 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
19Gamma Emission:Loss of a -ray (high-energy radiation that almost always accompanies the loss of a nuclear particle)
20e + C B e Positron Emission: Loss of a positron ( particle with same mass, but opposite charge than an electron)e +1C116B5+e1
21Positron emissionInvolves the conversion of a proton to a neutron emitting a positron.The atomic number decreases by one, mass number remains the same.
22Electron 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.p1+e−1n
23Rb Kr Electron capture Rb-81 Note that the electron goes in the side of the reactants. Electron gets consumed.Rb8137Kr36
24Patterns 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.
25Neutron-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 .
26Neutron-Proton Ratios As nuclei get larger, it takes a greater number of neutrons to stabilize the nucleus.
27Neutron-Proton Ratios For smaller nuclei (Z 20) stable nuclei have a neutron-to-proton ratio close to 1:1.
28Stable NucleiThe shaded region in the figure shows what nuclides would be stable, the so-called belt of stability.
29Stable Nuclei Nuclei above this belt have too many neutrons. They tend to decay by emitting beta particles. ( neutron becomes proton )
30Above the belt of stability Beta particle emission Too many neutrons. The nucleus emits Beta particles, decreasing the neutrons and increasing the number of protons.
31Stable 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)
32Stable NucleiElements with low atomic number are stable if # proton = # neutronsThere are no stable nuclei with an atomic number greater than 83.These nuclei tend to decay by alpha emission.
33Below the stability belt Increase the number of neutrons (by decreasing # protons) Positron emission more common in lighter nuclei.Electron capture common for heavier nuclei.
35Radioactive SeriesLarge 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).
36Predicting modes of nuclear decay Xe-118Pu-239In-120
37beta decayPositron emission or electron captureAlpha decay (too heavy, loses mass)Beta decay (ratio too low, gains protons)
38MAGIC NUMBERS 2, 8, 20, 28, 50, or 82Nuclei 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.
39Some TrendsNuclei with an even number of protons and neutrons tend to be more stable than nuclides that have odd numbers of these nucleons.
40Shell 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
41Transmutations 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
42Nuclear 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.
43Particle Accelerators (only for charged particles!) These particle accelerators are enormous, having circular tracks with radii that are miles long.
44CyclotronNuclear transformations can be induced by accelerating a particle and colliding it with the nuclide.
45NeutronsCan 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.
46Representing artificial nuclear transmutations 14N + 4He 7O + 1HTarget nucleus ( bombarding particle, ejected particle ) product nucleus14N (a, p) 17OWrite the balanced nuclear equations summarized as followed:16 O ( p, a) N27Al (n, a)24 Na
47Measuring 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.
48Mass defectThe 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) c2That energy is the Binding energy of the nucleus, and is the energy needed to separate the nucleus.
49Energy in Nuclear Reactions For example, the mass change for the decay of 1 mol of uranium-238 is − g.The change in energy, E, is thenE = (m) c2E = (−4.6 10−6 kg)(3.00 108 m/s)2E = −4.1 1011 J This amount is 50,000 times greater than the combustion of 1 mol of CH4
50Types of nuclear reactions fission and fusion The larger the binding energies, the more stable the nucleus is toward decomposition.Heavy nuclei gain stability (and give off energy) if they are fragmented into smaller nuclei. (FISSION)
51Even greater amounts of energy are released if very light nuclei are combined or fused together. (FUSION)
52Nuclear Fission How does one tap all that energy? Nuclear fission is the type of reaction carried out in nuclear reactors.
53Nuclear FissionBombardment of the radioactive nuclide with a neutron starts the process.Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.
54Nuclear FissionThis process continues in what we call a nuclear chain reaction.
55Nuclear FissionIf there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out.
56Nuclear FissionTherefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained: Critical Mass.
57Controlled vs Uncontrolled nuclear reaction Controlled reactions: inside a nuclear power plantUncontrolled reaction: nuclear bomb
58Nuclear ReactorsIn nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator.
59Nuclear ReactorsThe reaction is kept in check by the use of control rods.These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass.
60FUSION Combining small nucleii to form a larger one. Require millions of K of temperature
61Fusion 1H + 1H 2H + 1e + energy 1H + 2H 3He + energy 3He + 3He 4He + 21H + energyReaction that occurs in the sunTemperature 107 KHeavier elements are synthesized in hotter stars 108 K using Carbon as fuel
62Nuclear Fusion Fusion would be a superior method of generating power. The good news is that the products of the reaction are not radioactive.The bad news is that in order to achieve fusion, the material must be in the plasma state at several million kelvins.
63Nuclear Fusion (thermonuclear reactions) Tokamak apparati like the one shown at the right show promise for carrying out these reactions.They use magnetic fields to heat the material.3 million K degrees were reached inside but is not enough to begin fusion which requires 40 million K
64Rates of radioactive decay rate = k N N is the number of radioactive nuclei Activity: rate at which a sample decays. Expressed in disintegrations per unit time.Becquerel (Bq) SI unit : one nuclear disintegration per second.Curie (Ci) 3.7x1010 disintegrations per second, the rate of decay of 1g of Ra
65RADIOACTIVE DECAYAs a radioactive sample decays, the amount of radiation emanating for the sample decays as well.After one half life, half of the emanations!
66Half-LifeHalf-life is defined as the time required for one-half of a reactant to react.Because [A] at t1/2 is one-half of the original [A],[A]t = 0.5 [A]0.
67RADIOACTIVE DECAYIs a first order process. Its rate is proportional to the number of radioactive nuclei N in the sample rate= k NN0Ntln= ktTime elapsed =t k is the decay constantN0 is the original amountNt is the amount of sample at time t0.693 = kt1/2
68Half lifeThe half life of a reaction is useful to describe how fast it occurs.For a first order reaction (like nuclear decay!) it does not depend on the initial concentration of the reactants.HALF LIFE IS CONSTANT FOR A FIRST ORDER REACTION
69Half Life Decay of 10.0 g sample of Sr-90 t1/2= 28.8 y
70Problem 1 The half life of 210Pb= 25 y 1) How much left of a sample of 50 mg will remain after 100 y?2) Find number of half lives3) Find fraction left