Chapter 11 Nuclear Chemistry 11.1 Nuclear Reactions Nuclear reactions involve the particles located in the nucleus of the atom: The nucleus contains: An atom is characterized by: X A Z - Z the gives the number of protons in the nucleus written as a subscript to the left of the element symbol;. - A the gives the total number of nucleons written as a superscript to the left of the element symbol,. Atoms with - identical atomic numbers Z ( of protons) but - different mass numbers A ( of neutrons) - are called. This is the most common isotope of carbon. Problem: The other isotopes of carbon are C-13 and C-14. Write the element symbols and identify the number of protons, neutrons & electrons in each. What is different? Ch 11 Page 1
Nuclide: refers to the nucleus of a specific isotope of an element. Different isotopes have different nuclides. Nuclear reactions - reactions that change the nucleus. Elements involved in the reaction often change into new ones. Chemical reactions involved the electrons around the nucleus. Elements involved tin the reaction don t change. Differences between Nuclear and Chemical Reactions. Chemical Reaction 1. Change occur in distribution of the outer shell electrons Nuclear Reaction 1. Changes occur in an atom's 2. Different nuclides (isotopes) of an element have the same behavior 3. Rate depends on temp, pressure, catalyst, etc. 4. Reactions of an element depend on the compound it is contained in. 5. Small to moderate energy changes associated with reactions. 2. Different nuclides (isotopes) of an element have behaviors 3. rate at all conditions 4. Reactions are specific to the element isotope, not the compound. 5. Energy changes can be very Ch 11 Page 2
11.2 The Discovery and Nature of Radioactivity To be radioactive means that the nuclei spontaneously emit radiation. Antoine Henri Becquerel Radioactivity was first discovered by the French Physicist in 1896. He was studying phosphorescence (luminous glow of some minerals). Placed crystal of on wrapped photographic plate. The uranium exposed the film through the paper. Marie Sklodwska Curie She and husband Pierre studied this phenomenon. She came up with the name. Discovered 2 more radioactive elements: Earned 1903 Nobel Prize in Physics for work with Pierre and Henri. Earnest Rutherford Discovered two separate types of radiation: Additional form was found soon afterwards by one of his students (Chadwick). 11.3 Stable and Unstable Isotopes 1. Every element has at least one radioisotope (radioactive isotope). 2. This caused by having an nucleus. (Cause not clearly understood) 3. Radiation is emitted when radionuclide (unstable radioactive nucleus) changes into a more stable one. Ch 11 Page 3
Relative amounts of protons and neutrons For lighter elements, the most stable isotopes have a proton/neutron ratio of. For heavier elements, the ratio of neutrons to protons increases to almost. The area where stable isotopes are likely to be found is known as the For all elements above, all isotopes are radioactive. Stability favored by #p & #n All isotopes of the elements heavier than are artificial radioisotopes because they are not found in nature. They have been manmade by artificial transmutation in high-energy particle accelerators. 11.4 Nuclear Decay (and Types of Radiation) The spontaneous emission of a particle from an unstable nucleus is call nuclear decay or radioactive decay. The resulting change of one element into another element is called. Ch 11 Page 4
Nuclear decay: radioactive element new element + emission (usually) The Three Most Common Types : The radioactive source in the shielded box emits radiation, which passes between two electrically charged plates. Alpha radiation is deflected towards the negative plate. Beta radiation is deflected towards the positive plate. Gamma radiation passes right through. Alpha (a) Radiation 1. A stream of particles that consist of. 2. charged 3. Represented as. 4. Common in radio-isotopes. Example: Ch 11 Page 5
Steps to Balance a Nuclear Reaction. 1. Sums of the nucleons on both sides are equal. 2. Sums of the charges on both sides are equal. 3. Not concerned with ionic charges on atoms. Problem: Balance the following alpha decay nuclear equation: 4 208 84Po 2He + Problem: What product results from alpha emission by radon-222? Problem: What isotope is converted into radon-222 by alpha emission? Beta (b) Radiation - 1. A neutron spontaneously decays into a proton plus an electron, which is then ejected. 2. Represented as β or -1 0 e or β - Problem: Complete the following beta emission nuclear equations: 14 6C 55 24Cr Ch 11 Page 6
Gamma (g) Radiation 1. Electromagnetic radiation of very high energy and short wavelength. 2. Stream of high energy or light. 3. Almost always accompanies other emissions. a. mechanism for 4. gamma emission is often not shown in nuclear equations. 0 5. Represented by g or 0 γ Example: 60 60 0 27 Co 28Ni + -1e + 0 0 γ Summary of Properties of Common Radiation Types Radiation Identity Velocity Shielding Required alpha, a helium nucleus < 10% c paper, clothing beta, b electron < 90% c 30 cm wood, aluminum foil gamma, g high energy radiation c = the speed of light 100% c 10cm lead, 30 cm concrete Penetrating Power stopped by the skin ~ 1 cm of flesh, passes through body Positron Emission The conversion of a proton in the nucleus into a neutron plus an ejected positron. (CHARGE OF PROTON IS EJECTED) positron (1 0 e, or b+) - a particle with the same mass as an electron but opposite charge Ch 11 Page 7
+ 2 Positron Emission Tomography (PET) PET scanning is a nuclear medicine functional imaging technique that is used to observe metabolic processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radionuclide (tracer), which is introduced into the body on a biologically active molecule. Threedimensional images of tracer concentration within the body are then constructed by computer analysis. Radionuclides used in PET scanning are typically isotopes with short half-lives [2] such as carbon-11 (~20 min), nitrogen-13 (~10 min), oxygen-15 (~2 min), fluorine-18 (~110 min), gallium-68 (~67 min), zirconium-89 (~78.41 hours), or rubidium-82(~1.27 min). These radionuclides are incorporated either into compounds normally used by the body such as glucose (or glucose analogues), water, or ammonia, or into molecules that bind to receptors or other sites of drug action. Potassium-40 emits a positron when it decays. Positron Emission p Þ n by loosing a (+) charge. Problem: Write a nuclear equation for positron emission from calcium-38. Ch 11 Page 8
Electron Capture A proton in the nucleus captures an inner-shell electron which is converted into a neutron. (CHARGE OF PROTON IS NEUTRALIZED) Electron Capture p Þ n by gaining a (-) charge. Problem: Complete the following nuclear equations and identify the type(s) of radidoactivity. 62 30 204 84 118 54 210 83 66 29 a) Zn + e -1 0 b) Po + e -1 0 118 53 2 66 30 c) Po I + d) Po He + e) Cu Zn + 11.5 Radioactive Half Life Radioactive decay is characterized by a half-life, t½. 1. Time required for the number of radioactive nuclei in a sample to drop to of the initial value. 2. Each passage of a half-life causes the decay of one-half of whatever sample remains. 3. The half-life is the no matter what the size of the sample, the temperature, or any other external condition. Ch 11 Page 9
Example: The half-life of carbon-14 is 5730 years. What percentage of carbon-14 remains in a sample estimated to be 17, 000 years old? 17000 @ 5730 100% x ½ = 50% left after 1 half life 50% x ½ = 25% left after 2 half lives 25% x ½ = Carbon Dating Limitations: a) Doesn t work well for very recent or very old samples or very recent samples. b) Assumes C-14 abundance has always been the same. We can quantitatively determine the fraction of a radioactive isotope left. The fraction left = (1/2 ) n = (0.5 ) n n= the number of half-lives Problem: Radioisotope I-131, used to treat hypothyroidism, has a half-life of 8.02 days. If a patient is given a dose of 8.2 μg of this isotope, what mass remains after 28 days? What percent of the radioisotope remains? Start by calculating the number of half-lives elapsed. The fraction left = The amount left = The percentage left = Ch 11 Page 10
To figure out how much radioactive material was present at the beginning, based on current amounts, you need to do the reverse process. The multiplier of the amount remaining = 2 n OR original amt = (after decay amt) x 2 n Problem: If 0.18 μg of I-131 with a t-½ of 8.02 days remains after 52 days, what dose was the patient given? Start by calculating the number of half lives elapsed. Multiplier of amount left = Amount at start = Problem: If 0.88 mci of Tc-99m with a t-½ of 6.0 hrs remains after 24 hrs, what dose was Prof. Nuss given for scanning by a gamma camera? (1 Curie = 3.7x10 10 decay events/sec) Start by calculating the number of half-lives elapsed. Amt at start = Ch 11 Page 11
Radioactive Decay Series When a radioactive nuclide decays to a different element, the product element, known as a daughter product, may also be radioactive. Some elements undergo and extended decay series of nuclear disintegrations before they finally become stable (non-radioactive). For example, U-238 undergoes 14 sequential nuclear reactions before it finally becomes non-radioactive. The entire sequence takes thousands of years. 11.6 Ionizing radiation High-energy radiation of all kinds can be grouped together as ionizing radiation. Types include: alpha particles, beta particles, gamma rays, X-rays, and cosmic rays. alpha and beta are particles gamma and x-rays are high-energy electromagnetic radiation cosmic rays are not rays at all but a mixture of high energy particles that come from outer space. Energy = h c l As l goes, energy goes h = Planck s constant Ch 11 Page 12