Nuclear Chemistry PDF

I. LESSON TITLE NUCLEAR CHEMISTRY II. LESSON CONTENT In the context of nuclear science, protons and neutrons are called nucleons, because they reside in the nucleus. The atom’s mass number is called the nucleon number, and a particular type of nucleus, characterized b...

I. LESSON TITLE NUCLEAR CHEMISTRY II. LESSON CONTENT In the context of nuclear science, protons and neutrons are called nucleons, because they reside in the nucleus. The atom’s mass number is called the nucleon number, and a particular type of nucleus, characterized by a specific atomic number and nucleon number, is called a nuclide. Nuclides are represented in chemical notation by a subscript atomic number (Z) and superscript nucleon number (A) on the left side of the element’s symbol (X) 𝐴 𝑧𝑋 Example A nuclide that has 26 protons and 33 neutrons is used to study blood chemistry. Write in nuclide symbol in the form of 𝐴𝑍𝑋. Write two other ways to represent this nuclide Solution Because this nuclide has 26 protons, its atomic number, Z, is 26, identifying the element as iron, Fe. This nunclide of iron has 59 total nucleus (26 protons + 33 neutrons), so its nucleus number, A, is 59 59 59 26𝐹𝑒 𝑜𝑟 𝐹𝑒 𝑜𝑟 𝑖𝑟𝑜𝑛 − 59 Nuclear Stability Two forces act upon the particles within the nucleus to produce the nuclear structure 1. Electrostatic Force (electromagnetic force) The force that causes opposite electrical charges to attract each other and like charges to repel each other. In such that positively charged protons in the nucleus of an atom have an electrostatic force pushing them apart. 2. Strong Force Another force within the nucleus that hold nucleons (protons and neutrons) together If one proton were to encounter another, the electrostatic force pushing them apart would be greater than the strong force pulling them together, and the two protons would fly in separate directions. Therefore, nuclei that contain more than one proton and no neutrons do not exist. Neutrons can be described as the nuclear glue that allows protons to stay together in the nucleus. Because neutrons are uncharged, there are no electrostatic repulsions between them and other particles. At the same time, each neutron in the nucleus of an atom is attracted to other neutrons and to protons by the strong force. Therefore, adding neutrons to a nucleus increases the attractive forces holding the particles of the nucleus together without increasing the amount of repulsion between those particles. As a result, although a nucleus that consists of only two protons is unstable, a helium nucleus that consists of two protons and two neutrons is very stable. The increased stability is reflected in the significant amount of energy released when two protons and two neutrons combine to form a helium nucleus. Lighter Elements possess an equal number of protons and neutrons leads to stable atom such as carbon and oxygen. Larger atoms with more protons in their nuclei require a higher ratio of neutrons to protons to balance the increased electrostatic repulsion between protons. There are 264 stable nuclides found in nature. Collectively, these nuclides fall within what is known as the band of stability shown in the below graph A nuclide containing numbers of protons and neutrons that place it outside this band of stability will be unstable until it undergoes one or more nuclear reactions that take it into the band of stability. These unstable atoms are called radioactive nuclides, and the changes they undergo to reach stability are called radioactive decay. Note that the band of stability stops at 83 protons. All of the known nuclides with more than 83 protons are radioactive, but scientists have postulated that there should be a small island of stability around the point representing 114 protons and 184 neutrons. The relative stability of the heaviest atoms that have so far been synthesized in the laboratory suggests that this is true. Types of Radioactive Emission 1. Alpha Emission In alpha emission, radioactive nuclide changes into a different element, with an atomic number that is lower than 2 and a mass number that is lower than 4. These radioactive nuclides change to reach the band of stability releasing two protons and two neutrons in the form of helium nucleus which is called alpha particle. General Equation: 𝐴 𝐴−4 𝑍𝑋 → 𝑍−2𝑌 + 42𝐻𝑒 2. Beta Emission In beta emission, a neutron becomes a proton and an electron. The proton stays in the nucleus and the electron which is called beta particle is ejected from the atom. Electron is written as 𝛽, 𝛽 − or −10𝑒 General Equation: 𝐴 𝐴 0 𝑍𝑋 → 𝑍+1𝑌 + −1𝑒 3. Positron Emission In this emission, a proton becomes a neutron and an anti – matter electron or anti – electron. The latter is called a positron because it has a positive charge. The neutron stays in the nucleus, and the positron speeds out of the nucleus at high velocity. The electron is written as either 𝛽 + , +10𝑒 𝑜𝑟 01𝑒 General Equation: 𝐴 𝐴 0 𝑍𝑋 → 𝑍−1𝑌 + +1𝑒 4. Electron Capture In this process, the electron combines with the proton to form a neutron. It causes the radioactive nuclide to change to a new element, with an atomic number that is lower by 1 but with the same mass number General Equation: 0 −1𝑒 + 𝐴𝑍𝑋 → 𝐴 𝑍−1𝑌 Example Write nuclear equations for a. Alpha emission by polonium-210, used in radiation therapy b. Beta emission by gold-198, used to assess kidney activity, c. Positron emission by nitrogen-13, used in making brain, heart, and liver images, d. Electron capture by gallium-67, used to do whole body scans for tumors Solution a. The symbol for polonium-210 is 210 84𝑃𝑜 , and the symbol for an alpha particle is. Therefore, the beginning of our equation is 210 84𝑃𝑜 → _______ + 42𝐻𝑒 Determine first the subscript for the missing formula by asking what number would make the sum of the subscripts on the right of the arrow equal to the subscript on the left. The mass number for the product nuclide must be 206 210 206 84𝑃𝑜 → 82𝑃𝑏 + 42𝐻𝑒 b. The symbol for gold – 198 is 198 79𝐴𝑢 , and the symbol for a beta particle is 0 −1𝑒. Therefore the beginning of our equation is 198 0 79𝐴𝑢 → _______ + −1𝑒 To make the subscripts balance in our equation, the subscript for the missing nuclide must be 80, indicating the symbol for the product nuclide should be Hg, for mercury. The mass number stays the same in beta emission, so we write 198 198 198 0 79𝐴𝑢 → 80𝐻𝑔 + −1𝑒 c. The symbol for nitrogen – 13 is 137𝑁 , and the symbol for the positron is 0 +1𝑒. Therefore, the beginning of our equation is 13 → _______ + +10𝑒 7𝑁 To make the subscript balance, the subscript for the missing nuclide must be 6, so the symbol for the product nuclide is C, for carbon. The mass number stays the same in positron emission, so we write 13 13 13 0 7𝑁 → 6𝐶 + +1𝑒 d. The symbol for Gallium-67 is 67 31𝐺𝑎 , and the symbol for an electron is 0 −1𝑒. Therefore the beginning of our equation is 67 0 31𝐺𝑎 + −1𝑒 → ______ To balance the subscripts, the atomic number for our missing nuclide must be 30, so the symbol for the product Zinc is Zn 67 0 67 31𝐺𝑎 + −1𝑒 → 30𝑍𝑛 Radioactive Decay Series A radioactive decay series is a series of nuclear reactions that begins with an unstable nucleus and results in the formation of stable nucleus. Radioactive Decay Rates Radioisotopes have been decaying for about billions of years – the span of Earth’s existence. Naturally occurring radioisotopes are not uncommon on Earth. Some radioisotopes are continuously form in the upper atmosphere of Earth. Others are formed in the universe, during stellar nucleosynthesis for instance. Radioisotopes can also be synthesized in laboratories. The differing decay rates of isotopes also contribute to their presence on Earth. A half – life is the time required for one – half of a radioisotope’s nuclei to decay into its product. The remaining amount of radioactive element is equal to the initial amount times one – half raised the number of half – lives that have passed. 𝟏 𝒏 N is the remaining amount 𝑵 = 𝑵𝒐 ( ) No is the initial amount 𝟐 n is the number of half – lives that have passed The exponent n can also be replaced with the equivalent quantity 𝑡⁄𝑇, where t is the elapsed time and T is the duration of the half – life. Note that t and T must have the same units of time 𝒕 𝟏 ⁄𝑻 𝑵 = 𝑵𝒐 ( ) 𝟐 This type of expression is known as an exponential decay function. Example Krypton – 85 is used in indicator lights of appliances. The half – life og krypton – 85 is 11 years. How much of a 2.0 mg sample remains after 33 years? Solution Radiochemical Dating Chemical reaction rates are greatly affected by changes in temperature, pressure and concentration, and by the presence of a catalyst. In contrast, nuclear reaction rates remain constant regardless of such changes. In fact, the half – live of any particular radioisotope is constant. Because of this, radioisotopes can be used to determine the age of an object. The process of determining the age of an object by measuring the amount of a certain radioisotope remaining in that object is called radiochemical dating. Nuclear Reactions Nuclear reactions can be induced, in other words, produced artificially. The process, which involves striking nuclei with high – velocity particles, is called induced transmutation. Example Write a balanced nuclear equation for the induced transmutation of oxygen-16 into nitrogen-13 by proton bombardment. An alpha particle is emitted from the nitrogen atom in the reaction Solution Nuclear Reactions and Energy Einstein’s equation relates mass and energy. It states that any reaction produces or consumes energy due to a loss or gain in mass. Energy and mass are equivalent. Note that because c 2 is large, a small change in mass results in a large change in energy. Energy Equivalent of Mass ΔE is the change in energy, in Joules. Δm is the change ∆𝑬 = ∆𝒎𝒄𝟐 in mass, in kg. c is the speed of light The change in energy is equal to the change in mass times the square of the speed of light. Scientists have determined that the mass of the nucleus is always less than the sum of the masses of the individual protons and neutrons that comprise it. This difference in mass between a nucleus and its component nucleons is called the mass defect. Calculating Mass Defect Applying the equation ΔE = Δmc2, you can then derive the equivalent binding energy 𝑀𝑎𝑠𝑠 𝑑𝑒𝑓𝑒𝑐𝑡 = 𝑚𝑛𝑢𝑐𝑙𝑒𝑢𝑠 − [𝑁𝑝 𝑚𝑝 + 𝑁𝑛 𝑚𝑛 ] Where: 𝑚𝑛𝑢𝑐𝑙𝑒𝑢𝑠 Mass of the nucleus 𝑚𝑝 Mass of the proton 𝑚𝑛 Mass of the neutron 𝑁𝑝 Number of protons 𝑁𝑛 Number of neutrons If you start with the mass of the atom, you have to take into account the mass of the electrons. To do so, the mass of a hydrogen atom, which is composed of a proton and an electron, is used instead of the mass of a proton. The equation is then: 𝑀𝑎𝑠𝑠 𝑑𝑒𝑓𝑒𝑐𝑡 = 𝑚𝑖𝑠𝑜𝑡𝑜𝑝𝑒 − [𝑁𝑝 𝑚𝐻 + 𝑁𝑛 𝑚𝑛 ] Use the following values for the calculations: mH = 1.007825 amu mn = 1.008665 amu c = 3.0 x 108 m/s To calculate the energy in Joules, you can convert the masses into kilograms using 1 amu = 1.660540 x 10-27 kg Nuclear Fission and Electric Power Plant The process of splitting atoms to form a more stable, smaller atoms that releases energy is called fission. This is the process that fuels nuclear reactors used to make electricity. In a typical nuclear fission process, a neutron collides with a large atom, such as uranium-235, and forms a much less stable nuclide that spontaneously decomposes into two medium sized atoms and 2 or 3 neutrons. The nuclides produced composed of more than 200 different nuclides form, representing 35 different elements. Some nuclear reactions are used to power electrical generating plants. Energy is released when larger nuclides with lower binding energy per nucleon are converted to medium- sized nuclides with a higher binding energy per nucleon. An element can generate a lot of energy in a short period of time is that under the right circumstances, it can initiate a chain reaction, a process in which one of the products of a reaction initiates another identical reaction. To sustain a chain reaction for the fission, an average of at least one of the neutrons generated in each reaction must go on to cause another reaction. If this does not occur, the series of reactions slows down and eventually stops. In a nuclear reactor, the fuel rods are surrounded by a substance called a moderator that slows the neutrons as they pass through it. Several substances have been used as moderators, but normal water is most common. An efficient nuclear reactor needs to sustain the chain reaction but should not allow the fission reactions to take place too rapidly. For this reason, nuclear power plants have control rods containing substances such as cadmium or boron, which are efficient neutron absorbers. At the first sign of trouble, these control rods are inserted between the fuel rods, absorbing the neutrons that would have passed from one fuel rod to another and preventing them from causing more fission reactions. The deployment of the control rods stops the chain reaction. The control rods serve another purpose in the normal operation of the power plant. When fresh fuels rods are introduced, the control rods are partially inserted to absorb some of the neutrons released. Nuclear Fusion and the Sun A mass number of about 60 has the most stable atomic configuration. Thus it is possible to bind together two or more light (mass number less than 60) and less – stable nuclei to form a single more – stable nucleus. The combination of atomic nuclei is called nuclear fusion. Nuclear fusion reactions, which are responsible for the production of the heaviest element, are capable of releasing very large amounts of energy. You already have some everyday knowledge of this fact – the Sun is powered by a series of fusion reactions as hydrogen atoms fuse to form helium atoms. It is a promising source of energy and has several advantages compared to nuclear fission. Lightweight isotopes used to fuel the reactions such as hydrogen, are abundant. Nuclear fusion products are not generally radioactive. Nuclear fusion produces more of energy per unit of mass of fuel which could solve the problem of increasing needs for electricity in the world’s societies. However, fusion requires extremely high energies to initiate and sustain reaction. The required energy produce only at high temperature is needed to overcome the electrostatic repulsion between the nuclei in the reaction. Fusion, known as thermonuclear reactions, because of the energy requirement. A temperature of 5,000,000 K and even higher, is required to fuse hydrogen atoms to initiate fusion process. But, this approach is not practical for controlled electric power generation. Detecting Radioactivity 1. Geiger counter An ionizing detection device that is used to detect and measure radiation levels. Ionizing radiation produces an electric current in the counter. The current is displayed on a scaled meter, whereas a speaker produces audible sound 2. Scintillian counters Detect the presence of ionizing radiation. An ionizing radiation excites the electrons in the phospors (a certain type of atom or molecule). As the electron return to the ground states, they emit photons, which are then detected by the photodetector Uses of Radiation 1. Using Radioisotopes Radiotracer – a radioisotopes that emits non – ionizing radiation and is used to signal the presence of an element or specific substance  Chemical research  Use in medicine to detect disease 2. Treating Cancer Radiation therapy is used to treat cancer by destroying the cancer cells 3. Using Positron Emission Positron Emission Transaxial Tomography (PET) is radiation based diagnostic tool used to diagnose diseases or study parts of the brain Biological Effects of Radiation 1. Dose of Radiation Measuring Doses  rad (radiation – absorbed dose) – measure the amount of radiation that results in the absorption of 0.01 J of energy per kilogram of tissue  rem (roentgen equivalent for man) – the result of multiplying the dose in rad by a numerical factor that is related to the radiation’s effect on the tissue involved Effects of Short – term Radiation Exposure Dose (rem) Effects on Human 0 – 25 No detectable effects 25 – 50 Temporarily decrease in white blood cell population Nausea, substantial decrease in white blood cell 100 – 200 population 500 50% chance of death within 30 days of exposure Average Annual Radiation Exposure Source Average Exposure (mrem/y) Cosmic radiation 25 – 50 Radiation from the ground 25 – 175 Radiation from Buildings 10 – 160 Radiation from Air 20 – 260 Human body (internal) ~20 Medical and dental x rays 50 – 75 Nuclear weapon testing