discovered by Rutherford discovered by Chadwick Nucleus contains protons and neutrons. Nucleon: any particle in the nucleus (p +, n o )

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1 Proton: p + Neutron: n o discovered by Rutherford discovered by Chadwick Nucleus contains protons and neutrons. Nucleon: any particle in the nucleus (p +, n o ) Protons, neutrons, and electrons account for all of the mass and charge of an atom.

2 Atoms Atoms are composed of a nucleus (protons and neutrons) and electrons. Isotopes are different forms of some elements with a different number of neutrons in a nucleus. Nuclide: nucleus of an isotope Atomic number: the number of protons in a nucleus (Z) Mass number (nucleon number): the number of protons and neutrons in a nucleus (A) A Z X

3 Great forces bind protons and neutrons together in the nucleus. This is the STRONG NUCLEAR FORCE. The strong nuclear force does not exist outside the nucleus. Atoms that contain a large number of protons have large electrostatic forces (repulsion). The atom will have extra neutrons to counteract this. However, sometimes extra neutrons cannot counteract the repulsive electrostatic forces and the nucleus becomes unstable. The nucleus will disintegrate. At this point, the atom is radioactive.

4 Nuclear Binding Energy and Mass Defect The number of particles in a nuclear reaction or radioactive decay does not change but the mass of the products and reactants does. It takes a huge amount of energy to remove a nucleon (p + or n o ) from the nucleus. It takes 13.6 ev to remove an e - from the hydrogen atom. The energy required to separate all the nucleons in a nucleus is the binding energy. What happens to the binding energy, BE, you added to remove a nucleon? From Einstein s special theory of relativity: energy IS mass. E = mc 2 The mass of products is slightly less than that of the original reactants and this missing mass is equal to the kinetic energy of the products. This energy becomes very large when billions of atoms are considered.

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6 4 To remove an neutron from 2 He : He + energy 2 He n The differences between the masses is the mass defect ( m). Physicists use an atomic mass unit (u) because a kilogram is so large. 1 u = x kg

7 Example 1. Determine the binding energy in ev and J for an iron-56 nucleus given that the nuclear mass is u. Total mass = 26(m p ) + 30(m n ) = 26( u) + 30( u) = u m = u u = u * x kg/u = x kg E = mc 2 = x kg (3.00 x 10 8 m/s) 2 = x J = x J * 1 ev/1.6 x J = x 10 8 ev Energy per nucleon: = x 10 8 ev /56 = 8.82 x 10 6 ev/nucleon Example 2. Calculate the binding energy per nucleon for Mo given a nuclear mass of u.

8 Radioactivity Natural radioactivity: over 100 years ago, Becquerel and Pierre and Marie Curie noticed some substances spontaneously give off energy (radiation). These substances were later found to give off three major types of radiation. A) alpha (α) particles: positively charged particles (nuclei of helium atoms) which were given off at high speed but didn't travel far. Rutherford B) beta (β) particles: streams of high speed electrons - some approaching speed of light with more penetrating power. Rutherford C) gamma (γ) rays: EMR with high frequency and low wavelength with very high penetrating power. Villard Units of radioactivity are becquerel (Bq) 1 Bq = 1 emission/second

9 Alpha particles: penetrate 5 cm of air. They can be stopped by paper. They are not normally harmful unless inhaled or ingested. Beta particles: penetrate 10 m of air. They can be stopped by a thin sheet of metal. Gamma rays: penetrate 2 km of air or up to 10 cm of lead.

10 Positive alpha particles are attracted to the negative plate. Negative beta particles are attracted to the positive plate. Gamma rays are not attracted to either plate. They do not carry a charge.

11 Natural Transmutations some radioactive isotopes of particular elements will give off α, β, and γ emissions from their nuclei without any apparent reason and in os doing will change to a completely new element. They are said to undergo alpha, beta, or gamma decay. α decay: parent daughter Β - decay: Β + decay: γ decay: Rn 84 Po Na Mg A Z P He (α : helium nucleus) + 0 A + 1 e + Z 1 D e (β - : electron) 0 0 ν (β + : positron; neutrino) Usually accompanies α or β decay U 90 Th + 2 He + (γ ray) Many of these transmutations go through many steps called a decay series until a stable non-radioactive element is produced. 0 0 γ

12 Unstable nucleus will decay to a more stable form. Must obey conservation of: Mass energy Charge Momentum Nucleon Number Similarly to "balancing" chemical equations, nuclear equations have reactants and products. What must be the same before decay and after decay is the total mass and atomic numbers (nucleon numbers) on each side Pa 92 U +? γ

13 Decay Series

14 Radioactivity - Half Life and Activity Some radioactive substances decay and emit particles and radiation quickly (high activity) and some very slowly (low activity). The amount of time it takes for 1/2 of any amount of a radioactive substance to decay is called its half- life (T 1/2 ). Example. The half-life of Th 90 is 24 days so original amount 1 st half-life 2 nd half-life 100 g 50 g 25 g 100 atoms 50 atoms 25 atoms A typical graph of half-life or activity level will look like:

15 Radioactive Carbon Dating C-14 is a radioactive isotope of carbon found in all living things. When an organism dies it stops taking in any more carbon so as time passes, the amount of C-14 decreases. Examining the remains (fossilized wood, bones, etc..) of a once living organism and seeing how much (%) of C-14 remains is a way of dating how long it has been dead.

16 Radiation and Life too much exposure to radiation for prolonged periods of time can damage the DNA in human cells. however, radiation specifically directed at cancerous cells will kill them and stop their growth. Other uses for radioisotopes: o Cancer treatment: Co-60 decays and produces high energy gamma radiation. This radiation is much more powerful than the highest energy X-rays. o Medical diagnosis: Na-24 is a tracer in the human body. It is soluble in blood and used to measure the level of kidney function. o Industrial: radioisotopes are added to machine parts to determine the rate of wear. o Sterilization and food preservation: controlled doses can slow down the ripening of fruits, kill bacteria and micro-organisms that cause food to spoil. o Smoke detectors: when smoke interfers with the stream of alpha particles, there is no longer a current and the alarm goes off.

17 Nuclear Fission Breaking apart a large nucleus into smaller parts. Since more neutrons are produced as a result of the original fission, a chain reaction could occur (bomb). In a nuclear power plant control rods (graphite) absorb excess neutrons to control the rate of reaction. The major draw back for nuclear power produced in this method is high-level radioactive waste produced, many components of which have very long half-lives.

18 a very large nucleus splits into two large nuclei and two or more neutrons. The most common are U and Pu when a nucleus splits, or fissions, it produces huge amounts of energy. This energy is the kinetic energy of the neutrons and fission products. the smaller nuclei (products) have larger binding energies than the original nucleus. the large mass defect yields the large amount of energy U + n Ba+ Kr + n energy Products have the larger binding energy

19 Nuclear Fusion Producing a larger atom from two lighter ones. This actually produces more energy per gram of products than fission and produces no by-products. Why is it not used yet? More energy is required to initiate the reaction than it produces. Heat produced is so intense, containment vessels melt.

20 two nuclei combine to form one nucleus. if splitting a nucleus gives off energy, wouldn't fusing require energy?... when fusion occurs, it also results in a huge release of energy. The nuclides with small nucleon numbers have low binding energies compared to those with larger nucleon numbers. when fusion happens, the products have a larger binding energy than the reactants. This mass defect results in large amounts of energy.... Fusion requires energy? To fuse, two nuclei must be close enough for the strong nuclear force to draw them closer and keep them together. BUT, when the positive nuclei approach, the electrostatic force of repulsion is greater than the nuclear force. Therefore, the nuclei must be HIGHLY energetic to overcome the electrostatic force. This means high temperatures. This is difficult to generate while containing the atoms H + 1H 2He+ 0n + energy

21 Nuclear Reactors 1. Chain reactions: if more than one neutron is emitted when a nucleus fissions, those neutrons should be able to stimulate more nuclei to fission. if one neutron from one fission causes one more nucleus to fission... at a constant rate, the process is critical. if fewer than one neutron from one event causes another, the process is subcritical - the reactions will cease. if more than one neutron... the process is supercritical. 2. Fuel for fission: two nuclei that are likely to undergo fission are Pu and U Pu does not occur naturally but can be produced by bombarding U with neutrons. U occurs naturally but it only 0.7% abundant. Undergoing fission requires them to absorb a slow neutron. To do this, it has to undergo elastic collisions or be absorbed by nuclei. A moderator is used to slow down the neutron. Usually deuterium (H - 2), deuterium oxide (D 2 O), beryllium, or graphite.

22 3. Control rods: rods made of cadmium or boron are lowered into the core or withdrawn so that only one neutron from every fission reaction causes one more fission reaction. 4. Coolants: the primary coolant runs through the core of the reactor and removes heat from the rods and carries it to a boiler. The primary coolant pipes heat the water in the boiler. This water, the secondary coolant, is used to run the turbines. The primary and secondary coolant never mix. Coolants must be non-corrosive, have high BP, not absorb neutrons, not be radioactive, have good heat transfer properties, and pump easily. 5. Nuclear waste: fuels, U and Pu, are radioactive but emit alpha particles. But, the fission products are very unstable and highly radioactive. Extreme caution is required in handling and storage of them.

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