Read Sections 11.1, 11.2, and 11.3 before viewing the slide show. - PowerPoint PPT Presentation

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Read Sections 11.1, 11.2, and 11.3 before viewing the slide show.

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  1. Read Sections 11.1, 11.2, and 11.3 before viewing the slide show.

  2. Unit 31Nuclear Chemistry (Chapter 11) Radiation sources (11.1) Identification of nuclear particles (11.2) Completing nuclear equations (11.2) Half-lives (11.3)

  3. Radiation Sources (11.1) Above is a breakdown of the sources of radiation in our lives. Notice 82% is natural. The radiation we are most concerned with is called ionizing radiation - radiation that knocks electrons out of atoms and forms ions.

  4. If you are interested in knowing your personal radiation exposure, you can go to http://www.new.ans.org/pi/resources/dosechart/ to figure it out. The basic unit for measurement is the millirem – the average dosage in the USA is about 620 mrems. • Several of you indicated you were interested in teaching and, I am not sure it is age-appropriate, but the EPA sponsors an interactive activity called USA Radtown. Click the link and you can see the town and, clicking on any of the activities going on, gives you some information about radiation exposure due to that activity. Radiation Sources (11.1)

  5. To this point, all of the chemical reactions we have looked at dealt with the transfer or sharing of electrons. Nuclear reactions are focused on the reactions of the nucleus, typically disintegrations and sometimes the reaction between nuclei colliding at high energies. There are basically five types of nuclear processes we will look at, each involving the loss or gain of particles or energy from the nucleus. The following table contains a list of the types of decay and the particles or energy involved. Identification of Nuclear Particles (11.2)

  6. The alpha particle is comprised of two protons and two neutrons which is a helium nucleus. • The beta particle is an electron that has been emitted from the nucleus (We didn’t think about electrons being in the nucleus. Think of the neutron as a proton and an electron “stuck” together. If an electron leaves, it will leave a proton behind.) • The gamma ray is a burst of energy – not a particle. Thus, it has no mass and does not affect the charge. • Think of the positron as a positive electron – same mass as an electron, opposite charge. • Electron capture occurs when an electron (one of those you are used to in an electron shell) is captured by the nucleus. More Description of Nuclear Particles (11.2)

  7. Symbols for Isotopes (Review from Unit 11) • The same system you have seen previously is used to represent the various isotopes of elements. The base of the system is:where A represents the mass number and Z the atomic number • Consider the examples below:

  8. We can use the nuclide symbols from the previous page to write symbols for the decay particles in the table below. • Notice the alpha particle is identical to a helium nucleus – two protons, two neutrons. Symbols for Nuclear Change (11.2)

  9. Writing balanced nuclear equations is a matter of making sure the sum of the superscripts on the left side of equation matches the sum of superscripts on the right and that the subscripts also match. • For example: • Notice how the superscripts add up: 238 on the left = 234 + 4 on the right and the subscripts add up: 92 on the left = 90 + 2 on the right • The “trick” to completing nuclear equations is to ensure that, whatever the species missing is, its insertion will make the superscripts and the subscripts add up on each side. Writing Nuclear Equations (11.2)

  10. Suppose we are faced with the task of completing the following nuclear equation: • We must find out what nuclide corresponds to the X and its A and Z values. • On the left the superscripts add to 226, on the right to A+4. So, A has to be 222 to make those two sides equal. • On the left the subscripts add to 88, on the right to Z+4. So, Z has to be 86 to make those two sides equal. • The subscript identifies the element’s atomic number. The element is Rn since its atomic number is 86. • The completed equation is: Completing Nuclear Equations (11.2)

  11. Sometimes (actually most often in life) these sorts of situations present themselves as word problems. • As an example: What particle is emitted when thorium-231 decays to protactinium-231? • First write out an equation representing what you know (the subscripts are the atomic numbers of the respective elements): • Superscripts: 231 on the left, A + 231 on the right, so A = 0 • Subscripts: 90 on the left, Z + 91 on the right so Z = -1. • The equation looks like: • Based on the numbers in front of the question mark, the missing particle must be a beta particle, β. Notice that even though the numbers match the electron capture process, that process can only occur if the electron is on the left-hand side of the equation. Completing Nuclear Equations cont. (11.2)

  12. The rate of the decay process for a radioactive nuclide is often represented by its half-life. • The half-life is the time required for one-half of the original sample to decay and does not vary with the size of the original sample. • Half-lives for different radioactive isotopes can range from mere tiny fractions of a second to millions of years. • Since the sample size of the radioactive material gets cut in half for each half-life, finding the fraction of remaining sample after “n” half-lives is just a matter of multiplying ½ by itself “n” times. Half-lives (11.3)

  13. One of the most well-known uses of the half-life concept is radiocarbon dating. In the upper atmosphere, carbon-14 is formed from the bombardment of nitrogen-14 with a neutron according to the equation: • Carbon-14 has a half-life of 5730 years and is incorporated in all living organisms. Once the living organism has died, there is no mechanism to replenish the carbon-14 so its level continuously drops. • The carbon-14 level can be compared against that found in living organisms to determine how long something has been dead. • Carbon-14 dating is reliable to about 50000 years but longer than that and its level becomes too low to detect reliably. • Other isotopes can be used to determine the age of objects much older or younger than 50000 years. For example, hydrogen-3 (tritium) is reliable for 1 to 100 year measurements. Uranium-238 is good from 107 years back to the oldest Earth samples. Applications of Half-lives (11.4)

  14. A piece of wood from an Egyptian tomb has carbon-14 activity of 980 counts per hour. A piece of new wood of the same size gave 3920 counts per hour. What is the age of the wood from the tomb? • First figure out how many half-lives have passed. Notice that 980 is ¼ of 3920. That means two half-lives have passed. (980 is one-fourth of 3920: • ½ x ½ = ¼) • Since each half-life for carbon-14 is 5730 years, the time elapsed is 2 x 5730 years = 11460 years. Radioisotopic Dating Example (11.4)

  15. End of Unit 31 (and all of the) Slide Shows