Nuclear Chemistry. Chapter 24

Transcription

1 Nuclear Chemistry Chapter 24 1

2 Review (don t copy) All the chemistry we ve discussed so far has involved electrons. Questions: 1. If you tossed 128 coins in the air, about how many would you expect to land heads-up? 2. What do the mass number and atomic number represent? 3. Which subatomic particles are found in the nucleus? 2

3 Radioactivity Antoine Henri Becquerel (1896) experimented with uranium salts and discovered radioactivity Radioactivity (or nuclear decay): an unstable nucleus emits charged particles and energy Radioisotope: radioactive isotope - any atom that has an unstable nucleus. Examples: Uranium-238 (used in Becquerel s experiment) Carbon-14 (used often in radioactive dating) 3

4 Isotope symbols (Review. May copy) Isotopes are named using the element name followed by the mass number (see examples, slide #3) They can be called nuclides The symbol for isotopes includes the following parts Mass # on top Atomic # on bottom 238 U 92 Uranium C 6 Carbon Po Polonium-210 4

5 3 Types of Nuclear Radiation (In foldable. Don t copy) Nuclear radiation: charged particles and energy that are emitted from the nuclei of radioisotopes Radiation Type Symbol Charge Mass (amu) Common Source Penetrating Power Shielding Alpha particle 4 2 He a, 2+ 4 Radium- 226 Low Paper, clothing Beta particle 0 1 b, e 1- Carbon-14 Medium Metal foil Gamma ray g 0 0 Cobalt-60 Very High Lead, concrete 5

6 Alpha Decay (don t copy) 4 2 He Alpha particle, a 2 protons and 2 neutrons Positively charged Same as He nucleus Least penetrating type of nuclear radiation Travel only centimeters in air Can be stopped by a sheet of paper or clothing 6

7 Beta Decay (don t copy) 0 1 e Beta particle, b 1 electron Negatively charged Produced by a neutron that decomposes into a proton and an electron More penetrating than a particles Pass through paper Stopped by a thin sheet of metal 7

8 Gamma Decay (don t copy) Gamma ray, g Penetrating ray of energy Like X-rays and light, only very short wavelength Most penetrating form of the three types discussed Often accompanies alpha or beta decay Several centimeters of lead or several meters of concrete required to stop it 8

9 Writing and Balancing Nuclear Reactions Similar to chemical equations, but isotope symbols are used. Reactants Products U Th 4 2 He Th Pa 0 e 1 g In a balanced nuclear equation: Mass # on the left = sum of mass #s on the right Atomic # on the left = sum of atomic #s on the right You will need to use your PERIODIC TABLES! 9

10 Example: Math Skills p. 295 Write a balanced nuclear equation for the alpha decay of polonium Step 1: Define reactants and products. Use letters to represent the unknown values. Let Z atomic #, A mass #, and X chemical symbolof Po 2He + A ZX product isotope. Step 2: Write and solve equations to find unknown atomic and mass #s. 210 A 4 A Z Step 3: Look up the element symbol on the periodic table using the atomic #. Atomic # 82 = Pb (Lead) Step 4: Write the balanced nuclear equation and double-check your solution Po He Pb Z

11 Effects of Nuclear Radiation Background radiation: naturally occurring in the environment Sources: Radioisotopes in air, water, rocks & living things Cosmic radiation Generally at safe levels Nuclear radiation can ionize atoms. At levels significantly above background, this can damage DNA and proteins Which type of nuclear radiation is the least harmful? Which the most? 11

12 Detecting Nuclear Radiation (don t copy) Geiger counters Use gas-filled tubes to measure ionizing radiation Gas produces an electric current when exposed to ionizing radiation Film badges Photographic film wrapped in paper Film is exposed with exposure to radiation like photographic film is exposed with exposure to visible light 12

13 Rate of Nuclear Decay Nuclear decay rate describes how fast nuclear changes take place Nuclear decay rate stays the same for a given radioisotope Half-life: the time required for half of a radioisotope sample to decay 13

14 Radioisotope Remaining (%) Rates of Nuclear Decay (cont d) Nuclear Decay Rate Time (# of Half-lives) 14

15 Rates of Nuclear Decay (cont d) Different radioisotopes have different half-lives To determine how many halflives have elapsed for a sample, divide the total time of decay by the half-life Known decay rates are used in radioactive dating Radioisotope Radon-222 Iodine-131 Carbon-14 Thorium-230 Uranium-238 Half-life 3.82 days 8.07 days 5730 years 75,200 years 4.47x10 9 years 15

16 Radiocarbon dating (don t copy) Carbon-14 exists naturally in the atmosphere at a fairly constant ratio to C-12 CO 2 absorbed while living (including some C-14) As C-14 decays, it s replaced by C-14 absorbed from atmosphere Tree dies no more CO 2 absorbed to replace decaying C- 14 Age of fossil determines by comparing C-14/C-12 ratio in fossil to atmospheric ratio 16

17 Radiocarbon dating (cont d) Used for objects less than 50,000 years old For older objects, must use different isotope with longer half-life Think about it: (don t copy) What isotopes would work well to date a rock formation that is thought to be close to a trillion years old? 17

18 Artificial Transmutation (write definition only) Transmutation: conversion of atoms of one element into atoms of another Alchemists have attempted this for hundreds of years (but not through nuclear chemistry) First artificial transmutation: Ernest Rutherford (1919) turned nitrogen into oxygen-17 18

19 Artificial Transmutation (don t copy) Transmutation achieved by bombarding atomic nuclei with high-energy particles Protons, neutrons or alpha particles Example: Ernest Rutherford s transmutation used which particle? N 2He 8O 14 Transuranium elements Many produced by artificial transmutation of a lighter element All are radioactive 1 1 H 19

20 Nuclear Forces Strong nuclear forces: Electric forces: The strong nuclear force attracts protons and neutrons. Electric repulsions push protons apart. When a nucleus is large enough, the electric forces can overcome the strong nuclear forces. Nuclei are unstable at this point. Any atom with 83 or more protons is unstable and, therefore, radioactive. Small nucleus Large nucleus Proton from a small nucleus Proton from a large nucleus 20

21 (( Fission Fission: splitting of nucleus into two smaller parts Neutron Lise Meitner, Fritz Strassman and Otto Hahn s experiments (1939) first demonstrated nuclear fission. A small amount of the original mass is converted into 91 a lot of energy Kr (( 36 (( )) Energy U 92U (very unstable) Ba

22 Fission (cont d) About how much energy was released from 6.2 kg of Plutonium-239 in the second atomic bomb explosion? (Note: Only about 1 kg underwent fission the rest was scattered.) m 1kg c E mc 2 8 m s m s kg J 2 This quantity = 2.5 x kwh, or enough energy to power my house for over 3.6 million years! 22

23 Fission and Chain Reactions Fission can result in a chain reaction. Neutron Neutrons released from the first reaction can trigger another reaction, and so on similar to 142 a rumor spreading Kr U Energy 91 36Kr 56Ba Energy U Energy Kr 56Ba U Ba Energy Kr 56Ba U 23

24 Chain Reactions (cont d) For a chain reaction to happen, each split nucleus must produce at least one neutron with enough energy to split another nucleus This only happens when a specific mass of fissionable material is available called the critical mass. Controlled chain reactions are used to generate electricity in nuclear power plants. Uncontrolled chain reactions are used in nuclear weapons 24

25 Nuclear Fusion Fusion: nuclei of two atoms combine 2 1H 3 H The sun and other stars are powered by fusion of H into He Requires extremely HIGH temperatures What state is matter in at such high temperatures? PLASMA 4 1 He + + ENERGY (17.6 MeV) n Fusion 25