Chapter 19
1 RADIOACTIVITY Radioactivity is the emission of high energy released when the of atoms change. Radioactivity can be or. TYPES OF RADIATION OR EMITTED ENERGY IN NUCLEAR CHANGES Radiation is the generic name for. Radiation does not imply nuclear radiation. Do microwaves give off radiation?. Type of Nuclear Radiation actual particle and symbol á â - â + ã proton neutron In general, the higher the charge the the penetration particle air penetration stopped by ability to ionize molecules á â ã few cm few m long distances
2 MAGIC NUMBERS Listed below are the stable (non-radioactive) isotopes of the first 40 elements: number of protons number of neutrons number of protons number of neutrons 1 0,1 21 24 2 1,2 22 24,25,26,27,28 3 3,4 23 28 4 5 24 26,28,29,30 5 5,6 25 30 6 6,7 26 28,30,31,32 7 7,8 27 32 8 8,9,10 28 30,32,33,34,36 9 10 29 34,36 10 10,11,12 30 34,36,37,38,40 11 12 31 38,40 12 12,13,14 32 38,40,41,42,44 13 14 33 42 14 14,15,16 34 40,42,43,44,46,48 15 16 35 44,46 16 16,17,18,20 36 42,44,46,47,48,50 17 18,20 37 46,48 18 18,20,22 38 46,48,49,50 19 20,22 39 50 20 20,22,23,24,26,28 40 50,51,52,54,56
3 # protons # neutrons # stable isotopes known even even 140 even odd 53 odd even 48 odd odd 5 total 246 Most naturally occurring isotopes of elements have of either protons or neutrons. There are over 3000 known isotopes of the elements. Therefore, most isotopes are. Many of the unstable isotopes have even numbers of protons and or neutrons, so having even numbers of protons and or neutrons guarantee that an isotope is stable. The isotopes that are stable may actually be unstable (radioactive) but with extremely long half lives so that their decays haven t been observed. Particular numbers of protons and neutrons are especially stable: 2,8,20,28,50,82, 114 protons; 2,8,20,28,50,82,126, and 184 for. These are called the numbers. 40 Example 1: Would you expect Ca to be stable?
4 BAND OF STABILITY As the number of protons increase, the number of neutrons in stable nuclei. The last element that has a non-radioactive naturally occurring isotope is. The last element that has a stable isotope with equal numbers of protons and neutrons is. 60 Zn is radioactive because the number of neutrons is. 69 Cu is radioactive because the number of neutrons is. 68 As is radioactive because the number of neutrons is.
5 BALANCING NUCLEAR EQUATIONS 1. The sum of the numbers must be equal on both sides of the equation. 2. The sum of the numbers must be equal on both sides. 3. The chemical reactant in a nuclear reaction is often called a and the chemical products are called. Example 1: n ------> p (Neutrons have a half life of about 15 minutes when free. Protons in the nucleus act to stabilize neutrons. When the number of neutrons/protons becomes too high, neutrons tend to decay even within the nucleus.) 59 - Example 2: Fe -------> â + What happens to the n/p ratio? 210 Example 2: Po > á + What happens to the n/p ratio? 40 40 Example 3: K + -----> Ar What happens to the n/p ratio? process á â â ã electron capture (EC, or K -capture) - + n/p ratio increases decreases increases no change increases Positron emission and EC are often competing processes and often occur simultaneously but with differing probabilities. Alpha decay only happens when the nucleus is very large.
6 Examples: 60 69 68 Write decay schemes for Zn, Cu, and As that ultimately produce a stable nuclide. FUSION AND FISSION Fission, the breaking of nucleus into ones of similar. Fission can be classified as induced (fission induced by a collision with another particle, typically because ) or spontaneous (fission that occurs without the nucleus being hit by another particle). The spontaneous fission rate of most nuclides is. Most nuclides do not react by the process of induced fission. Nuclear weapons use nuclides that exhibit fission. The only naturally occurring nuclide that reacts by the induced fission process is. nuclide % abundance induced fission? 238 U 99.3 no 235 U 0.7 yes 235 94 139 U ------> Kr + Ba + 2 n + energy Fission is not the same as decay. 238 234 U --------> Th + á Why is this not fission.
7 Fusion, the combining of nuclei into ones, occurs with elements: DECAY MECHANISMS 2 2 4 H + H -------> He + energy Just like chemical reactions, nuclear reactions can occur in a single step or as a series of elementary reactions. Overall Rxn: Rate determining step:
8 HALF LIFE AND ENERGY RELEASED As a general rule (but there are exceptions), the larger the amount of energy released by the reaction the the half life. You can only compare the energies of two reactions undergoing the same type of decay when making comparisons. You can t compare an á decay - + and a â decay or an EC with a â, etc. Look at the decay scheme on page 7. Which á decay is expected to give off the most energy? - Which â decay gives off the least amount of energy?. The shorter the half life, the dangerous the isotope is to us because 222 238. Rn is formed from U in the soil. Which substance poses a greater health risk?
9 RATE OF DECAY Most radioactive decay reactions follow order kinetics. rate laws t 1/2 Half life of decay reactions is Unlike chemical reactions, k is not dependent on external factors such as 90 Example: Sr is radioactive and present in radioactive fallout produced during the fission of 235 U that was used in some of the first nuclear bombs. It has a half life of 29 years. What fraction of the original Sr-90 that was formed during an above ground nuclear test explosion occurring in 1950 still remains today? RADIOMETRIC DATING 14 Example: Radioactive C is produced in the atmosphere by the capture of a neutron by 14 N. 14 14 The half life of C is 5730 years. The C may unite with oxygen to form radioactive CO 2 that enters the food chain and becomes incorporated into living matter. When the living matter dies 14 no new C is incorporated. The radiometric clock is now set. As time increases the amount of 14 14 C is reduced. By measuring the amount of C left we get an estimate of the amount of time that has elapsed since the living matter. 14 How old is a wooded bowl if it contains 1/25 as much C as found in wood presently?
10 SOURCE OF RELEASED ENERGY - BINDING ENERGY E = 8 c = 2.9979 x 10 m/s Example 1: If a penny (3 grams) could be converted totally to energy, how much energy would it produce? How does this compare to the energy released during chemical changes? BINDING ENERGY Binding energy is another energy (like lattice energy, hydration energy) that refers to the energy involved in a specific template reaction: These reactions are always and the binding energy is always. particle mass amu 4 He 4.00260 p 1.00728 n 1.00866 - e 0.00054858 1gram = 6.022e23 a m u 1 H 1.0078 Total BE: 1 M e V = 1.60217646E-13 J 1 amu = 931.46 MeV BE/nucleon
11 BINDING ENERGY AS A FUNCTION OF ELEMENT So why hasn t everything converted into Fe?
12 Example: is involved? Is the reaction below, endo or exothermic? How much energy, in units of MeV, Alpha Emission: Nuclide 222 Rn 218 Po atomic mass (amu) 222.0175705 218.0089658 Being lighter, the á particle carries most of the energy given off. How does this energy compare to the activation energy of typical reactions and to the bond strength of typical bonds?. What is likely to happen if a molecule is hit by one of the á particles? Look at page 6: â emission: - 69 Cu EC: 68 As â + 68 As + In order for â emission to occur, the mass defect must be greater than amu..