Nuclear Chemistry

1. Nuclear Chemistry Just the basics…. By J.M.Soltmann

2. What is Nuclear Chemistry • As its name implies, nuclear chemistry is the study of the nucleus and reactions between nuclei. • Remember that virtually all of the mass of an atom resides in the nucleus, as does all of the positive charge. • Nuclear energy is a much greater form of energy than bond energy.

3. Radioactivity • While most nuclei are stable, many nuclei are unstable and spontaneously emit particles and electromagnetic radiation. • These nuclei are refered to as radionuclides.

4. Nuclear Equations • In a nuclear equation, mass numbers and atomic numbers are balanced instead of elements. • The example here to the right depicts a radioactive decay; specifically an alpha decay. • The helium ion is called an alpha particle.

5. 3 Common types of Radioactive Decay • Alpha decay • Beta decay - a ß- particle is a subatomic nuclear particle essentially equivalent to an electron and a ß+ particle is a positively charge electron, called a positron. • Gamma decay - high energy photons are emitted which have virtually no mass nor charge.

6. Nuclear electrons? • Modern theory has shown that a neutron is actually comprised of a proton and an electron. • So, if a nucleus emits an electron, it has really transformed a neutron into a proton. • Also, if a nucleus absorbs an electron, it will convert a proton into a neutron.

7. Common Particles in Nuclear Reactions • Neutrons (10n) • Protons (11p or 11H) • Electrons (0-1e) • Alpha Particles (42He or 42) • Beta- Particles (0-1e or 0-1) • Gamma (00) - Gamma radiation consists of high-energy photons, with a mass far too little for consideration. • Positron (01e) - A positron is a positively charged electron. It has the mass of an electron but a positive charge.

8. Differentiating the Radiations • Alpha emissions are the heaviest and thus have the least penetrating power. • Beta emissions have masses much smaller than protons or neutrons, so they have more penetrating power. In terms of penetrating power, =100* . • Gamma emissions have essentially no mass, so they are the most powerful. In terms of penetrating power.  = 100* .

9. Try this • Write a nuclear equation for the process when mercury-201 undergoes electron capture.

10. To answer this question: • First we have to understand what mercury-201 is. Since mercury is always atomic number 80, this isotope is 20180Hg. • Since we are capturing an electron, the electron must be a reactant. • Now we add up mass numbers and atomic numbers. (201 + 0 = 201 and 80 + -1 =79). • Element 79 is gold, so the answer is: • 20180Hg + 0-1e -->20179Au

11. Try another • Thorium-231 decays into protactinium-231. • What is the balanced equation? • What other particle(s) is/are involved in the reaction?

12. The answers are: • 23190Th --> 23191Pa + 0-1e • The extra particle is an electron, but because it is being emitted, it would be called a Beta emission.

13. Nuclear Transformations • The first manmade conversion of one nucleus into another was performed by Sir Ernest Rutherford (1919). • Rutherford bombarded a nitrogen-14 atom with alpha particles to produce an oxygen-17 atom plus a proton. • 147N + 42He --> 178O + 11H • The shorthand version of this reaction is • 147N(,p)178O WHY???

14. Now try this one: • Write the balanced nuclear equation for the process noted by the shorthand: • 2713Al(n,)2411Na

15. Now try this one: • Write the balanced nuclear equation for the process noted by the shorthand: • 2713Al(n,)2411Na • 2713Al + 10n --> 2411Na + 42He

16. Nuclear Stability • Why are some nuclei more stable than others? • To be honest, there are several factors, most of which are beyond the scope of this course. • However, there are a few easy to see indications of nuclear stability.

17. Did you ever wonder…? • We know that like charges repel each other, yet a nucleus can have dozens of positively charged protons held together. Why? • Neutrons are a major reason. All nuclei with 2 or more protons have neutrons. The neutrons and the protons meld by a force of nature, different than gravity or electromagnetism, called the strong (nuclear) force. • Because of the way this force binds the protons and neutrons together, the ratio of protons to neutrons is an issue.

18. Smaller Atoms vs Bigger Atoms • In smaller atoms, most stable atoms have neutron to proton ratios of about 1.00. • As isotopes increase in atomic number, most stable isotopes have increasingly larger ratios of neutrons to protons. • To our knowledge, any isotope with an atomic number greater than or equal to 84 would be radioactive.

19. Some stability trends • Of the 265 known stable isotopes: • 157 of them have even numbers of protons and neutrons. • 53 of them have an even number of protons but an odd number of neutrons. • 50 of them have an odd number of protons but an even number of neutrons. • Only 5 of them have odd numbers of both protons and neutrons.

20. Magic Numbers • For some reason, nuclei with 2,8,20,28,50 or 82 protons and/or 2,8,20,28,50,82, or 126 neutrons are generally more stable than isotopes without these numbers. • When we think of substances that shield radiation, we tend to think of lead. The most common isotope of lead is 20882Pb; that means it has 82 protons and 126 neutrons.

21. Decays and Half-lifes • When a radioactive substance decays, the amount of that particular isotope will decrease. • We call the rate of decay the half-life, because it is the time needed for exactly 1/2 of the isotope to decay.

22. More on Half-life • If we examine the graph to the right, we see that we started with 50 g of the isotope. Each subsequent point represents half of the previous mass (50 to 25 to 12.5 to 6.25 to 3.125 to 1.5625 to .78125). • Each point is approximately 24 days apart; The half-life for this substance is 24 days.

23. Calculations with half-life • Although it is possible to determine the amount remaining of a radioisotope using natural logs { ln(Nt/N0)=-kt } we do not need to do this. • We only work with whole number increments of the half-life.

24. For example • The half-life of an isotope is 8 days. If we start with 100 grams of the isotope, how much is present in 32 days? • 32 days/(8 days/half-life) = 4 half-lives. • Each half-life divides the previous mass in half. • 100g/2 = 50g/2 = 25g/2 = 12.5g/2 = 6.25g • There would be 6.25 g of that isotope left.

25. You try one • The half-life of Bismuth-211 is 185 years. How much time would it take for a 360 g sample to decay to 11.25 g?

26. You try one • The half-life of Bismuth-211 is 185 years. How much time would it take for a 360 g sample to decay to 11.25 g? • 360g/2=180g/2=90g/2=45g/2=22.5g/2=11.25g. • That is 5 half-lives. • 5 half-lives*185 years/half-life = 925 years.

27. Fusion vs. Fission • Fission and Fusion are two types of highly exothermic nuclear reactions, different than the decays covered earlier. • Fusion means to bring two smaller nuclei together to make a larger nucleus. • 136C + 136C --> 2511Na + 11p • Fission means to break a larger nucleus into 2 or more smaller nuclei. • 23592U + 10n --> 13752Te + 9740Zr + 2 10n

28. How much Energy are we talking about? • In fusion and fission, a tiny, almost meaningless mass of each affected nucleus is converted to energy. • Einstein theorized that the amount of energy was dependent on the mass lost and the square of the speed of light. E = mc2.

29. So, how much energy is that? • Well if each uranium atom in a given fission process loses the mass of one electron (9.11x10-31 kg): • E = mc2 • E = (9.11x10-31 kg) *(3.0x108 m/s)2 • E = 8.2x10-14 J

30. But that seems like a small number! • 8.2x10-14 J is a small amount, but that was for just one atom or uranium. If we had 1 mg of uranium (about the mass of a cystal of salt), that would contain roughly 2.5 x 1018 atoms of uranium. • 8.2x10-14 J/atom * 2.5 x 1018 atoms =2.1x105 J • That is almost enough energy to handle the electrical needs of this school for a day - from a tiny starting mass.

31. Think about this • If we could convert the .25 kg mass of a banana peel (using our Mr. Fusion power supply) into pure energy, • E = mc2 • E = (.25 kg)*(3.0x108 m/s)2 • E = 2.25x1016 J • That’s enough energy to run New York City for a year (with enough energy left over to go Back to the Future)!