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Special Topics for SOL 2 1 st Power Point Nuclear Chemistry (Chap 28)

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1 Special Topics for SOL 2 1 st Power Point Nuclear Chemistry (Chap 28)

2 Objective A What is radioactivity? What is radioactivity? What is radiation? What is radiation? What is radioactive decay? What is radioactive decay? X-ray is a type of radiation (that’s what “ray” means). “X” just means unknown, because when it was discovered it was something new, and so it was called an X-ray. The name stuck.

3 Objective A These things all sound really bad and really dangerous. These things all sound really bad and really dangerous. But they are not. But they are not. These things are all associated with nuclear reactions. These things are all associated with nuclear reactions. Nuclear reactor in India. The last nuclear reactor built in the US was constructed in 1977.

4 Objective A Nuclear reactions are just reactions that occur in the nucleus. Chemical reactions occur by electrons interacting with each other. Nuclear reactions are just reactions that occur in the nucleus. Chemical reactions occur by electrons interacting with each other. Nuclear reactions can release a lot of energy, as seen in the photograph. However, to do that, you need a “critical mass” of radioactive material. That’s usually quite a lot. Nuclear reactions can release a lot of energy, as seen in the photograph. However, to do that, you need a “critical mass” of radioactive material. That’s usually quite a lot. In a nuclear reaction, an element CAN change into another kind of element. This doesn’t violate Dalton’s theory, because it’s not a chemical reaction. In a nuclear reaction, an element CAN change into another kind of element. This doesn’t violate Dalton’s theory, because it’s not a chemical reaction. E = mc 2. c is the speed of light and c 2 is a HUGE number. So even a little bit of mass can convert to a LOT of energy.

5 Objective A Nuclear reactions occur for one reason and one reason only: the nucleus is unstable. They become stable by giving off radiation, and ultimately by changing into a more stable nucleus. When the nucleus becomes stable, the nuclear reaction is over.    238 U decays in a series of 14 steps to 206 Pb. Lead is stable.

6 Objective A Radioactive decay is the process by which unstable nuclei (plural of nucleus) become stable. They become stable by giving off radiation (particles or energy or both). Radiation or nuclear radioactivity comes in 3 types: alpha, beta and gamma. α = alpha β = beta γ = gamma

7 Objective B Alpha particles are the same as a helium nucleus. It has 2 protons and 2 neutrons. It has no electrons, and so it has a +2 charge. Since an α particle has 2 protons, Z = 2. Since it also has 2 neutrons, the mass number is 4.

8 Objective B So when a nucleus loses an alpha particle (called alpha radiation), it loses 2 protons and 2 neutrons. It changes into a DIFFERENT element. It’s atomic number decreases by 2. It’s mass decreases by 4. Z = 95Z = 93

9 Objective B Beta particles are basically just an electron. The mass number of a beta particle is 0. For the atomic number, we say that it is “-1.” We say it’s -1, because losing a beta particle causes the nucleus to GAIN a proton. We’ll see how in a little bit. Yes, it’s in German. Not all the best stuff is in English. Notice that an electron is “produced” when a neutron splits into a proton and an electron. Being negative, the electron is immediately spit out of the positive nucleus. “Strahlung” means radiation.

10 Objective B So when a nucleus loses a beta particle (called β radiation), it GAINS one proton and the mass remains the same. It changes into a DIFFERENT element (in this case from H to He). Don’t worry about the antineutrino. We won’t worry about that for now.

11 Objective B The third type of radiation is gamma radiation. Gamma is NOT a particle, like alpha or beta. Gamma rays (or γ radiation) are pure energy. It has 0 mass and the atomic number is 0 as well. Gamma radiation is often released along with alpha or beta radiation. The nucleus loses energy (it’s this energy that can be harnessed to do productive work…like nuclear power plants…or destructive things, like an atomic bomb). When the nucleus releases gamma radiation, the mass doesn’t change and the element’s identity doesn’t change either. Only alpha and beta radiation cause the element to turn into a different element.

12 Objective B Alpha particles have the LEAST amount of energy. Paper can stop alpha particles. Beta particles have more energy than alpha, but less than gamma. Aluminum foil or a thin piece of wood can stop beta particles. Gamma particles have the most energy by far. Several meters of concrete will stop them as will several centimeters of lead. They easily pass through the human body, of course.

13 Objective B You have to be able to write nuclear reactions. Luckily, they are very easy, if you can do some simple arithmetic. Alpha radiation Beta radiation 263 Sg  259 Rf + 4 α 14 C  14 N + 0 β Mass: 263 = Z: 106 = Mass: 14 = Z: 6 = 7 + (-1)

14 Objective C There are 109 elements, but over 1,500 different possible isotopes for those elements. Of those, only about 264 are stable. The rest decay, by some form radioactive decay, to BECOME stable.

15 Objective C Many other things have half lives of minutes or days or years or decades or centuries, or even millions of years. How do you know IF an isotope is stable. Let’s talk about the Band of Stability. The stable isotopes can be calculated using a simple formula. IsotopeHalf LifeInformation Carbon-145,730 yearsUsed in dating of ancient artifacts Tritium (H-3)12.3 yearsProduced from weapons testing Iodine daysUsed to treat thyroid disease Technetium x 10 5 years Beta decay product of Mo-99. Used for medical diagnoses. Used as a γ-free source of β particles.

16 The Band of Stability Find the value of # Neutrons / # Protons If that number is greater than or equal to 1 AND less than or equal to 1.5, the isotope is stable. So, anything less than 1 is unstable. Anything greater than 1.5 is unstable. #N/#P = 1.5

17 The Band of Stability This formula really only works for elements with an atomic number > 20. For example, 14 C is radioactive, but the ratio 8 N / 6 P = Elements with Z < 20 generally decay by β decay. Elements with Z > 83 are always radioactive. In other words, for Po (Polonium, named by Marie Curie for her native Poland) and higher there are NO naturally occurring stable isotopes. Elements with a HIGH atomic number that are unstable usually have too many neutrons. They decay by α decay, primarily until they reach 206 Pb, which is a stable isotope of lead.

18 Objective C Transuranium elements are the elements that have an atomic number greater than U. All of the transuranium elements are man-made. Most of them only exist for a second or less and then they decay into something else. Many of them were made by Dr. Seaborg and his team of researchers. 93 Np for example decays by beta decay into 94 Pu, which then decays by α decay into 92 U. Uranium then decays in a series of 14 steps to 206 Pb. Know why Plutonium was abbreviated Pu and not Pl? Basically, because top scientists really are geeks. At least, so said Alex Trebek on Jeopardy! Remember when you were a kid, and you said “P-u.” That’s why. It probably made him laugh every time he thought of it.

19 Objective D Beta decay occurs when a neutron in the nucleus splits into a proton and an electron. You know there are no electrons in the nucleus. Where did it came from? How did it get there? (Z goes up by 1; mass stays the same.) Closely related is “positron emission,” also called Beta-plus decay. This occurs when a proton in the nucleus splits into a neutron and a positron. A positron has a positive charge and is about the same mass as an electron. (Z goes down by 1; mass stays the same.)

20 Objective D picture cropped Half life (t ½ ) is the time required for one half of the nuclei in a radioactive sample to decay. As we have seen, half lives can range from fractions of a section to billions of years.

21 El Samplo Problemo hey, I took French in high school Co-60 decreases from g to g in a period of 10.5 years. From this information, what is the half life. A fter one half life, half should have decayed g  g After two half lives, we find ourselves at the amount listed in the problem g  g (1 st half life) g  g (2 nd half life) So, 10.5 years = 2 half lives. Therefore 5.25 years = 1 half life. Easy!

22 Second Objective D (yes, it should be Objective E, but it’s not, but I just want you to know these two definitions) Nuclear fission is the splitting of a large nucleus into smaller fragments. They break apart to form smaller elements. U-235 combined with a neutron to split into Ba-144 and Kr neutrons. Those neutrons can then react with other U-235 atoms, causing a “chain reaction.” Note that =

23 Second Objective D (yes, it should be Objective E, but it’s not, but I just want you to know these two definitions) Nuclear fusion is when smaller nuclei combine to produce a larger nucleus. They “fuse” together, like if you take 2 pieces of clay and “mush” them together. There is some interesting stuff in the study guide on nuclear fusion and He-3 on the moon. None of that will be on the test, but it is kind of interesting. H-2 + He-3  p + He-4 + a LOT of energy… 25 tons of He-3 could replace MILLION tons of fossil fuels with no pollution and no harmful radiation!

24 The End


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