VO Atomic bombs, nuclear power plants, and the sun are powered as the result of releasing of nuclear energy.

Transcription

1 Physics and Chemistry 1501 Nuclear Science Part I Atomic bombs, nuclear power plants, and the sun are powered as the result of releasing of nuclear energy. (Read objectives on screen.) In this program, we re going to take a look at a fascinating, mysterious, and sometimes scary field of chemistry and physics, nuclear science. Why is the nucleus so mysterious? Well, in chemistry, you learned that the nucleus of the atom takes up only a few quadrillionths of the volume of the atom. In fact, if an atom were the size of a major league baseball stadium, the nucleus would be the size of a flea in the middle of the stadium. And yet, the nucleus contains just about all the mass of the atom. That s kind of bizarre when you think about it. But listen to this. In physics, you learned that like electrical charges repel. Well, the nucleus is made of positively charged protons and neutral neutrons. So the protons repel each other, right? Why doesn t the nucleus fly apart then? What holds it together? And another thing Sometimes, during radioactive decay, ELECTRONS come out of the nucleus. How did electrons get in there? In fact, over two hundred various types of particles have been detected coming out of the nucleus, other than just the basic protons and neutrons. Mysterious, huh? Today, we ll try to unravel some of the mysteries associated with the nucleus. Mind you, I said some. There is just too much information for me to give you anything more than an overview of nuclear science. You may want to do some more research on your own. Let s start with a description of the nucleus itself. What is packed into this extremely small space? So let s first describe the particles occupying the nucleus. (green chalkboard on screen) The particles that make up the nucleus are called nucleons. The two major nucleons are positively charged protons and neutral neutrons. Protons and neutrons have almost two thousand times the mass of an electron. In recent years scientists have discovered that protons and neutrons are not the basic building blocks of matter, but are made of smaller elementary particles called quarks and leptons. You may want to do some reading on quarks and leptons, which make up the over two hundred types of particles that have been detected coming out of the nucleus. But we re going to stick to the basic protons and neutrons in this program. Do you remember this notation from chemistry? It was used to tell the number of protons, neutrons, and electrons in an atom. Well, we re going to stick to nucleons only.

2 (atomic symbol on screen) The bottom number is the atomic number of the atom, the Z number, is the number of protons in the nucleus. This also is the charge of the nucleus. And the top number is the atomic mass, the A number, which equals the number of protons plus the number of neutrons in the nucleus. That is, of course, the total number of nucleons. Use this notation to tell your teacher the number of protons, neutrons, and nucleons in an aluminum nucleus. (Pause Tape Now graphic) (symbol on screen) The bottom number is the number of protons, so there are 13 protons in this nucleus. And the top number is the mass, which equals the number of protons plus neutrons. So we subtract 13 from 27 and get 14 neutrons. And since nucleons are both protons and neutrons, this number also is the number of nucleons: 27. Now tell your teacher which of these numbers can change and still represent an aluminum nucleus. The identity of an element depends on the number of protons in the nucleus, so this number cannot change. The top number can change. What if the mass number changed to 28? What would it mean? Tell your teacher. This time, we d have an aluminum nucleus with 13 protons and 15 neutrons, for a mass number of 28. Now here s another question for you. What do we call atoms of the same element with different numbers of neutrons? Tell your teacher. Did you remember isotopes? Isotopes are atoms of the same element with different numbers of neutrons and different masses. Isotopes aren t very important when you re studying chemical reactions, because one isotope of an element acts pretty much like another. But as far as nuclear behavior goes, isotopes are very important. Consider this. (atomic symbol on screen) Carbon-12 is the most common isotope of carbon, with six protons and six neutrons. This is a stable isotope. But carbon-14, with six protons and eight neutrons, is unstable. It is radioactive. You may have heard of the role of carbon-14 in determining the age of fossils through radioactive dating. No, radioactive dating is not having a blind date with the incredible Hulk. So a couple of neutrons can make a big difference in the stability of a nucleus. Why is that? And what do unstable isotopes do that others don t? Let s start with what makes an isotope unstable? It has something to do with the mystery of what holds protons together in the nucleus. It s not gravity, 2

3 and it certainly isn t an electrical force. Watch this. ( The strongest field force is not gravitational or electromagnetic, but rather a nuclear force called the strong force. As you know, the center of an atom is made up of positive protons and neutral neutrons. You also know that like charges repel one another. If it were not for the strong force, all of the positively charged protons would blow apart. Obviously, nuclear strong forces are more powerful than the electromagnetic force of the protons. The strong force is a force of attraction that holds nucleons together. This force is independent of charge. The strong force between two protons is the same as the strong force between two neutrons. And unlike electrical forces, the strong force acts only over a very short range. So a strong force only exists between the closest neighbors in large nuclei. In every nucleus, there are two kinds of forces acting against each other the strong force holding the neighboring nucleons together versus the electric repulsions of the protons trying to tear the nucleus apart. If the repulsions are greater than the strong forces, the nucleus will be unstable. Maybe a diagram will help you understand. (diagram on screen) In this diagram, the protons are red and the neutrons are blue. Now, look at protons A and B. They repel because they are both positively charged, and they attract because they are close neighbors. So the strong nuclear force is in effect. But look at protons A and C. They still repel because electrical forces are long-range. But they are too far apart for the strong forces to act. So they do not attract each other. All the protons in this nucleus repel all the other protons. But not all the protons attract through strong forces. This means that large nuclei experience more repulsions than attractions and tend to be unstable. Now, another factor in nuclear stability is the neutron to proton ratio of the nucleons. Look at these stable nuclei. For a small nucleus, such as carbon, the neutron to proton ratio is one to one. But as the number of protons in the nucleus increases, the number of neutrons has to increase even more to add enough strong forces to maintain stability. Think of neutrons as the glue that holds the protons together. This graph shows the neutron to proton ratio of stable nuclei. Each dot represents a stable nucleus. The straight line is a one to one ratio. You can see that smaller nuclei follow the one to one ratio, but the larger the nucleus, the higher the neutron to proton ratio has to be. For Z numbers greater than 82, the repulsive forces between protons cannot be overcome by the addition of more neutrons. So elements of atomic numbers 83 and above are not stable. 3

4 So very heavy isotopes and ones with the wrong neutron to proton ratio are unstable. We say that they are naturally radioactive. (green chalkboard on screen) A radioactive isotope has an unstable nucleus. To become more stable, a radioactive isotope emits a particle and changes into the nucleus of another element. This is called radioactive decay. Another term for radioactive decay is transmutation. In 1896, a French physicist, Henri Becquerel, discovered that compounds containing uranium emitted penetrating rays. Two years later, Marie and Pierre Curie were the first to isolate two undiscovered radioactive elements, polonium and radium. So what are these particles that radioactive atoms emit? Have you ever heard of alpha, beta, and gamma radiation? Well, alpha and beta particles are emitted during radioactive decay, and so are gamma rays, which, as you know, aren t particles but electromagnetic waves. You have a chart in your note-taking guide, and you need to fill it in now. (table on screen) The first type of radioactive emission is the alpha particle. The symbol is the Greek letter, alpha. I m going to show you another way of writing the symbol in a minute, so leave a space. An alpha particle is made up of two protons and two neutrons. This makes it identical to a helium nucleus. So the symbol for helium is often used, with a charge of two and a mass of four. You ll use this notation in writing and balancing nuclear reaction equations, so it s important to know. The next particle is the beta particle, with the Greek letter beta as it symbol. A beta particle is actually an electron. You ll see how an electron can come out of the nucleus later. Another way of writing the symbol is e for electron, with the charge of negative one at the bottom and a mass of zero, since the electron is so light its mass is negligible. There is another type of beta emission called a positron, but we re going to keep things simple and omit it. Beta particles have higher penetrating power than alpha. They go right through paper and are stopped by a sheet of aluminum. The third type of radioactive emission is the gamma ray. We use the Greek letter gamma as its symbol. Since gamma rays are energy, not matter, there is no charge or mass. Gamma rays go right through aluminum but are stopped by a thick sheet of lead. What happens to an element that undergoes radioactive decay? It changes into another element. And if that nucleus is still unstable, another radioactive particle will be emitted as the element changes into yet another element. We can write equations to show what happens to both the charge and mass of the nucleus. Let s try an example together. (equations on screen) This isotope of polonium is unstable because it s too large. Remember that elements over atomic 4

5 number 82 are radioactive. And many of them emit alpha particles, as this one does. We write the symbol for alpha as helium, with a charge of two and a mass of four. To find the element that polonium becomes, we just balance the equation, top and bottom, left and right. So the mass of our new isotope is 214, and the charge of the nucleus is 82. This means that this particular atom has 82 protons, and its atomic number is 82. The periodic table shows us that the element is lead. Our next example involves beta decay. This isotope of lead is unstable because its neutron to proton ratio is wrong. So it emits a beta particle, which is an electron with a charge of negative one and a mass of zero. When we balance the top and bottom numbers on the left and right of the arrow, we get a new element with an atomic number of 83 and the same mass as before, 209. But the element has changed to atomic number 83, which is bismuth. Now let s answer the question, How does an electron come out of the nucleus? During beta decay, a neutron changes into a proton. And our balanced equation shows that a particle with no mass and a charge of negative one is produced. That s a beta particle, or an electron. You ll notice that we didn t show you an example of gamma decay. The reason is that when gamma rays are emitted, neither the charge nor the mass of the nucleus is affected. Remember that gamma radiation is a type of electromagnetic radiation, like heat and light. Often, gamma rays are given off during alpha and beta decay, as the nucleus becomes more stable and gives off energy. Now, before we can talk about the uses and consequences of nuclear radiation, you need to learn about half-life. (green chalkboard on screen) Half-life is defined as the time it takes for half the mass of a radioactive sample to decay into another element. Half-lives range from a millionth of a second for extremely unstable nuclei to billions of years for elements such as uranium 238. The half-life of an element is a constant number that cannot be changed by external conditions, no matter how drastic. Factors such as chemical reactions, temperature, and pressure cannot alter the rate at which a nucleus decays. Of course, the shorter the half-life of an isotope, the faster it decays and the more alpha, beta, and gamma rays are given off each second. This makes these isotopes dangerous since exposure to radiation greater than normal background radiation can do damage to our cells. And unlike harmful chemicals that can be neutralized by chemical reactions, there s nothing we can do to neutralize a highly radioactive isotope. We just have to get them away from us, which is a big environmental problem. Do we bury them, sink them to the bottom of the ocean, launch them into space or what? Maybe some of you can come up with answers in the future. But let s talk about some good uses for radioactive isotopes. The radiation given off by these isotopes can be detected by various devices like Geiger counters. This means that small amounts of radioactive isotopes can be used as tracers in scientific research, industry, and medical diagnosis. And greater concentrations can be used in medical treatments for cancer because cancer cells are 5

6 killed by radiation before healthy cells. And another use for radioactive isotopes is in determining the age of rocks and fossils. For example, uranium-238 undergoes a series of transmutations, ending in the stable, but uncommon isotope, lead By measuring the percentages of these two isotopes in a rock, and using the half-life of U-238, the age of the rock can be calculated. Rocks dated by this method have been found to be as old as 3.7 billion years. That s really older than dirt. In the next program you ll learn about artificial radioactivity, fission, and fusion. But for now, let s Show What You Know.. Write down your choice for each question. Your teacher will go over the correct answers with you. (Read Show What You Know questions on screen) 6