Inside the Atom. Every person is different. What are some things that make one person look different from another person?

Transcription

1 chapter 14 3 Inside the Atom section 2 The Nucleus Before You Read Every person is different. What are some things that make one person look different from another person? What You ll Learn what radioactive decay is what half-life means how radioactive isotopes are used Read to Learn Identifying Numbers The electron cloud model is an example of what an average nuclear atom looks like. But what makes atoms of different elements different? The atoms of different elements have different numbers of protons. The atomic number of an element is the number of protons in the nucleus of an atom of that element. The element hydrogen has the smallest atomic number. It has only one proton in its nucleus, so hydrogen s atomic number is 1. The element uranium has the greatest atomic number of a naturally occurring element. It has 92 protons in its nucleus. Its atomic number is 92. How many neutrons are in the nucleus? A certain type of atom can have different numbers of neutrons in its nucleus. For example, most carbon atoms have six protons and six neutrons. But, some have seven or eight neutrons. All of these atoms are carbon atoms because they all have six protons. These carbon atoms with different numbers of neutrons are called isotopes. Isotopes (I suh tohps) are atoms of the same element that have different numbers of neutrons. Carbon-12 is an isotope that has 6 protons and 6 neutrons. Carbon-13 has 6 protons and 7 neutrons. Carbon-14 has 6 protons and 8 neutrons. Together, the protons and neutrons make up most of the mass in an atom. Highlight Main Ideas As you read, highlight the main ideas under each heading. After you finish reading, review the main ideas of the lesson. C Build Vocabulary Make the following Foldable and write the definitions of each of the terms from this lesson. Half-Life Reading Essentials 215

2 What is the mass number? The mass number of an isotope is the number of neutrons plus protons. The table below shows the mass number and particles for the isotopes of carbon. For example, carbon-12 has six protons and six neutrons, so its mass number is 12. Notice that all isotopes of carbon have six protons. The atomic number of carbon is 6. Applying Math 1. Evaluate How many more neutrons does Carbon-14 have than Carbon-12? Isotopes of Carbon Mass number 4 n-14 Number of proto 6 Number of neutr 8 Number of electr 6 Atomic number 6 2. Explain What keeps the protons in an atom from repelling each other? What is the strong nuclear force? How do you hold things together? You might use tape or glue. What holds the protons and neutrons together in the nucleus of an atom? Remember that protons have a positive charge. You might think that the protons would repel each other. But, when the protons and neutrons in a nucleus are tightly packed together, an even stronger binding force takes over. This force is called the strong nuclear force. The strong nuclear force keeps the protons from repelling each other. This force works only in the nucleus of an atom. Radioactive Decay The nuclei of most elements are stable because they have about the same number of protons as neutrons. For example, carbon-12 is stable because its atoms have six protons and six neutrons. Some nuclei are unstable because they have too many or too few neutrons. So the particles in the nucleus try to repel each other. The nucleus must eject, or release, a particle to become stable. When a nucleus lets a particle go, it gives off energy. Radioactive decay is the release of particles and energy from the nucleus. 216 Inside the Atom

3 Transmutation When protons are released from the nucleus, the atomic number of the atom changes. So one element changes into another. Transmutation is the changing of one element into another through radioactive decay. What happens when alpha particles are lost? Most smoke detectors contain the element americium-241 (a muh RIH shee um). This element is unstable and undergoes radioactive decay. Americium-241 transmutates into another element by ejecting an alpha particle and energy. An alpha particle is a particle that is made up of two protons and two neutrons. The energy and alpha particle that are ejected are called nuclear radiation. In a smoke detector, the alpha particles make it possible for the air to conduct an electric current. As long as the electric current flows, the detector is silent. Smoke will interrupt the flow of the electric current and the alarm will go off. 3. Explain What happens to an element if it undergoes transmutation? 4. Define What are alpha particles made of? How does an element change its identity? Americium has 95 protons. After transmutation, it only has 93 protons and becomes the element neptunium. Neptunium has an atomic number of 93. Notice in the figure that the mass and atomic numbers of neptunium and the alpha particle add up to the mass and atomic number of americium. No particles were destroyed during transmutation. Americium 95 p 146 n Neptunium 93 p 144 n What are beta particles? Not all transmutations cause the nucleus to eject an alpha particle. Some eject an electron called a beta particle. A beta particle is a high-energy electron that comes from the nucleus, not the electron cloud. But, the nucleus contains only protons and neutrons. How can it give off an electron? In this kind of transmutation, a neutron becomes unstable. It splits into an electron and a proton. The electron, or beta particle, is ejected with a large amount of energy. p n n p Alpha particle 2 p 2 n Applying Math 5. Calculate The element actinium has an atomic number of 89. How many alpha particles would americium need to lose before it became actinium? Reading Essentials 217

4 What happens to the proton? After the electron is ejected, the proton stays in the nucleus. Now there is one more proton in the nucleus and the atomic number increases by one. The figure shows unstable hydrogen-3. One neutron splits into a proton and an electron. The electron (e ), or beta particle, is ejected. Now the nucleus has two protons. Hydrogen-3 turns into helium-3. The mass of the atom stays almost the same because the mass of the electron it loses is so small. Picture This 6. Identify Circle the beta particle in this diagram e Hydrogen-3 Helium-3 e Applying Math 7. Calculate If you start with 4 g of iodine-131, how many grams would there be after 24 days? Show your work. 218 Inside the Atom Rate of Decay Have you ever watched popcorn pop? You never know which kernel will pop next. But, if you have popped a lot of popcorn before, you might be able to predict how long it will take for half the kernels to pop. Radioactive decay also is random. That s why radioactive decay is measured using its half-life. The half-life of a radioactive element is the amount of time it takes for half of a sample of the element to decay. How do you calculate half-life decay? The half-lives of radioactive isotopes range from fractions of a second to billions of years. Iodine-131 has a half-life of eight days. If you start with 4 g of iodine-131, after eight days you have only half the amount, or 2 g. After eight more days, you have only 1 g. The radioactive decay of unstable atoms happens at a steady rate that nothing can change. How are objects dated using half-life? Scientists use radioactive decay to find the age of fossils. Carbon-14 is a radioactive isotope of carbon. Its half-life is 5,730 years. It is used to find the age of dead animals, plants, and humans. Living things have carbon-14 in them because they take in and release carbon. When a living thing dies, the amount of carbon-14 inside it begins to decrease because of radioactive decay. Scientists can measure the amount of carbon-14 in an ancient item. Using the half-life of carbon-14, scientists can calculate when the animal, plant, or human lived.

5 Why are long half-lives sometimes a problem? Some radioactive isotopes have half-lives that are thousands, millions, or billions of years. Waste products that have these isotopes can be dangerous because they still release radiation. These waste products must be kept away from people and the environment. Special disposal sites are used to store this waste for long periods. Many of these sites are deep underground. Making Synthetic Elements There are only 92 elements found in nature. Other elements are made through transmutation. Scientists can smash alpha and beta particles into the nuclei of existing atoms to make new elements. Since these new elements are made by humans, they are called synthetic. Synthetic elements have greater numbers of protons and neutrons. They have atomic numbers greater than 92. What are the uses of radioactive isotopes? Scientists have made many useful isotopes. These isotopes, called tracer elements, can be placed in the body or released into the air. Then, scientists can use instruments to look for radiation while the tracer elements decay. Tracer elements have been used to diagnose diseases and study the environment. Tracer elements with short half-lives are the best to use. The short half-lives do not expose living organisms to radiation for long periods of time. Diagnose Diseases Iodine is an element that is used by the thyroid gland. Radioactive iodine-131 can be given to a patient with a thyroid problem. The tracer element is absorbed by the thyroid gland. The radiation can create a picture of the thyroid. Doctors can then find out if the patient s thyroid is working properly. Study the Environment Tracer elements are used in the environment, too. Scientists inject them into the roots of plants to see how the roots absorb food. Others are put into pesticides. The tracer elements can then be followed to find out what happens to the pesticide in the environment. 8. Define What are synthetic elements? 9. Infer Why might we want to know what happens to pesticides in the environment? Reading Essentials 219

6 After You Read Mini Glossary atomic number: the number of protons in the nucleus of an atom of an element beta particle: a high-energy electron that comes from the nucleus, not the electron cloud half-life: the amount of time it takes for half of a sample of the element to decay isotopes: atoms of the same element that have different numbers of neutrons mass number: the number of neutrons plus protons radioactive decay: the release of particles and energy from the nucleus transmutation: the changing of one element into another through radioactive decay 1. Review the terms and their definitions in the Mini Glossary. Explain why the mass number and atomic number of an element are different. Use complete sentences. 2. Complete the table to explain what causes radioactive decay and what happens when particles are ejected. Radioactive Decay Type of Particle Released Decay Process Result Alpha particle Beta particle A neutron in the nucleus splits into a proton and an electron. The electron, or beta particle, is ejected. The atom changes into a different element. The atom now has a lower atomic number. End of Section 220 Inside the Atom Visit blue.msscience.com to access your textbook, interactive games, and projects to help you learn more about the nucleus of an atom.