# Radioactivity and Nuclear Reactions

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Radioactivity and Nuclear Reactions
Read page 256. Complete Explore Activity on page 257. Complete Foldable on page 257.

Review What are atoms composed of? What is the charge of each of the particles? Where is each particle located? What is the atomic number? Atoms are composed of protons, neutrons, and electrons. Neutrons have no charge. Protons have a positive charge. Electrons have a negative charge. Protons and neutrons are found in the nucleus of the atom. Electrons are found surrounding the nucleus. The atomic number is the total number of protons in the nucleus. It gives the overall charge of the nucleus. The atomic weight (or mass number) tells you the number of protons + electrons. Usually, the number of electrons equals the number of protons and the atom’s charge is neutral. Read last paragraph on page Remind students of the Explore Activity.

The Strong Force The strong force causes protons and neutrons to be attracted to each other. The strong force is about 100 x greater than the electric force (the force that would cause the protons to repel each other). The strong force is a short range force that quickly becomes extremely weak as protons and neutrons get farther apart. Before 1st bullet: If opposites attract, the alikes should repel. How can the protons stay so close to each other in the very small nucleus? After 1st bullet: Read Teacher FYI on page 259. After 2nd bullet: However, protons and neutrons have to be close together, like they are in the nucleus, to be attracted by the strong force. After 3rd bullet: The electric force is a long-range force, so protons that are far apart still are repelled by the electric force. See Figure 3 on page 259. Read “Attraction and Repulsion” and “Forces in a Large Nucleus” on page 260.

All nuclei that contain more than 83 protons are radioactive. Almost all elements with more than 92 protons don’t exist naturally on Earth. They have been produced only in laboratories. Read 1st paragraph under “Radioactivity” on page 260. After 1st bullet: Large nuclei tend to be unstable and can break apart or decay. After 2nd bullet: Many other nuclei that contain less than 83 are also radioactive. Even some nuclei with only one or a few protons are radioactive. After 3rd bullet: These are called synthetic elements. They are unstable and decay soon after they are created. With pic: The first element to be prepared synthetically was technetium. The element was discovered in 1937 by the Italian mineralogist Carlo Perrier and the Italian-born American physicist E. G. Segrè (Palermo/Italy) in a sample of molybdenum that was bombarded with deuterons in a cyclotron at the Univ. of California at Berkeley. Read “Why is there an end to the periodic table”? on page 260.

Isotopes isotopes—nuclei having the same number of protons but different numbers of neutrons The atoms of isotopes of an element have the same number of electrons and the same chemical properties. Read 1st paragraph until mark on page 261. After 1st bullet: The element carbon has 3 isotopes. Nuclei with too many or too few neutrons compared to the number of protons are radioactive. Radioactive isotopes are sometimes called radioisotopes.

Nucleus Numbers This is a shorthand way to write the important information about an element. (mass # on top, symbol to the side, and atomic # on bottom) This isotope is called Carbon-12. The number of neutrons in the nucleus is the mass number – the atomic number. Compare to Carbon-14 Have the students complete Visual Learning on page 261.

Henri Becquerel accidentally discovered radioactivity in 1896. Pierre and Marie Curie, shown here in their laboratory in Paris, helped pioneer the study of radioactivity, unaware of the deadly hazards. Even with deteriorating health, the Curies continued their research. Much of today’s nuclear science is based on their work. Radioactivity cannot be detected with our senses. Read all of page 262. Homework: page 262 questions 1-6

Section 2: Nuclear Decay
When an unstable nucleus decays, particles and energy are emitted from the decaying nucleus. These particles and energy are called nuclear radiation. The three types of nuclear radiation are alpha, beta, and gamma radiation.

Alpha Particles An alpha particle is made of two protons and two neutrons bound together. charge: +2 atomic mass: 4 When alpha radiation occurs, an alpha particle is emitted from the decaying nucleus. After 1st bullet: It is actually the same as a helium nucleus. The symbol for an alpha particle is the same for a helium nucleus. At the end: Read about Smoke Detectors on page Read Visual Learning.

Transmutation in Alpha Radiation
When an atom loses an alpha particle, it no longer has the same number of protons, so it no longer is the same element. Transmutation is the process of changing one element to another through nuclear decay. After 1st bullet: If you change the number of protons, you change the whole element. In the example, we went from polonium-210 to lead Notice the change in atomic number. Also, notice that the charge of the original nucleus equals the charges of the new nucleus and the alpha particle combined.

Beta Particles Sometimes in an unstable nucleus, a neutron decays into a proton and releases an electron. beta particle: the electron emitted from the nucleus during beta decay Beta decay is caused by the weak force. After 3rd bullet: Remember the weak force is involved in the neutron decay process.

Transmutation in Beta Radiation
During beta decay, the atom now has one more proton so it becomes the element with an atomic number one greater than that of the original element. Since the neutron decays into a proton, the total number of protons and neutrons does not change during beta transmutation, so the atomic mass number stays the same.

Gamma Rays Gamma rays are a form of radiation called electromagnetic waves (so they carry energy). They have no mass and no charge. They travel at the speed of light and are usually released along with alpha and beta particles. Because gamma rays only transfer energy and not particles, no transmutation occurs. After 1st bullet: Remember waves carry energy not particles. After 4th bullet: This means that the element does not change (it remains the same element). The mass number and atomic number stay the same.

Comparison of the Three Types of Nuclear Radiation
Compared to beta particles and gamma radiation, alpha particles are much more massive. Alpha particles also have the most electric charge. +2 compared to -1 compared to no charge After 1st bullet: This makes sense b/c beta radiation releases an electron and alpha radiation releases a molecule made up of 2 p and 2 n. We have already discussed that protons and neutrons are much more massive than electrons. Also, gamma rays have no mass b/c they are just carrying energy so of course alpha particles are more massive than gamma rays. What is the electric charge of alpha particles? +2 What is the electric charge of beta particles? -1 What is the electric charge of gamma rays? No charge

Comparison of the Three Types of Nuclear Radiation—continued
Alpha particles are the least penetrating form of nuclear radiation. Alpha particles cannot even pass through a sheet of paper. Before 1st bullet: Read marked part of book on page 263. Read last paragraph on page 263.

Comparison of the Three Types of Nuclear Radiation—continued
Beta particles are much faster and more penetrating than alpha particles. They can pass through paper but are stopped by a sheet of aluminum foil or plastic.. Just like alpha particles, beta particles can damage cells when they are emitted by radioactive nuclei inside the human body.

Comparison of the Three Types of Nuclear Radiation—continued
Thick blocks of dense material, such as lead and concrete, are required to stop gamma rays. After 1st bullet: However, gamma rays cause less damage to biological molecules as they pass through living tissue.

Comparison of the Three Types of Nuclear Radiation—continued

The half-life of a radioactive isotope is the amount of time it takes for half the nuclei in a sample of the isotope to decay. The nucleus left after the isotope decays is called the daughter nucleus. Read marked paragraph on page 266. Read last paragraph under Radioactive Half-life on page 266. Have students get out a sheet of paper. Have them demonstrate half-life while reading Use an Analogy on page 266.

Radioactive Dating Carbon Dating
The half-life of a carbon-14 atom is 5,730 years. The age of the remains of plants and animals that lived within the last 50,000 years can be measured using carbon-14. Uranium Dating can be used to measure the age of rocks Have the students read aloud about Radioactive Dating on pages 266 and 267. For Bonus: Ask Discussion question on page 266.

Carbon Dating Homework: Have students complete questions 1-5 and 7 on page 267.

Section 4: Nuclear Reactions
Fission means breaking things apart. Fusion means putting things together.

Nuclear Fission nuclear fission—process of splitting a nucleus into two nuclei with smaller masses Only large nuclei, such as the nuclei of uranium and plutonium atoms, can undergo fission. Read 1st paragraph on page 273

The products of fission usually include several individual neutrons in addition to the smaller nuclei. The small amount of missing mass is converted to a tremendous amount of energy.

Chain Reactions A chain reaction is an ongoing series of fission reactions. Billions of reactions can occur each second during a chain reaction, resulting in the release of tremendous amounts of energy. Read 1st paragraph until “Chain Reaction” on page 274. After 2nd bullet: When controlled, the large amounts of energy released in nuclear fission reactions can be used to generate electricity or in nuclear weapons.

Critical Mass The critical mass is the amount of fissionable material required so that each fission reaction produces approximately one more fission reaction. Nuclear power plants use control rods made of nonfissionable material that can absorb neutrons. The control rods are moved in and out of the fissionable material to control the rate of the chain reaction. Read under “Critical Mass” until definition on page 274. After 1st bullet: If less than the critical mass of reaction material is present, a chain reaction will not occur.

Nuclear Fusion In nuclear fusion, two nuclei with low masses are combined to form one nucleus of larger mass. Temperature and Fusion Tremendous amounts of energy can be released in nuclear fission. In fact, splitting one U-235 nucleus produces about 30 million times more energy than chemically reacting one molecule of dynamite. But even more energy can be released in nuclear fusion. After 2nd bullet: Read Temperature and Fusion on page 275 and then Teacher FYI

The Sun Most of the energy given off by the Sun is produced by a process involving the fusion of hydrogen nuclei. Earth receives a small amount of this energy as heat and light. The Sun is composed mainly of hydrogen. Read last paragraph on page 275.

Using Nuclear Reactions in Medicine
What is a tracer? What can tracers be used for? What happens to the cells in a person’s body when they have cancer? How can radiation be used to treat cancer? How can physicians be sure that only cancer cells will absorb radiation? Homework: page 278 questions 1-5