Section 1: Radioactivity

• The Nucleus

  • Every second you are being bombarded by energetic particles.

  • The total amount of charge in a nucleus is determined by the number of protons, which also is called the atomic number.

  • Negatively charged electrons are electrically attracted to the positively charged nucleus and swarm around it.

  • The region outside the nucleus in which the electrons are located is large compared to the size of the nucleus.

    • If an atom were enlarged so that it was 1 km in diameter, its nucleus would have a diameter of only a few centimeters.

• The Strong Force: causes protons and neutrons to be attracted to each other

  • The strong force is one of the four basic forces in nature and is about 100 times stronger than the electric force.

  • The attractive forces between all the protons and neutrons in a nucleus keep the nucleus together.

  • The strong force is a short-range force that quickly becomes extremely weak as protons and neutrons get farther apart.

  • The electric force is a long-range force, so protons that are far apart still are repelled by the electric force

  • If a nucleus has only a few protons and neutrons, they are all close enough together to be attracted to each other by the strong force.

    • Because only a few protons are in the nucleus, the total electric force causing protons to repel each other is small.

  • Because only the closest protons and neutrons attract each other in a large nucleus, the strong force holding them together is about the same as in a small nucleus.

  • Because the repulsive force increases in a large nucleus while the attractive force on each proton or neutron remains about the same, protons and neutrons are held together less tightly in a large nucleus.

• Radioactivity: The process of nuclear decay

  • When the strong force is not large enough to hold a nucleus together tightly, the nucleus can decay and give off matter and energy.

  • Large nuclei tend to be unstable and can break apart or decay.

    • All nuclei that contain more than 83 protons are radioactive.

    • Many other nuclei that contain fewer than 83 protons also are radioactive. Even some nuclei with only one or a few protons are radioactive.

  • Almost all elements with more than 92 protons don’t exist naturally on Earth.

  • The atoms of an element all have the same number of protons in their nuclei.

  • Nuclei that have the same number of protons but different numbers of neutrons are called isotopes.

  • The atoms of all isotopes of an element have the same number of electrons, and have the same chemical properties.

  • The ratio of neutrons to protons is related to the stability of the nucleus.

    • In less massive elements, an isotope is stable if the ratio is about 1 to 1.

    • Isotopes of the heavier elements are stable when the ratio of neutrons to protons is about 3 to 2.

    • Nuclei with too many or too few neutrons compared to the number of protons are radioactive.

  • A nucleus can be described by the number of protons and neutrons it contains.

  • A nucleus can be represented by a symbol that includes its atomic number, mass number, and the symbol of the element it belongs to.

  • The number of neutrons in the nucleus is the mass number minus the atomic number.

Section 2: Nuclear Decay

• Nuclear Radiation

  • When an unstable nucleus decays, particles and energy called nuclear radiation are emitted from it.

  • The three types of nuclear radiation are alpha, beta, and gamma radiation.

    • Alpha and beta radiation are particles. Gamma radiation is an electromagnetic wave.

• Alpha Particle: made of two protons and two neutron

  • An alpha particle is the same as the nucleus of a helium atom and has a charge of +2 and an atomic mass of 4.

  • Compared to beta and gamma radiation, alpha particles are much more massive.

  • Alpha particles lose energy more quickly when they interact with matter than the other types of nuclear radiation do.

  • When alpha particles pass through matter, they exert an electric force on the electrons in atoms in their path.

  • Alpha particles can be stopped by a sheet of paper.

  • Alpha particles can be dangerous if they are released by radioactive atoms inside the human body.

  • A single alpha particle can damage many fragile biological molecules. Damage from alpha particles can cause cells not to function properly, leading to illness and disease.

  • When an atom emits an alpha particle, it has two fewer protons, so it is a different element.

  • Transmutation: the process of changing one element to another through nuclear decay.

  • In alpha decay, two protons and two neutrons are lost from the nucleus.

  • The charge of the original nucleus equals the sum of the charges of the nucleus and the alpha particle that are formed.

• Beta Particle: electron emitted from the nucleus in a neutron decay.

  • A second type of radioactive decay is called beta decay.

  • Beta decay is caused by another basic force called the weak force.

  • Atoms that lose beta particles undergo transmutation.

  • 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.

  • Just like alpha particles, beta particles can damage cells when they are emitted by radioactive nuclei inside the human body.

• Gamma Rays: electromagnetic waves with the highest frequencies and the shortest wavelengths in the electromagnetic spectrum.

  • The most penetrating form of nuclear radiation is gamma radiation.

  • They usually are emitted from a nucleus when alpha decay or beta decay occurs.

  • They have no mass and no charge and travel at the speed of light.

  • Thick blocks of dense materials, such as lead and concrete, are required to stop gamma rays.

  • Gamma rays cause less damage to biological molecules as they pass through living tissue.

• Radioactive Half-Life

  • Some radioisotopes decay to stable atoms in less than a second.

  • The nuclei of certain radioactive isotopes require millions of years to decay.

  • Half-Life: 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.

  • Half-lives vary widely among the radioactive isotopes.

• Radioactive Dating

  • The number of half-lives is the amount of time that has passed since the isotope began to decay.

  • It is also usually the amount of time that has passed since the object was formed, or the age of the object.

  • Different isotopes are useful in dating different types of materials.

  • The radioactive isotope carbon-14 often is used to estimate the ages of plant and animal remains.

  • Carbon- 14 has a half-life of 5,730 years and is found in molecules such as carbon dioxide.

  • The decaying carbon-14 in a plant or animal is replaced when an animal eats or when a plant makes food.

  • When an organism dies, its carbon-14 atoms decay without being replaced.

  • Radioactive dating also can be used to estimate the ages of rocks.

Section 3: Detecting Radioactivity

• Radiation Detectors

  • Because you can’t see or feel alpha particles, beta particles, or gamma rays, you must use instruments to detect their presence.

  • Cloud Chamber: used to detect alpha or beta particle radiation

  • When a radioactive sample is placed in the cloud chamber, it gives off charged alpha or beta particles that travel through the water or ethanol vapor.

  • Bubble Chamber: holds a superheated liquid, which doesn’t boil because the pressure in the chamber is high.

  • Particles of nuclear radiation can be detected as they leave trails of bubbles in a bubble chamber.

  • Nuclear radiation moving through the air can remove electrons from some molecules in air.

• Measuring Radiation

  • It is important to monitor the amount of radiation a person is being exposed to because large doses of radiation can be harmful to living tissue.

  • Geiger Counter: a device that measures the amount of radiation by producing an electric current when it detects a charged particle.

  • Electrons that are stripped off gas molecules in a Geiger counter move to a positively charged wire in the device. This causes current to flow in the wire. The current then is used to produce a click or a flash of light.

  • The intensity of radiation present is determined by the number of clicks or flashes of light each second.

• Background Radiation

  • Background Radiation is low-level radiation emitted mainly by naturally occurring radioactive isotopes found in Earth’s rocks, soils, and atmosphere.

  • Traces of naturally occurring radioactive isotopes are found in the food, water, and air consumed by all animals and plants.

  • Background radiation comes from several sources.

    • The largest source comes from the decay of radon gas.

  • Some background radiation comes from high-speed nuclei, called cosmic rays, that strike Earth’s atmosphere.

  • Some of the elements that are essential for life have naturally occurring radioactive isotopes.

  • The amount of background radiation a person receives can vary greatly. The amount depends on the type of rocks underground, the type of materials used to construct the person’s home, and the elevation at which the person lives, among other things.

Section 4: Nuclear Reactions

• Nuclear Fission: The process of splitting a nucleus into several smaller nuclei

  • The word fission means “to divide.”

  • Only large nuclei, such as the nuclei of uranium and plutonium atoms, can undergo nuclear fission.

  • The products of a fission reaction usually include several individual neutrons in addition to the smaller nuclei.

  • The total mass of the products is slightly less than the mass of the original nucleus and the neutron.

  • A small amount of mass can be converted into an enormous amount of energy.

  • When a nuclear fission reaction occurs, the neutrons emitted can strike other nuclei in the sample, and cause them to split.

  • Chain Reaction: The series of repeated fission reactions caused by the release of neutrons in each reaction.

  • If the chain reaction is uncontrolled, an enormous amount of energy is released in an instant

  • If enough neutrons are absorbed, the reaction will continue at a constant rate.

  • Critical Mass: the amount of material required so that each fission reaction produces approximately one more fission reaction.

• Nuclear Fusion: the amount of material required so that each fission reaction produces approximately one more fission reaction.

  • For nuclear fusion to occur, positively charged nuclei must get close to each other.

  • If nuclei are moving fast, they can have enough kinetic energy to overcome the repulsive electrical force between them and get close to each other.

  • Most of the energy given off by the Sun is produced by a process involving the fusion of hydrogen nuclei.

  • As the Sun ages, the hydrogen nuclei are used up as they are converted into helium.

• Using Nuclear Reactions in Medicine

  • Radioactive isotopes can be located by detecting the radiation they emit.

  • Tracer: When a radioisotope is used to find or keep track of molecules in an organism.

  • Scientists can use tracers to follow where a particular molecule goes in your body or to study how a particular organ functions.

  • Radioactive iodine- 131 accumulates in the thyroid gland and emits gamma rays, which can be detected to form
    an image of a patient’s thyroid.

  • Radiation can be used to stop some types of cancerous cells from growing.

  • Because cancer cells grow quickly, they are more susceptible to absorbing radiation and being damaged than healthy cells are.