Chapter 25: Nuclear Changes 

Section 1: Radioactivity

• The Nucleus

  • The nucleus of an atom contains the protons, which have a positive charge, and neutrons, which have no electric charge.
  • Atoms usually contain the same number of protons as electrons.
  • Negatively charged electrons are electrically attracted to the positively charged nucleus and swarm around it.
  • Protons and neutrons are packed together tightly in a nucleus.
  • The size of a nucleus in an atom can be compared to a marble sitting in the middle of an empty football stadium.

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

  • The particles in the nucleus are attracted to each other by the strong force.
  • The strong force is one of the four basic forces in nature and is about 100 times stronger than the electric force
  • The strong force is a short-range force that quickly becomes extremely weak as protons and neutrons get farther apart.
  • The total force between two protons depends on how far apart they are.
  • Some atoms, such as uranium, have many protons and neutrons in their nuclei.
  • 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 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: process of nuclear decay

  • A nucleus that decays is called a radioactive nucleus.
  • Nuclei that contain large numbers of protons and neutrons tend to be unstable.
  • In fact, all nuclei that contain more than 83 protons are radioactive. However, many other nuclei that contain fewer than 83 protons also are radioactive.
  • Almost all elements with more than 92 protons don’t exist naturally on Earth.
  • When an unstable nucleus decays, energy is emitted.
  • As an unstable nucleus decays, a small amount of mass is converted into energy.
  • A large amount of energy is produced by the conversion of only a small amount of mass.
  • 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.
  • A nucleus can be described by the number of protons and neutrons it contains.
  • The number of protons in a nucleus is called the atomic number.
  • A nucleus can be represented by a symbol that includes its atomic number, mass number, and the symbol of the element it belongs to.

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 Particle: made of two protons and two neutrons

  • Compared to beta and gamma radiation, alpha particles are much more massive.
  • When alpha particles pass through matter, they exert an electric force on the electrons in atoms in their path.
  • Alpha particles are the least penetrating form of nuclear radiation.
  • Alpha particles can be dangerous if they are released by radioactive atoms inside the human body.
  • Damage from alpha particles can cause cells not to function properly, leading to illness and disease.
  • Some smoke detectors give off alpha particles that ionize the surrounding air.
  • When alpha particles collide with molecules in the air, positively charged ions and electrons result. The ions and electrons move toward charged plates, creating a current in the smoke detector.
  • 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.

• Beta Particle: The electron emitted from the nucleus.

  • A second type of radioactive 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.
  • 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.

  • They have no mass and no charge and travel at the speed of light
  • Gamma rays cause less damage to biological molecules as they pass through living tissue.
  • The gamma ray produces fewer ions because it has no electric charge.

• Radioactive Half-Life

  • Some radioisotopes decay to stable atoms in less than a second.
  • A measure of the time required by the nuclei of an isotope to decay is called the half-life.
  • Half-Life: the amount of time it' takes for half the nuclei in a sample of the isotope to decay.
  • Half-lives vary widely among the radioactive isotopes.

• Radioactive Dating

  • Some geologists, biologists, and archaeologists, among others, are interested in the ages of rocks and fossils found on Earth. The ages of these materials can be determined using radioactive isotopes and their half-lives.
  • The number of half-lives is the amount of time that has passed since the isotope began to decay.
  • The radioactive isotope carbon-14 often is used to estimate the ages of plant and animal remains.
  • The decaying carbon-14 in a plant or animal is replaced when an animal eats or when a plant makes food.

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

Section 3: Detecting Radioactivity

• Radiation Detectors

  • Some tools that are used to detect radioactivity rely on the fact that radiation forms ions in the matter it passes through.
  • Cloud Chamber: used to detect alpha or beta particle radiation
  • A cloud chamber is filled with water or ethanol vapor.
  • If a sample of radioactive material is placed in a cloud chamber, a trail of condensed vapor will form along the paths of the emitted particles.
  • 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.
  • When an electroscope is given a negative charge, its leaves repel each other and spread apart.
  • 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.
  • It has a tube with a positively charged wire running through the center of a negatively charged copper cylinder.
  • The intensity of radiation present is determined by the number of clicks or flashes of light each second.

• Background Radiation

  • It 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. Radon, which emits an alpha particle when it decays, is produced in Earth’s crust by the decay of uranium-238.
  • Some background radiation comes from high-speed nuclei, called cosmic rays, that strike Earth’s atmosphere. They produce showers of particles, including alpha, beta, and gamma radiation.
  • 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.
  • However, because it comes from naturally occurring processes, background radiation never can be eliminated.

Section 4: Nuclear Reaction

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

  • When a neutron hits a uranium-235 nucleus, the uranium nucleus splits into two smaller nuclei and two or three free neutrons. Energy also is released.
  • Only large nuclei, such as the nuclei of uranium and plutonium, undergo nuclear fission
  • A fission reaction usually produces several individual neutrons in addition to the smaller nuclei.
  • Albert Einstein proposed that mass and energy were related in his special theory of relativity.
  • 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.
  • Critical Mass: the amount of material required so that each fission reaction produces approximately one more fission reaction.

• Nuclear Fusion: two nuclei with small masses combine to form a nucleus of larger mass.

  • Nuclear fission reactions release millions of times more energy than can be released by chemical reactions.
  • The reason nuclear reactions release so much more energy than chemical reactions is that the strong force is much stronger than the electric force.
  • 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.
  • The Sun is composed mainly of hydrogen. 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.
  • The thyroid gland is located in your neck and produces chemical compounds called hormones.
  • The beta particles are absorbed by the surrounding tissues, but the gamma rays penetrate the skin.
  • 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.
  • When a person has cancer, a group of cells in that person’s body grows out of control and can form a tumor. Radiation can be used to stop some types of cancerous cells from growing.
  • When possible, a radioactive isotope such as gold-198 or iridium-192 is implanted within or near the tumor.
  • Because cancer cells grow quickly, they are more susceptible to absorbing radiation and being damaged than healthy cells are.

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