Chapter 24

What are the three isotopes of hydrogen called? What distinguishes one isotope from another?

1. Protium (H-1). One proton. 2. Deuterium (H-2). One proton and one neutron. 3. Tritium (H-3). One proton and two neutrons. Different amounts or numbers of neutrons.

X-Rays

A form of high-energy electromagnetic radiation. Not produced by radioactive sources and have low energy. They are emitted when inner electrons are knocked out and electrons from higher levels drop down to fill the vacancy.

Positron

A particle with the same mass as an electron but an opposite charge. Represented by ß+ or e^+.

Radiotracer

A radioisotope that emits non-ionizing radiation and is used to signal the presence of an element or specific substance. This is possible because all of an element's isotopes have the same chemical properties, thus replacing a stable atom of that element in a reaction with one of its isotopes does not alter the reaction. This can be used by drinking Iodine-131 to detect diseases in the thyroid gland.

Beta Decay

A radioisotope that lies above the band of stability is unstable because it has too many neutrons relative to its number of protons. It then undergoes beta decay, which decreases the number of neutrons in the nucleus. It increases the atomic number and lowers the n/p ratio, moving the atom closer, if not into, the band of stability.

Transmutation

A reaction in which an atom's atomic number is altered, or the conversion of an element into another element.

Critical Mass

A sample that is massive enough to sustain a chain reaction. A sample that is not massive enough to sustain a chain reaction is said to have subcritical mass. If this happens, neutrons escape from the sample before they can start the chain reaction by striking other nuclei. When a critical mass is present, the neutrons released in one fission cause other fissions to occur. If much more mass than the critical mass is present, the chain reaction rapidly escalates and leads to violent nuclear explosion. A sample of fissionable material with a mass greater than the critical mass is said to have supercritical mass.

Radioactive Decay Series

A series of nuclear reactions that begins with an unstable nucleus and results in the formation of a stable nucleus.

Strong Nuclear Force

Acts on subatomic particles that are extremely close together. Overcomes the electrostatic repulsion between protons and is the reason why all nucleons then remain bound in the dense nucleus. The number of neutrons in a nucleus is important because nuclear stability is related to the balance between electrostatic and strong nuclear forces. Proton and Proton, Proton and Neutron, Neutron and Neutron.

What kinds of elements are made inside the stars?

All elements except hydrogen and helium.

Alpha Decay

All nuclei with more than 82 protons are radioactive and decay spontaneously. Both the number of neutrons and the number of protons must be reduced in order to make these radioisotopes stable. These very heavy nuclei often decay by emitting alpha particles.

Three Most Common Types of Radiation

Alpha, beta, and gamma radiation.

Uses of Radiation: Using Positron Emission

Another radiation-based medical diagnostic tools is called positron emission transaxial tomography (PET). A radiotracer that decays by positron emission is injected into a patient's bloodstream. Positrons emitted from the radiotracer cause gamma-ray emissions that are then detected by an array of sensors around the patient. PET scans can be used to diagnose diseases or study parts of the brain activated under given circumstances.

Energy Equivalent of Mass

Any reaction produces or consumes energy due to a loss or gain in mass. Energy and mass are equivalent. A small change in mass results in a large change in energy. E=mc^2 where E is equal to the change in energy, m is the change in mass, and c is the speed of light.

Band of Stability

Area on the graph of n/p ratio where all stable nuclei are found. All nuclei outside the band of stability are radioactive and must undergo decay in order to gain stability. Ends at lead-208 (atomic number of 82).

Nuclear Change

Atoms change their identity because the number of protons in the nucleus is changed. Large amounts of energy involved because mass is converted into energy (E=mc^2) EX: fission (splitting of nuclei), fusion (union of nuclei), radioactive emissions (cosmic rays or background emission)

Physical Change

Atoms of elements retain their identity and characteristic properties. EX: mixing, freezing, melting, distilling, condensing, boiling

Chemical Change

Atoms rearrange to form new substances with new properties. Atoms keep their identity but give up their properties. Electrons are lost/gained (ionic bonds) or shared (covalent bonds). EX: gas given off, precipitate forms, color change, temperature change

Strong Nuclear Force

Attraction that keeps the protons and neutrons together in the nucleus. Acts over short (10^-15 m) distances. Particle is gluon or meson. Strongest.

Thermonuclear Reactions

Because of the energy requirements, fusion reactions are known as these.

More about Nuclear Reactors

Because of the hazardous radioactive fuels and fission products in a nuclear power plant, a dense concrete structure is usually built to enclose the reactor. This can shield personnel and nearby residents from harmful radiation. As the reactor operates, the fuel rods are gradually depleted and products from the fission reactions accumulate. The reactor then must be serviced periodically with spent fuel rods being extracted, reprocessed, and repackaged to make new fuel rods. Some fission products are extremely radioactive and must be stored as nuclear waste. Risks of accidents are to be taken in account, but the storage of highly radioactive nuclear waste takes sometimes thousand of years before it can be biologically exposed. Scientific research goes into the disposal of radioactive wastes, with the waste being treated with advanced technologies so the materials will not deteriorate over a long period of time and the waste is stored in sealed containers buried underground.

Scintillation Counter

Brief flashes of light produced when ionizing radiation excites the electrons in certain types of atoms or molecules called phosphors. Often contains a base material containing phosphors, which the energy excites the electrons in. As the electrons return to their ground states, light is emitted and is transmitted through the base material by a photodetector that converts the light to an electrical signal. The number and brightness of the scintillations give a measure of the amount of ionizing radiation.

What happens to a star that is greater than 1.44 times the mass of our sun?

Contracts so violently that the core implodes. In a matter of seconds, the star reaches 10 billion degrees. A massive shock wave tears through the star and it explodes as a supernova. Dozens of new elements are formed.

Positron Emission

For nuclei with low neutron-proton ratios because it increases the n/p ratio so the atom is closer to, if not in, the band of stability. A radioactive decay process that involves the emission of a positron from a nucleus. A proton in the nucleus is converted into a neutron and a positron, and then the positron is emitted. The mass number remains the same while the atomic number decreases by 1.

What accounts for the "weak rungs" in nucleosynthesis?

Fragile nuclei that are overstressed by the repulsion of proton for proton and may exist only for a second before ripping themselves apart or disintegrating in a collision with another nucleus.

Gravity

Greater the masses, greater the force. Large distance effect, but decreases with distance. Particle is the graviton. Weakest.

Steam in a Nuclear Reactor

Heat from the nuclear fission is used to generate steam, which then drives turbines that produce electricity.

Weak Nuclear Force

Holds nucleus together. Shorter range than strong nuclear force (10^-18 m). Particle is boson and W&Z particles. Governs radioactive decay (alpha, beta). 2nd strongest.

Half-Life

How radioactive decay rates are measured. The time required for 1/2 of a radioisotope's nuclei to decay into its products. The equation for the remaining amount of a radioactive element is N=N0 (1/2)^n where N is the remaining amount, N0 is the initial amount, and n is the number of half lives that have passed or the elapsed time/half-life (with the same units of time). Each radioisotope has its own characteristic half-life.

At 5 billion degrees, what is the last element that can be formed inside of stars? Relate this to homework problem #27 in Chapter 24 of your text.

Iron (Fe). Homework problem #27 states that iron is the most stable nuclei because it is highest on the curve in the graph. Iron is the hardest atom to pull apart and create new elements because of how stable its nuclei is.

Radioisotopes

Isotopes of atoms with unstable nuclei. These emit radiation to attain more stable atom configurations in a process called radioactive decay.

Electromagnetic Force

Keeps electrons close to the nucleus. Governs chemical reactions. Particle is photon. Stream of photons is called electromagnetic radiation (EM). 3rd strongest.

Grand Unified Theory

Modern forces.

Penetrating Power

The ability of radiation to pass through matter. Alpha is the lowest, beta is higher, and gamma is highest (needing a thick block of lead).

Nuclear Fusion

The combination of atomic nuclei. It is possible to bind together two or more light (mass number less than 60) less-stable nuclei to form one single more-stable nucleus. Responsible for the production of the heaviest elements and is capable of releasing large amounts of energy (EX: Sun). With nuclear fusion, it is a promising source of energy because lightweight isotopes are abundant, the products are not generally radioactive, and produces large amounts of energy (more than fission reactions). However, fusion requires extremely high energies to initiate and sustain a reaction and requires extremely high temperatures to overcome the electrostatic repulsion between the nuclei. The temperature has been achieved using an atomic explosion, but this is not practical. There is also no way to confine the reaction and no materials capable of withstanding these temperatures.

Biological Effects of Radiation

The damage produced by ionizing radiation absorbed by the body depends on several factors, such as the type of radiation, its energy, the type of tissue absorbing the radiation, the penetrating power, and the distance from the source. Radiation can disrupt cell processes and damage skin.

Transuranium Elements

The elements immediately following uranium in the periodic table (atomic number 93 and greater). They have been produced in the laboratory by induced transmutation and are radioactive.

What happens to a star that is 1.44 times the mass of our sun or less?

The expanded outer layers will drift off in a wispy ring called a planetary nebula. The remnant of the star, called a white dwarf, will burn more and more quickly, get dimmer and dimmer, and eventually go out.

Fission in Nuclear Reactors

The fission within a nuclear reactor is started by a neutron-emitting source and is stopped by positioning the control rods to absorb all of the neutrons produced in the reaction. The reactor core contains a reflector that acts to reflect neutrons back into the core, where they will react with the fuel elements, called fuel rods. A coolant, usually water, circulates through the reactor core to carry off the heat generated by the reactions. The hot coolant heats water that is used to power steam-driven turbines, which produce electric power.

Intensity and Distance

The intensity of radiation depends on the distance from the source as shown by the equation I1d1^2=I2d2^2 where d1 and d2 are distances from the source and I1 is the intensity at d1 and I2 is the intensity of d2. The farther away the source, the lower the intensity. The intensity of radiation is measured in the amount of radiation per unit of time and/or surface.

Describe how carbon is formed.

Two helium molecules combine during helium burning to create beryllium, which can combine with another helium to make carbon. Basically, 3 heliums combine or collide at the same time in the triple alpha process.

What is the last naturally occurring element?

Uranium.

Beta Particles

Very fast-moving electrons that are emitted when a neutron in an unstable nucleus converts into a proton. Represented by ß or e^- and have a 1- charge. Mass is so small it is approximated to zero. Beta radiation consists of a stream of fast-moving electrons. They have greater penetrating power (metal foil is required).

Discovery of Radioactivity

Nuclear chemistry is concerned with the structure of atomic nuclei and the changes they undergo and nuclear reactions involve much larger energy changes. In 1895, Bequerel decided to build on Roentgen's work of X-rays and found that uranium salts produced spontaneous emissions that darkened photographic plates. Marie and Pierre Curie then isolated the components emitting the rays and concluded that the uranium atoms were giving off the rays in a process called radioactivity where radiation are rays and particles emitted.

Nuclear Reactors

Nuclear fission produces the energy generated by nuclear reactors. This energy is primarily used to generate energy at nuclear power plants. A common fuel is fissionable uranium oxide encased in corrosion resistant rods in U-238 or U-235, which can undergo induced fission when hit by a neutron and is 3% of the fuel which is the amount required to sustain a chemical reaction, called enriched uranium. Additional rods, made of cadmium or boron, control the fission process inside the reactor by absorbing neutrons released during the reaction. Keeping the chain reaction going while preventing it from going our of control requires precise monitoring and continual adjusting of the rods. Losing control of the nuclear reactor can result in the accidental release of harmful levels of radiation, such as the Three Mile Island accident and the Chernobyl accident.

What is the prerequisite for forming molecules or compounds?

Nuclei need electrons to attach to each other to form atoms. Atoms could combine by sharing and exchanging their electrons.

What forces does a star experience in its lifetime?

Nucleosynthesis. Gravity.

Detecting Radioactivity

People who work near radioactive sources may wear a thermoluminescent dosimeter, which contains a tiny crystal. Radiation excites electrons in the crystal and the electrons return to their ground states, emitting lights. This light works as a measure of the radiation dose to which a worker has been exposed.

Gamma Rays

Photons, high-energy short-wavelength electromagnetic radiation. Denoted by weird y. Have no mass and no charge, meaning that the emission of gamma rays does not change the atomic number of mass number of an atom. Almost always accompany alpha and beta radiation, and account for most of the energy loss that occurs as a nucleus decays. Customary to omit from nuclear equations.

Nucleons

Protons and neutrons (in the nucleus).

Uses of Radiation: Treating Cancer

Radiation can damage or destroy healthy cells, but it can also destroy unhealthy cells, such as cancer cells. Cancer cells are more susceptible to destruction by radiation than healthy ones. Unfortunately, in the process of destroying unhealthy cells, radiation also destroys some healthy cells.

Ionizing Matter

Radiation energetic enough to ionize matter with which it collides. A Geiger counter is an ionizing radiation detecting device and consists of a metal tube filled with gas, with a wire in the center of the tub that is connected to a power supply. When ionizing radiation penetrates the end of the tube, the gas inside the tube absorbs the radiation and forms ions and free electrons. The free electrons are attracted to the wire, creating an electric current. A meter built into the Geiger counter measures the current flow through the ionized gas and is used to determine the amount of ionizing radiation present.

Electron Capture

Radioactive decay process that decreases the number of protons in unstable nuclei lying below the band of stability. Occurs when the nucleus of an atom draws in a surrounding electron, usually one from the lowest energy level. This electron then combines with a proton to form a neutron. The mass number remains the same while the atomic number decreases by 1. An X-ray photon is also released.

Uses of Radiation: Using Radioisotopes

Radioisotopes can be used to follow the course of an element through a chemical reaction.

Radioactive Decay Rates

Radioisotopes can occur naturally or be synthesized, and radioisotopes have differing decay rates. These all contribute to their presence on Earth.

Breeder Reactors

Reactors able to produce more fuel than they use. There is a limited supply of Uranium-235, so there is an option to build these reactors that produce new quantities of fissionable fuels.

Dose of Radiation

Refers to the amount of radiation a body absorbs from a radiative source. The units rad and rem are used to measure doses. The rad (radiation-absorbed dose) is a measure of the amount of radiation that results in the absorption of 0.01 J of energy per kilogram of tissue. The dose in rads is multiplied by a factor related to the radiation's effect and tissue involved. This results in the rem.

Alpha Particles

Same composition as a helium nucleus, two protons and two neutrons, and has the symbol of 4 2 He and the charge of 2+. Alpha radiation is a stream of alpha particles. Because of their mass and charge, alpha particles are relatively slow-moving and not very penetrating, even a single sheet of paper can stop alpha particles.

What generation star is our sun?

Second or third generation.

Chain Reactions

Self-sustaining process in which one reaction initiates the next. EX: Uranium-235 fission produces additional neutrons which can then produce additional fissions with other Uranium-235 atoms and so on Atomic Bomb=uncontrolled chain reaction

Mass Defect

The mass of the nucleus is always less than the sum of the masses of the individual protons and neutrons that comprise it. When nucleons combine together to form an atom, the energy corresponding to the mass defect is released. The nuclear binding energy is equal to this mass defect, and is the amount of energy needed to break one mole of nuclei into individual nucleons. The larger the binding energy per nucleon, the more strongly the nucleons are held together, and the more stable the nucleus is. Elements with a mass number near 60 are the most stable. In chemical reactions, the accompanying changes in mass to energy produced or consumed is very small, but in nuclear reactions the energy changes and associated mass changes are very large.

Induced Transmutation

The process in which striking stable nuclei with high velocity particles, such as neutrons of high-energy alpha, beta, or gamma radiation forces transmutation, or the conversion of one element to another. The particles must be moving at extremely high speeds to overcome the electrostatic repulsion between themselves and the target nucleus. Scientists have developed methods to do this by using very strong electrostatic and magnetic fields or particle accelerators. This can be used to synthesize hundreds of new isotopes.

Radiochemical Dating

The process of determining the age of an object by measuring the amount of a certain radioisotope remaining in that object. This can be done using the half-life of that radioisotope. EX: Carbon Dating (Carbon-14), Uranium-238 to Lead-206 Dating Old Things, Rocks, and Meteorites

Nuclear Fission

The splitting of a nucleus into fragments. Atoms with a mass number greater than 60 often fragment into smaller atoms to increase their stability. The fission of a nucleus is accompanied by a very large release of energy. Nuclear power plants use nuclear fission to generate power.

Neutron to Proton Ratio

The stability of a nucleus can be correlated to its neutron-to-proton (n/p) ratio. For atoms with low atomic numbers (less than 20), this ratio is about 1:1. As atomic number increases, more and more neutrons are needed to produce a strong nuclear force that is sufficient to balance the electrostatic repulsion force between protons. The n/p ratio therefore gradually increases, reaching about a maximum of 1.5:1 for the largest atoms.

Balanced Nuclear Equations

The sum of the mass numbers and the sum of the atomic numbers on each side of the arrow are equal. Charges are also balanced.