Chemistry Ch. 15
Remember, isotopes are...
Atoms with the same atomic number Z, but different neutron numbers N and mass numbers A.
Nuclear instability
- Excess mass : alpha decay : slight increase in N/Z ratio - N/Z too high : beta decay : decrease in N/Z ratio - N/Z too low : positron emission : increase in N/Z ratio - N/Z too low : electron capture : increase in N/Z ratio - Energetically excited : gamma emission : no change in N/Z ratio
Natural radiation associated with radioactive decay can be placed into three classes...
-Alpha particles -Beta particles -Gamma rays
Electron capture
A proton and an electron combine to form a neutron. The mass number stays the same, but the atomic number decreases by 1. Very few nuclides undergo this transformation.
Chain reactions
A reaction in which the product of one step is the reactant in another step. In order for a chain reaction to sustain itself, the amount and shape of the sample of fissionable material must be such that the neutrons will not escape due to energy that is higher than optimum for inducing further fission. A chain reaction should maintain a constant rate.
Breeder reactors
A reactor that produces fuel that can be used in other reactors
Plasma
An ionized gas that must be created and controlled at temperature of about 10^8 K. Melts most container material.
Medical applications using radioactivity
Many medical applications exist that use radioactivity: -power generators -medical diagnoses -position emission topography -cancer therapy
Particulate radiation
Mass given off from unstable atoms with the energy of motion
Fission reactors
Nuclear power plants use fission to produce electric energy. If the chain reaction is going too quickly, movable control rods made of these elements are inserted into a core of uranium fuel in fission reactors.
Nuclear bombardment reactions
Nuclei are hit with a beam of nuclei or nuclear particles to trigger a nuclear reaction. Occurs when a nuclear reaction is not spontaneous and is produced intentionally by artificial means. Used to synthesize transuranium elements, those following uranium on the periodic table.
Alpha particles
Nuclei of helium- 4 atoms. Contains 2 protons and 2 neutrons. Least harmful to animal and human tissue.
Band of stability
Of the more than 3000 nuclides known, about 250 are stable. The rest decompose over a period of time, emitting radiation in the process of creating new nuclides. The stable nuclides have approximately equal numbers of protons and neutrons (N/Z ratio = 1) in the lighter elements (Z= 1-20) and more neutrons than protons in the heavier elements (N/Z ratio > 1).
Particle accelerators
Particle accelerators are used for nuclear bombardment reactions. The synchroton, perhaps the most successful accelerator, uses a circular path for the accelerating particles.
2 types of beta particles
Positron and Electron. Positron has the same mass as an electron with an opposite charge.
Critical mass
The smallest amount of fissionable material necessary to support a continuing chain reaction.
Radioactive decay
The spontaneous emission of electromagnetic or other types of radiation
Spontaneous nuclear decay reactions
The tendency for the neutron/proton (N/Z) ratio to move toward the band of stability, explains the nuclear reactions of naturally radioactive nuclides. For every process except "y" emission, the change that occurs for an unstable nuclide takes it closer to the observed band of stability. Radioactive nuclides convert spontaneously over time to form stable nuclides.
Half-life
The time required for half of a sample of a nuclide to decay to a different nuclide. It takes the same time for a fresh sample to decay to one-half the original number of atoms of that nuclide as it does one-half to decay to one-fourth and so on. The shorter the half-life of a nuclide, the more intense the radiation that it emits.
Nuclear reactions
Two conditions must be met to balance a nuclear equation. 1) conservation of mass number 2) conservation of nuclear charge (atomic number)
Radioactive atoms
Unstable atoms that give off excess matter, energy, or both as ionizing radiation
Detecting radiation
Various instruments have been deveoped to give speedier and more accurate measures of radiation intensity: -Geiger-muller counter -Scintillation counter
Beta particle (electron) emission
When a nucleus emits a beta particle (electron), its atomic number increase by 1 and its mass number remains unchanged. When a nucleus emits a beta particle (positron), its atomic number decrease by 1 and its mass number remains unchanged.
Alpha particle emission
When a nucleus emits an alpha particle, it loses 2 protons and 2 neutrons, so its atomic number decreases by 2 and its mass number decreases by 4.
Remember this about all the "numbers"...
Z signifies the atomic number, the number of protons in the nucleus of an atom. N signifies the neutron number, the number of neutrons in the nucleus of an atom. Sum of N and Z is A (N+Z = A), the mass number.
Fusion
Combination of light nuclei to form heavier nuclei. A major fusion reaction occurs continuously in the Sun and other stars
Two types of radiation
Electromagnetic and particulate
Radiation
Energy that comes from a source and travels through matter or space
Nucleons
General term used to describe nuclear particles, protons, and neutrons.
Gamma rays
High energy electromagnetic radiation energy without charge or mass. Highest energy and most penetrating type of radiation.
Fusion reaction terms
Ignition temperature Breeder reactors Plasma Until recently, fusion in reactors required more energy than was given off. In order to achieve fusion, the gaseous reactants must be condensed to a small volume at high temperatures.
Radiation
In a nuclear reaction, an emission of radiation usually accompanies changes in the composition of the nucleus. The three classes of natural radiation behave differently in an electric field, as shown below.
Gamma ray emission
In all nuclear reactions, the nucleus changes from a state of higher energy to a state of lower energy. Gamma rays are pure electromagnetic energy. Results in no change in mass or atomic number.
Radioactive decay series
In heavier elements, often the product of radioactive decay is itself radioactive. In such cases, a series of alpha and beta decay steps ultimately leads to a stable nuclide. Accounts for most of the radioactive decay among elements 83 through 92.
Electromagnetic radiation
Includes light, gamma rays, and x-rays
Nuclides
Isotopes that exist for a measurable length of time and have a defined energy state. An atom of a particular atomic number, mass number, and neutron number.
Ignition temperature
Temperature required to initiate a fusion reaction
Biological effects of radiation
Radiation can have one of four effects on the functioning of a cell: 1) The radiation can pass through the cell with no damage. 2) The cell can absorb the radiation and be damaged, but it can subsequently repair the damage and resume normal functioning. 3) The cell can be damaged so severely that it cannot repair itself. New cells formed from this cell will be abnormal. This mutant cell can ultimately cause cancer if it continues to proliferate. 4) The cell can be so severely damaged that it dies.
Ionizing radiation
Radiation of either type that can produce charged particles in matter
Archaeological dating
Radio-carbon dating. Using carbon-14 to measure time on an archaeological scale. As long as a plant or animal is alive, its carbon-14 content should match that in the atmosphere. After it dies, its carbon-14 content should match that in the atmosphere.
Biological effects of radiation (cont)
Radon: a rare noble gas which has also been implicated as a possible cause of lung cancer. Accumulates in houses from particular kinds of soils or rock strata
Beta particles
Small, charged particle that can be emitted from unstable atoms at speeds approaching the speed of light. Penetrate through skin into tissue.
Nuclear energy: fission
Splitting of a heavy nucleus into two or more lighter nuclei and some number of neutrons
