Radioisotope: Isotopes of atoms with unstable nuclei.
X-ray: High-energy electromagnetic radiation not produced by radioactive sources.
Penetrating power: The ability of radiation to pass through matter.
Review Vocabulary
Element: A pure substance that cannot be broken down into simpler substances by physical or chemical means.
The Discovery of Radioactivity
Nuclear chemistry studies the structure of atomic nuclei and their changes.
Table 1: Comparison of Chemical and Nuclear Reactions
Chemical Reactions
Occur when bonds are broken and formed.
Involve only valence electrons.
Associated with small energy changes.
Atoms keep the same identity, may gain, lose, or share electrons, and form new substances.
Temperature, pressure, concentration, and catalysts affect reaction rates.
Nuclear Reactions
Occur when nuclei combine, split, and emit radiation.
Can involve protons, neutrons, and electrons.
Associated with large energy changes.
Atoms of one element are often converted into atoms of another element.
Temperature, pressure, and catalysts do not normally affect reaction rates.
Wilhelm Roentgen discovered X-rays in 1895.
Henri Becquerel, Marie Curie, and Pierre Curie pioneered the fields of radioactivity and nuclear chemistry.
In 1898, the Curies identified two new elements, polonium and radium, based on their radioactivity.
Types of Radiation
Isotopes of atoms with unstable nuclei are called radioisotopes.
Unstable nuclei emit radiation to attain more stable atomic configurations in a process called radioactive decay.
During radioactive decay, unstable nuclei release energy by emitting radiation.
The three most common types of radiation are alpha α, beta β, and gamma γ.
Ernest Rutherford identified alpha, beta, and gamma radiation when studying the effects of an electric field on emissions from a radioactive source.
*Alpha Particles
An alpha particle α has the same composition as a helium nucleus—two protons and two neutrons.
The charge of an alpha particle is 2+ due to the presence of the two protons.
Alpha radiation consists of a stream of alpha particles.
Alpha particles are not very penetrating; a single sheet of paper stops alpha particles.
*Example Decay:
Radium-226 (88226Ra) nucleus undergoes alpha decay to form radon-222 and an alpha particle.
88226Ra→86222Rn+24He
*Beta Particles
A beta particle is a very fast-moving electron emitted when a neutron in an unstable nucleus converts into a proton.
Beta particles are represented by the symbol β or e−. They have a 1- charge.
Beta radiation consists of a stream of fast-moving electrons.
A thin sheet of metal foil is required to stop beta particles.
*Example Decay:
Iodine-131 nucleus undergoes beta decay to form xenon-131 and a beta particle.
53131I→54131Xe+β
*Gamma Rays
Gamma rays are high-energy electromagnetic radiation denoted by the symbol γ.
Emission of gamma rays does not change the atomic number or mass number of a nucleus.
Gamma rays almost always accompany alpha and beta radiation.
They account for most of the energy loss as a nucleus decays.
Some, but not all, gamma rays are stopped by a thick layer of concrete or lead.
*X-rays
X-rays are high-energy electromagnetic radiation that are not produced by radioactive sources.
Their energy is lower than that of gamma rays.
X-rays are emitted when inner electrons are knocked out, and electrons from higher energy levels drop down to fill the vacancy.
Penetrating Power
The ability of radiation to pass through matter is called penetrating power.
Penetrating power is quantified as the depth of water that stops 50 percent of incoming radiation.
Gamma rays have the highest penetrating power because they have no charge or mass.