The smallest particle of an element. Atoms are made up of protons, neutrons, and electrons. Atoms that have the same number of protons—that is, are of the same element—but differ in number of neutrons are known as isotopes.
An SI unit (abbreviated amu), equal to 1.66 · 10 −24 g, for measuring the mass of atoms.
The number of protons in the nucleus of an atom. Since this number is different for each element, elements are listed on the periodic table of elements in order of atomic number.
A figure used by chemists to specify the mass—in atomic mass units—of the average atom in a large sample.
A substance made up of only one kind of atom. Hence an element cannot be chemically broken into other substances.
Negatively charged particles in an atom, which spin around the protons and neutrons that make up the atom's nucleus.
The length of time it takes a substance to diminish to one-half its initialamount.
An atom or atoms that has lost or gained one or more electrons, and thus has a net electric charge.
Atoms that have an equal number of protons, and hence are of the same element, but differ in their number of neutrons. This results in a difference ofmass. Isotopes may be either stable or unstable. The latter type is known as aradioisotope.
A subatomic particle that has no electric charge. Neutrons are found at the nucleus of an atom, alongside protons.
The center of an atom, a region where protons and neutrons are located, and around which electrons spin. The plural of "nucleus" is nuclei.
Another name for isotopes.
A chart that shows the elements arranged in order of atomic number.
A positively charged particle in an atom. Protons and neutrons, which together form the nucleus around which electrons spin, have approximately the same mass—a mass that is many times greater than that of an electron. The number of protons in the nucleus of an atom is the atomic number of an element.
In a general sense, radiation can refer to anything that travels in astream, whether that stream be composed of subatomic particles or electromagnetic waves. In a more specific sense, the term relates to the radiation from radio active materials, which can be harmful to humanbeings.
A term describing a phenomenon whereby certain materials are subject to a form of decay brought about by the emission of high-energy particles or radiation, including alpha particles, beta particles, or gamma rays.
An isotope subject to the decay associated with radioactivity. A radioisotope is thus an unstable isotope.
An atom or group of atoms whose participation in a chemical, physical, or biological reaction can be easilyobserved. Radioisotopes are often used astracers.
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