The smallest particle of an element that retains all the chemical and physical properties of that element.
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.
An old term for atomic mass. Since weight varies depending on gravitational field, whereas mass is the same throughout the universe, scientists typically use the term "atomic mass" instead.
A figure used by chemists to specify the mass—in atomic mass units—of the average atom in a large sample.
A figure, named after Italian physicist Amedeo Avogadro (1776-1856), equal to 6.022137 × 10 23 . Avogadro's number indicates the number of atoms or molecules in a mole.
A substance made up of atoms of more than one element. These atoms are usually joined in molecules.
An alternate term for atomic mass units, used in biochemistry and microbiology for describing the mass of large organic molecules. The dalton (abbreviated Da or D) is named after English chemist John Dalton (1766-1844), who introduced the concept of the atom to science.
A substance made up of only one kind of atom, which cannot be chemically broken into other substances.
The length of time it takes a substance to diminish to one-half its initialamount.
An atom or group of atoms that has lost or gained one or more electrons, and thus has a net electric charge.
Atoms of the same element (that is, they have the same number of protons) that differ in terms of mass. Isotopes may be either stable or unstable. The latter type, known as radioisotopes, are radioactive.
The amount of matter an object contains.
The mass, in grams, of1 mole of a given substance. The value in grams of molar mass is always equal to the value, in atomic mass units, of the average atomic mass of that substance. Thus carbon has a molar mass of 12.01 g, and anaverage atomic mass of 12.01 amu.
The SI fundamental unit for "amount of substance." A mole is, generally speaking, Avogadro's number of atoms or molecules; however, in the more precise SI definition, a mole is equal to the number of carbon atoms in 12.01 g of carbon.
A group of atoms, usually, but not always, representing more than one element, joined in a structure. Compounds are typically made of up molecules.
A chart that shows the elements arranged in order of atomic number, along with chemical symbol and the average atomic mass (in atomic mass units) for that particular element.
A term describing a phenomenon whereby certain isotopes known as radioisotopes are subject to a form of decay brought about by the emission of high-energy particles. "Decay" does not mean that the isotope "rots"; rather, it decays to form another isotope, until eventually (though this may take a long time), it becomes stable.
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