The smallest particle of an element that retains all the chemical and physical properties of the 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.
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 one-or two-letter abbreviation for the name of an element.
A substance made of two or more elements that have bonded chemically. These atoms are usually, but not always, joined in molecules.
A negatively charged particle in an atom. The configurations of valence electrons define specific groups on the periodic table of elements, while the principal energy levels of those valence electrons define periods on the table.
A substance made up of only one kind of atom, which cannot be chemically broken into other substances.
Another term for chemical symbol.
Columns on the periodic table of elements. These are ordered according to the numbers of valence electrons in the outer shells of the atoms for the elements represented.
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, thus acquiring 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, known as radioisotopes, are radioactive.
The SI fundamental unit for "amount of substance." A mole is, generally speaking, Avogadro's number of atoms, molecules, or other elementary particles; 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 by chemical bonds. Compounds are typically made of up molecules.
A subatomic particle that has no electric charge. Neutrons, together with protons, account for the majority of average atomic mass. When atoms have the same number of protons—and hence are the same element—but differ in their number of neutrons, they are called isotopes.
The center of an atom, a region where protons and neutrons are located. The nucleus accounts for the vast majority of the average atomic mass.
A pattern of probabilities regarding the regions that an electron can occupy within an atom in a particular energy state. The higher the principal energy level, the more complex the pattern of orbitals.
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.
Rows of the periodic table of elements. These represent successive principal energy levels for the valence electrons in the atoms of the elements involved.
A value indicating the distance that an electron may move away from the nucleus of anatom. This is designated by a whole-number integer, beginning with 1 and moving upward. The higher the principal energy level, the greater the energy in the atom, and the more complex the pattern of orbitals.
A positively charged particle in an atom. The number of protons in the nucleus of an atom is the atomic number of an 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—either of the same element or another—until eventually it becomes stable. This stabilizing process may take a few seconds, or many years.
Electrons that occupy the highest energy levels in anatom. These are the electrons involved in chemical bonding.
Comment about this article, ask questions, or add new information about this topic: