The smallest particle of an element that retains all the chemical and physical properties of the element. Anatom can exist either alone or in combination with other atoms in a molecule. Atoms are made up of protons, neutrons, and electrons.
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 one-or two-letter abbreviation for the name of an element.
A substance made up of atoms of more than one element. These atoms are usually joined in molecules.
Negatively charged particles in an atom. Electrons, which spin around the protons and neutrons that make up the atom's nucleus, constitute a very small portion of the atom's mass. The number of electrons and protons is the same, thus canceling out one another; on the other hand, if an atom loses or gainselectrons, it becomes an ion.
Another term for chemical symbol.
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.
The sum of protons and neutrons in an atom's nucleus.
A group of atoms, usually (but not always) representing more than one element, joined in a structure. Compounds are typically made up of molecules.
A subatomic particle that has no electric charge. Neutrons are found at the nucleus of an atom, alongside protons.
A generic term for the heavy particles—protons and neutrons—that make up the nucleus of an atom.
Another term for mass number.
The center of an atom, a region where protons and neutrons are located, and around which electrons spin.
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. Vertical columns within the periodic table indicate groups or "families" of elements with similar chemical characteristics.
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.
A particle believed to be a component of protons and neutrons. A quarkmay either have a positive electric charge of less than 1+, in which case it is called an "up quark"; or a negative charge of less than 1−, in which case it is called a "down quark."
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 isotopes are subject to a form of decay brought about by the emission of high-energy particles or radiation, such as alpha particles, beta particles, or gamma rays.