alpha particle A helium nucleus that is emitted from some radioactive elements.
atom The smallest bit of matter than can be identified as a certain chemical element.
atomic mass The mass of an atom of a chemical element expressed in atomic mass units. For naturally occurring elements, it is approximately equal to the number of protons plus the average number of neutrons in the nuclei.
beta particle or beta ray An electron that is emitted from some radioactive elements.
compound A substance made of only one kind of molecule that consists of more than one kind of atom. For example, water is made of molecules that contain two atoms of hydrogen and one atom of oxygen.
electron A very light subatomic particle (the first to be discovered) that carries a negative charge and is responsible for chemical properties of matter.
element A substance made of only one kind of atom.
fusion A process in which lighter nuclei combine to form heavier nuclei and release energy. This process powers the stars and produces neutrinos that stream outward into space.
gamma ray A high-energy photon that is emitted from some radioactive elements.
gluon A particle that is exchanged between quarks, resulting in their being bound together.
ionization A process in which neutral atoms are turned into electrically charged ions by gaining or losing electrons.
lepton A subatomic particle that does not respond to the strong nuclear force. The leptons include electrons, muons, taus, and their corresponding neutrinos.
molecule The smallest bit of matter that can be identified as a certain chemical compound.
neutrino A subatomic particle with very little mass and no electric charge that is emitted along with an electron in beta radiation.
neutron A subatomic particle with neutral electric charge found in the nucleus of atoms.
nucleus The very tiny, positively charged central part of an atom that carries most of its mass.
photon A particle that carries electromagnetic energy, such as light energy.
physics The science of matter and energy.
pion A subatomic particle that carries the strong nuclear force, binding protons and neutrons together in the nucleus.
proton A subatomic particle with a positive electric charge found in the nucleus of atoms.
quantum electrodynamics (QED) A mathematical description of the electromagnetic force that accounts for quantum mechanical phenomena.
quantum mechanics A field of physics, developed to describe the relationships between matter and energy, that accounts for the dual wave-particle nature of both.
quantum number One of several numbers that specifies the state of a property of a subatomic particle, such as its orbital characteristics within an atom or its spin.
quark A sub-subatomic particle that exists in several forms that combine to make protons, neutrons, and some other subatomic particles.
radioactivity A property of unstable atoms that causes them to emit alpha, beta, or gamma rays.
scattering An experimental technique used to detect the shape or properties of an unseen object by observing how other objects deflect from it.
spin A property of subatomic particles expressed by a quantum number, also called spin, that describes the way it may align in a magnetic field.
strong nuclear force or strong force A fundamental force of nature that acts to hold the protons and neutrons in a nucleus together.
transmutation A transformation of the nucleus of one element into another by emission of an alpha or beta particle.
weak nuclear force, or weak force A fundamental force of nature that is responsible for beta decay of a radioactive nucleus.