alchemy A predecessor field to chemistry through which many people hoped to transform less valuable metals into gold but never succeeded.
alpha particle or alpha ray 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 or atomic weight The mass of a particular atom compared to a standard which sets the mass of a carbon-12 atom to be exactly 12. For a particular isotope, that value is approximately the number of protons plus the number of neutrons in its nucleus. For a naturally occurring element, that value is approximately the number of protons plus the average number of neutrons in the nuclei of naturally occurring isotopes.
atomic number The number of protons in the nucleus of an atom, which determines its chemical identity as an element.
beta particle or beta ray An electron that is emitted from some radioactive elements.
color In the theory of the strong nuclear force, this term is used to refer to the property that particles have that makes them respond to the force, just as electric charge is a property of particles that makes them respond to electromagnetic forces.
electromagnetism A fundamental force of nature, or property of matter and energy, that includes electricity, magnetism, and electromagnetic waves, such as light.
electron A very light subatomic particle (the first to be discovered) that carries negative charge and is responsible for chemical properties of matter.
gamma ray A high-energy photon that is emitted from some radioactive elements.
greenhouse gas A gas that keeps the heat of Earth’s surface from escaping into space.
molecule The smallest bit of matter that can be identified as a certain chemical compound.
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.
periodic table of the elements An arrangement of the elements in rows and columns by increasing atomic number, first proposed by Dmitri Mendeleyev, in which elements in the same column have similar chemical properties.
photon A particle that carries electromagnetic energy, such as light energy.
pi meson A particle that is interchanged by nucleons protons and electrons to produce the strong nuclear force.
proton A subatomic particle with positive electric charge found in the nucleus of atoms.
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.
radioactivity A property of unstable atoms that causes them to emit alpha, beta, or gamma rays or to undergo fission.
scattering An experimental technique used to detect the shape or properties of an unseen object by observing how other objects deflect from it.
spectrum The mixture of colors contained within a beam of light, or the band produced when those colors are spread out by a prism or other device that separates the colors from each other.
theory of relativity A theory developed by Albert Einstein that deals with the relationship between space and time. Its most famous equation E = mc2 describes the relationship between mass and energy.
transmutation The transformation of a nucleus of one element into another by radioactive emission.