alchemy A field of study that preceded 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. 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.
Brownian motion The jiggling motion of a piece of dust or pollen suspended in a fluid, first observed by Robert Brown and eventually shown by Albert Einstein to demonstrate the existence of atoms and molecules.
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 (H2O).
electromagnetic An adjective to describe 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 the chemical properties of matter.
element A substance made of only one kind of atom.
emission Sending out something that has been produced, such as the emission of an alpha, beta, or gamma ray from a radioactive atom.
fusion The joining of two nuclei to form a new nucleus.
gamma ray A high-energy photon that is emitted from some radioactive elements.
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 and 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 Dmitry Mendeleyev, in which
elements in the same column have similar chemical properties.
photoelectric effect A phenomenon in which light can, under some circumstances, knock electrons out of atoms.
photon A particle that carries electromagnetic energy, such as light energy.
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.
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.
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.
strong nuclear force or strong force A fundamental force of nature that acts to hold the protons and neutrons in a nucleus together.
theory of relativity A theory developed by Albert Einstein that dealt with the relationship between space and time. Its most famous equation (E = mc2) described the relationship between mass and energy.
transmutation The transformation of one element to another by a change in its nucleus, such as by alpha or beta emission.
weak nuclear force or weak force A fundamental force of nature that is responsible for beta decay of a radioactive nucleus.