A.
> alpha particle - one of the three types of radiation resulting from natural radioactivity.
> antiparticle - elementary particle corresponding to an ordinary particle such as the proton, neutron, or electron, but having the opposite electrical charge and magnetic moment.
> atom - basic unit of matter.
> atomic number - often represented by the symbol Z, the number of protons in the nucleus of an atom
> atomic mass - the mass of a single atom

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   B.
> beta particle - one of the three types of radiation resulting from natural radioactivity.
> black body - in physics, an ideal black substance that absorbs all and reflects none of the radiant energy falling on it.

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   C.
> charge - property of matter that gives rise to all electrical phenomena.
> Compton effect - (for A. H. Compton), increase in the wavelengths of X rays and gamma rays when they collide with and are scattered from loosely bound electrons in matter.
> cosmic rays - charged particles moving at nearly the speed of light reaching the earth from outer space.

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   D.
> diffraction - bending of waves around the edge of an obstacle.

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   E.
> electron - elementary particle carrying a unit charge of negative electricity.
> electron-volt - abbr. eV, unit of energy used in atomic and nuclear physics; 1 electron-volt is the energy transferred in moving a unit charge, positive or negative and equal to that charge on the electron, through a potential difference of 1 volt.
> elementary particles - the most basic physical constituents of the universe.

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   G.
> gamma radiation - high-energy photons emitted as one of the three types of radiation resulting from natural radioactivity.
> Geiger counter - instrument for the detection and quantitative determination of ionizing radiation such as the alpha and beta rays given off by radioactive minerals and cosmic rays.

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   L.
> light - visible electromagnetic radiation. Of the entire electromagnetic spectrum, the human eye is sensitive to only a tiny part, the part that is called light.

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   M.
> mass number - often represented by the symbol A, the total number of neutrons and protons in the nucleus of an atom.
> meson - class of elementary particles whose masses are generally between those of the lepton class of lighter particles and those of the baryon class of heavier particles.

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   N.
> neutron - uncharged elementary particle of slightly greater mass than the proton.
> nuclear energy - the energy stored in the nucleus of an atom and released through fission, fusion, or radioactivity.
> nucleus - in physics, the extremely dense central core of an atom.

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   P.
> periodic law - statement of a periodic recurrence of chemical and physical properties of the elements when the elements are arranged in order of increasing atomic number.
> periodic table - chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley [view table].
> photoelectric effect - emission of electrons by substances, especially metals, when light falls on their surfaces.
> photon - the particle composing light and other forms of electromagnetic radiation, sometimes called light quantum.
> pion - lightest of the meson family of elementary particles.
> proton - elementary particle having a single positive electrical charge and constituting the nucleus of the ordinary hydrogen atom.

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   Q.
> quantum electrodynamics - (QED), quantum field theory that describes the properties of electromagnetic radiation and its interaction with electrically charged matter in the framework of quantum theory.
> quantum mechanics (quantum theory) - modern physical theory concerned with the emission and absorption of energy by matter and with the motion of material particles; the quantum theory and the theory of relativity together form the theoretical basis of modern physics.

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   R.
> radiation - term applied to the emission and transmission of energy through space or through a material medium and also to the radiated energy itself. In its widest sense the term includes electromagnetic, acoustic, and particle radiation, and all forms of ionizing radiation.
> radioactivity - spontaneous disintegration or decay of the nucleus of an atom by emission of particles, usually accompanied by electromagnetic radiation.
> relativity - physical theory, introduced by Albert Einstein, that discards the concept of absolute motion and instead treats only relative motion between two systems or frames of reference.
> Roentgen ray (X ray) - invisible, highly penetrating electromagnetic radiation of much shorter wavelength (higher frequency) than visible light.

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   V
> valence - combining capacity of an atom expressed as the number of single bonds the atom can form or the number of electrons an element gives up or accepts when reacting to form a compound. Atoms are called monovalent, divalent, trivalent, or tetravalent, according to whether they form one, two, three, or four bonds.

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   X.
> X ray - see Roentgen ray

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