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The Structure of the Atom

Dalton's atomic theory

A theory proposed by John Dalton in 1808, based on numerous scientific experiments, that marked the beginning of the development of modern atomic theory.


The smallest particle of an element that retains all the properties of that element; is electrically neutral, spherically shaped, and composed of electrons, protons, and neutrons.

cathode ray

A ray of radiation that originates from the cathode and travels to the anode of a cathode ray tube.


A negatively charged, fast-moving particle with an extremely small mass that is found in all forms of matter and moves through the empty space surrounding an atom's nucleus.


The extremely small, positively charged, dense center of an atom that contains positively charged protons, neutral neutrons, and is surrounded by empty space through which one or more negatively charged electrons move.


A subatomic particle in an atom's nucleus that has a positive charge of 1+.


A neutral subatomic particle in an atom's nucleus that has a mass nearly equal to that of a proton.

atomic number

The number of protons in an atom.


Atoms of the same element with the same number of protons but different numbers of neutrons.

mass number

The number after an element's name, representing the sum of its protons and neutrons.

atomic mass unit (amu)

One-twelfth the mass of a carbon-12 atom.

atomic mass

The weighted average mass of the isotopes of that element.

nuclear reaction

A reaction that involves a change in the nucleus of an atom.


The process in which some substances spontaneously emit radiation.


The rays and particles - alpha and beta particles and gamma rays - that are emitted by radioactive materials.

radioactive decay

A spontaneous process in which unstable nuclei lose energy by emitting radiation.

alpha radiation

Radiation that is made up of alpha particles; is deflected toward a negatively charged plate when radiation from a radioactive source is directed between two electrically charged plates.

alpha particle

A particle with two protons and two neutrons, with a 2+ charge; is equivalent to a helium-4 nucleus, and is emitted during radioactive decay.

nuclear equation

A type of equation that shows the atomic number and mass number of the particles involved.

beta radiation

Radiation that is made up of beta particles; is deflected toward a positively charged plate when radiation from a radioactive source is directed between two electrically charged plates.

beta particle

A high-speed electron with a 1- charge that is emitted during radioactive decay.

gamma ray

High-energy radiation that has no electrical charge and no mass, is not deflected by electric or magnetic fields, usually accompanies alpha and beta radiation, and accounts for most of the energy lost during radioactive decay.

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