[ rey-dee-oh-ak-tiv-i-tee ]
/ ˌreɪ di oʊ ækˈtɪv ɪ ti /

noun Physics, Chemistry.

the phenomenon, exhibited by and being a property of certain elements, of spontaneously emitting radiation resulting from changes in the nuclei of atoms of the element.
Also called activity.

Origin of radioactivity

First recorded in 1895–1900; radio- + activity

Example sentences from the Web for radioactivity

British Dictionary definitions for radioactivity

/ (ˌreɪdɪəʊækˈtɪvɪtɪ) /


the spontaneous emission of radiation from atomic nuclei. The radiation can consist of alpha, beta, and gamma radiation

Medical definitions for radioactivity

[ rā′dē-ō-ăk-tĭvĭ-tē ]


Spontaneous emission of radiation, either directly from unstable atomic nuclei or as a consequence of a nuclear reaction.
The radiation, including alpha particles, nucleons, electrons, and gamma rays, emitted by a radioactive substance.

Scientific definitions for radioactivity

[ rā′dē-ō-ăk-tĭvĭ-tē ]

The emission of radiation by unstable atomic nuclei undergoing radioactive decay.

A Closer Look

In the nuclei of stable atoms, such as those of lead, the force binding the protons and neutrons to each other individually is great enough to hold together each nucleus as a whole. In other atoms, especially heavy ones such as those of uranium, this energy is insufficient, and the nuclei are unstable. An unstable nucleus spontaneously emits particles and energy in a process known as radioactive decay. The term radioactivity refers to the particles emitted. When enough particles and energy have been emitted to create a new, stable nucleus (often the nucleus of an entirely different element), radioactivity ceases. Uranium 238, a very unstable element, goes through 18 stages of decay before becoming a stable isotope of lead, lead 206. Some of the intermediate stages include the heavier elements thorium, radium, radon, and polonium. All known elements with atomic numbers greater than 83 (bismuth) are radioactive, and many isotopes of elements with lower atomic numbers are also radioactive. When the nuclei of isotopes that are not naturally radioactive are bombarded with high-energy particles, the result is artificial radioisotopes that decay in the same manner as natural isotopes. Each element remains radioactive for a characteristic length of time, ranging from mere microseconds to billions of years. An element's rate of decay is called its half-life. This refers to the average length of time it takes for half of its nuclei to decay.

Cultural definitions for radioactivity


The emission of elementary particles by some atoms when their unstable nuclei disintegrate (see half-life). Materials composed of such atoms are radioactive. (See alpha radiation, beta radiation, and gamma radiation.)