[ak-tiv-i-tee] 
| 1. | the state or quality of being active: There was not much activity in the stock market today. He doesn't have enough physical activity in his life. |
| 2. | a specific deed, action, function, or sphere of action: social activities. |
| 3. | work, esp. in elementary grades at school, that involves direct experience by the student rather than textbook study. |
| 4. | energetic activity; animation; liveliness. |
| 5. | a use of energy or force; an active movement or operation. |
| 6. | normal mental or bodily power, function, or process. |
| 7. | Physical Chemistry. the capacity of a substance to react, corrected for the loss of reactivity due to the interaction of its constituents. |
| 8. | Physics.
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| 9. | an organizational unit or the function it performs. |
| ac·tiv·i·ty (āk-tĭv'ĭ-tē) n. pl. ac·tiv·i·ties
|
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.)
activity
activity ac·tiv·i·ty (āk-tĭv'ĭ-tē)
n.
A physiological process.
The presence of neurogenic electrical energy in electroencephalography..
An ideal concentration for which the law of mass action will apply perfectly.
The intensity of a radioactive source.
The ability to take part in a chemical reaction.
radioactivity ra·di·o·ac·tiv·i·ty (rā'dē-ō-āk-tĭv'ĭ-tē)
n.
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.
| radioactivity (rā'dē-ō-āk-tĭv'ĭ-tē) Pronunciation Key
The emission of radiation by unstable atomic nuclei undergoing radioactive decay. Our Living Language : 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. |
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