radioactivity
Americannoun
noun
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.
Etymology
Origin of radioactivity
Example Sentences
Examples are provided to illustrate real-world usage of words in context. Any opinions expressed do not reflect the views of Dictionary.com.
There, Erikson teased out the stealthy, almost sinister peculiarities of modern man-made hazards, like radioactivity and toxicity, and their special, traumatic effects.
From The Wall Street Journal • Dec. 18, 2025
It could release vast amounts of energy with little associated radioactivity.
From BBC • Dec. 18, 2025
That process releases huge amounts of energy, no carbon emissions and limited radioactivity.
From The Wall Street Journal • Dec. 18, 2025
Cabrera Services also said it’s “unlikely” the radioactivity is from large deposits of alum mud.
From Los Angeles Times • Dec. 2, 2024
The leak may have saved unit 2 from explosion, but it was also releasing radioactivity into the air.
From "Meltdown" by Deirdre Langeland
Definitions and idiom definitions from Dictionary.com Unabridged, based on the Random House Unabridged Dictionary, © Random House, Inc. 2023
Idioms from The American Heritage® Idioms Dictionary copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.
