How to define radioactivity ?

Radioactivity is defined by the emission of radiation by unstable atomic nuclei undergoing radioactive decay.

 

A Closer Look (source definition from The American Heritage, Science dictionary 2005 copyright by Houghton Mifflin Company): 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 (see figure below).

Schematic picture of the natural 238U decay chain with the 18 stages of decay between 238U (radioactive element) and 206Pb (stable element). For each intermediate radioactive element, half-lives are expressed in years (y), days (d) and minutes (mn). Schematic picture of the natural 238U decay chain with the 18 stages of decay between 238U (radioactive element) and 206Pb (stable element). For each intermediate radioactive element, half-lives are expressed in years (y), days (d) and minutes (mn).

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.