'''Radioactivity''' is the emission of high [[energy]] [[particles]] through the natural phenomenon of the decay of unstable [[isotopes]] of [[chemical elements]] into more stable forms, which are called ''daughter products.'' This type of emission is generally called nuclear radiation.
The most common types of nuclear radiation are called alpha, and betaradiation, and gamma radiationthe processes for each are respectively alpha decay and beta decay.<ref>http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/radact.html</ref> There can also be gamma radiation associated with a nuclear decay. Alpha particles are [[helium]] nuclii(two protons and two neutrons); beta particles are high-energy [[electron]]s; gamma rays are high-energy [[photon]]s. Alpha particles can normally be stopped by a sheet of paper, and beta particles can normally be stopped by healthy human skin, but gamma rays can penetrate one's body to cause great harm. Gamma radiation is also a form of electro-magnetic radiation, like X-rays or visible light.
All forms of nuclear decay follow the fundamental rules of mass and energy balance. ==IAnalogy Alpha Decay==As stated above, an alpha particle is the nucleus of a Helium atom, i.e., two protons and two neutrons. This arrangement means the alpha particle has a charge of +2, and an atomic mass of 4, the symbol for which is <math>{{}_2^4}He^{+2}</math>. For example, the most common [[isotope]] of Uranium is Uranium-238. Since all Uranium atoms have 92 protons, the remaining mass is composed of neutrons (238 - 92 = 146 neutrons). The neutron/proton ration is therefore 146 / 92 = 1.59. The initial step in Uranium-238 decaying (eventually) into Lead-206 is an alpha decay: <center><math>{{}_{92}^{238}}U </math> → <math>{{}_2^4}He + {{}_{90}^{234}}Th</math></center> Note that the particle count is conserved, meaning on the left there are 92 protons, and on the right there are 2 + 90 = 92 protons as well. Likewise, the atomic masses are balanced, since on the left the total is 238, and on the right the total is 4 + 234 = 238. However, from just this equation, it is difficult to understand how energy is balanced, too. Because this is a naturally occurring decay, we expect that the energy contained within the alpha particle and the Thorium atom will be <b>less than</b> the energy contained within the Uranium atom (because of the law of Entropy). So, where is that extra energy? It is released from the reaction in the form of a weak gamma emission. ==Analogy to disease==
In [[H. G. Wells]]' 1909 novel [[Tono-Bungay]], the narrator muses:
:To my mind radio-activity is a real disease of matter. Moreover, it is a contagious disease. It spreads. You bring those debased and crumbling atoms near others and those too presently catch the trick of swinging themselves out of coherent existence. It is in matter exactly what the decay of our old culture is in society, a loss of traditions and distinctions and assured reactions. ...I am haunted by a grotesque fancy of the ultimate eating away and dry-rotting and dispersal of all our world. So that while man still struggles and dreams his very substance will change and crumble from beneath him. I mention this here as a queer persistent fancy. Suppose, indeed, that is to be the end of our planet; no splendid climax and finale, no towering accumulation of achievements, but just—atomic decay!<ref>Wells, H. G. (1909) [http://www.gutenberg.org/files/718/718-h/718-h.htm Tono-Bungay], online Project Gutenberg text; search for text string "real disease"</ref>