Radioactivity is a property of certain elements - such as uranium - of emitting energy in the form of radiation as a result of some sort of nuclear decay of an unstable atom. Nuclei that exhibit radioactivity are known as radioactive nuclei. Additionally, radioactivity or simply activity can be used as a measurement to describe how many decays a radioactive atom goes through in a period of time. These decays result in an ejection of particles from the nucleus as it changes form and obtains a greater nuclear stability. Radioactivity can also be referred to as radioactive decay or nuclear decay.
The most common forms of radiation include alpha, beta, and gamma radiation, but other types of radioactive decay exist such as proton emission or neutron emission, or spontaneous fission of large nuclei.
The radioactivity of elements can be used in a number of different applications, such as in medicine and these elements act as the fuel in nuclear power plants to generate electricity. As well, the radiation from these elements can be used to irradiate foods and keep them from spoiling.
Whether or not a nuclide is radioactive has to do mainly with its nuclear stability. Nuclear isotopes that are unstable emit radiation as a result of the conflict in the strength of the repulsive Coulomb force between protons in the nucleus and the attractive strong nuclear force between nucleons. If the Coulomb force and strong nuclear force do not balance, the nuclide in question lays outside the belt of stability and is radioactive. A number known as the neutron-to-proton ratio or N/Z ratio can be used to quickly see if the Coulomb force and strong nuclear force remain fairly balanced or out of balance. For smaller elements near the top of the periodic table, the ratio for stability is nearly 1:1. As the nuclei get larger, the N/Z ratio increases slightly for stability. If a nucleus has too many protons or neutrons, it will likely undergo some sort of transmutation to reach a more stable state (where it changes to some new nuclide with a "better" N/Z ratio).
There are also more complex ways to decide whether or not a nucleus will exhibit radioactivity. For example, if a nucleus has either an odd number of protons or an odd number of neutrons, it tends to be radioactive regardless of whether or not the Coulomb and strong nuclear forces balance. The reason for a greater instability in nuclei with odd numbers of neutrons or protons is essentially because protons and neutrons like to "pair up" with particles of the same type, boosting stability. As well, there is something special about having certain numbers of protons or neutrons in a nucleus that promote stability. These numbers are known as magic numbers.
Most radioactive isotopes tend to be large, and the last completely stable nucleus is bismuth (which has 83 protons). These large radioactive elements tend to undergo alpha decay as it quickly lowers the number of protons and neutrons in the nucleus. Most nuclei that are found in nature are not radioactive, because all of the short-lived radioactive nuclei have already decayed, leaving a vast majority of stable nuclei. There are only 50 naturally occuring isotopes that exhibit radioactivity while there are around 270 stable isotopes. Hundreds of short-lived isotopes have been created in laboratories and particle accelerators.
Radioactivity can also be used to describe how much ionizing radiation is released by a radioactive material. The SI unit of radioactivity is the becquerel (Bq), equal to one decay per second. The curie (Ci) was the original unit for radioactivity and is equal to 3.7×1010 Bq. Geiger counters can be used to measure the radioactivity of a substance and these devices are widely known for the "clicking" noise they make when they detect a decay producing ionizing radiation.
Another way to measure how radioactive something is is to investigate its half-life, since the half-life of an isotope is related to its radiation risk.
One common misconception about radioactivity is that any radioactive object is harmful to human health. This is not the case, however, as small doses radiation are not harmful to humans. In fact, there are many radioactive products that can be purchased and pose no health threats to humans. Bananas, smoke detectors, some ceramic dishware, cat litter, beer, and brazil nuts are all radioactive.
However, in larger doses radiation does have negative effects on health. When radioactive materials decay, they produce ionizing radiation. Simply put, this type of radiation can strip electrons away from atoms or break chemical bonds (to make ions). This causes damage to living tissues that cannot always be repaired. Chronic exposure to radiation can lead to cancer (as a result of damage at the cellular or molecular level) or other mutations that can be harmful to fetuses. Effects from acute exposure to radiation appear quickly, and include burns and radiation poisoning. The symptoms of radiation poisoning include nausea, weakness, hair loss, and diminished organ function and this radiation sickness can result in death if the dose is high enough.
As well, some radiation is only harmful in certain circumstances. For example, smoke detectors generally contain a source of alpha particles known as americium (which is radioactive). The americium is used to detect the smoke. In the smoke detector itself, this source is radioactive but not harmful. However, because of the nature of alpha particles, the Americium is very dangerous if ingested. Smoke detectors save many lives every year, and changing the batteries once a year is a good idea. Dissecting smoke detectors however can be dangerous.