Radioactive element. an element subject to spontaneous degeneration of its nucleus accompanied by the emission of alpha particles, beta particles, or gamma rays. All elements with atomic numbers greater than 83 are radioactive. Naturally occurring radioactive elements include radium, thorium, and uranium.

polonium is the most radioactive element

Radioactive decay(also known asnuclear decay,radioactivityornuclear radiation) is the process by which an unstableatomic nucleusloses energy (in terms of mass in itsrest frame) by emittingradiation, such as analpha particle,beta particlewithneutrinoor only a neutrino in the case ofelectron capture, or agamma rayorelectronin the case ofinternal conversion. A material containing such unstable nuclei is consideredradioactive. Certain highly excited short-lived nuclear states can decay throughneutron emission, or more rarely,proton emission.Radioactive decay is astochastic(i.e. random) process at the level of single atoms. According toquantum theory, it is impossible to predict when a particular atom will decay,[1][2][3]regardless of how long the atom has existed. However, for a collection of atoms, the collection's expected decay rate is characterized in terms of their measureddecay constantsorhalf-lives. This is the basis ofradiometric dating. The half-lives of radioactive atoms have no known upper limit, spanning a time range of over 55orders of magnitude, from nearly instantaneous to far longer than theage of the universe.A radioactive nucleus with zerospincan have no defined orientation, and hence emits the totalmomentumof its decay productsisotropically(all directions and without bias). If there are multiple particles produced during a single decay, as inbeta decay, theirrelativeangular distribution, or spin directions may not be isotropic. Decay products from a nucleus with spin may be distributed non-isotropically with respect to that spin direction, either because of an external influence such as anelectromagnetic field, or because the nucleus was produced in a dynamic process that constrained the direction of its spin. Such a parent process could be a previous decay, or anuclear reaction.[4][5][6][note 1]The decaying nucleus is called theparentradionuclide(orparent radioisotope[note 2]), and the process produces at least onedaughter nuclide. Except for gamma decay or internal conversion from a nuclearexcited state, the decay is anuclear transmutationresulting in a daughter containing a different number ofprotonsorneutrons(or both). When the number of protons changes, an atom of a differentchemical elementis created.The first decay processes to be discovered were alpha decay, beta decay, and gamma decay.Alpha decayoccurs when the nucleus ejects an alpha particle (helium nucleus). This is the most common process of emittingnucleons, but highly excited nuclei can eject single nucleons, or in the case ofcluster decay, specific light nuclei of other elements.Beta decayoccurs in two ways: (i) beta-minus decay, when the nucleus emits an electron and an antineutrino in a process that changes a neutron to a proton, or (ii) beta-plus decay, when the nucleus emits apositronand a neutrino in a process that changes a proton to a neutron. Highly excited neutron-rich nuclei, formed as the product of other types of decay, occasionally lose energy by way of neutron emission, resulting in a change from oneisotopeto another of the same element. The nucleus may capture an orbiting electron, causing a proton to convert into a neutron in a process called electron capture. All of these processes result in a well-defined nuclear transmutation.By contrast, there are radioactive decay processes that do not result in a nuclear transmutation. The energy of an excited nucleus may be emitted as a gamma ray in a process calledgamma decay, or that energy may be lost when the nucleus interacts with an orbital electron causing its ejection from the atom, in a process calledinternal conversion.Another type of radioactive decay results in products that vary, appearing as two or more "fragments" of the original nucleus with a range of possible masses. This decay, called spontaneousfission, happens when a large unstable nucleus spontaneously splits into two (or occasionally three) smaller daughter nuclei, and generally leads to the emission of gamma rays, neutrons, or other particles from those products.For a summary table showing the number of stable and radioactive nuclides in each category, seeradionuclide. There are 28 naturally occurring chemical elements on Earth that are radioactive, consisting of 33 radionuclides (5 elements have 2 different radionuclides) that date before the time of formation of the solar system. These 33 are known asprimordial nuclides. Well-known examples areuraniumandthorium, but also included are naturally occurring long-lived radioisotopes, such aspotassium-40. Another 50 or so shorter-lived radionuclides, such asradiumandradon, found on Earth, are the products ofdecay chainsthat began with the primordial nuclides, or are the product of ongoingcosmogenicprocesses, such as the production ofcarbon-14fromnitrogen-14in the atmosphere bycosmic rays. Radionuclides may also beproduced artificiallyinparticle acceleratorsornuclear reactors, resulting in 650 of these with half-lives of over an hour, and several thousand more with even shorter half-lives. (SeeList of nuclidesfor a list of these sorted by half-life.)

Radioactive decay(also known asnuclear decay,radioactivityornuclear radiation) is the process by which an unstableatomic nucleusloses energy (in terms of mass in itsrest frame) by emittingradiation, such as analpha particle,beta particlewithneutrinoor only a neutrino in the case ofelectron capture, or agamma rayorelectronin the case ofinternal conversion. A material containing such unstable nuclei is consideredradioactive. Certain highly excited short-lived nuclear states can decay throughneutron emission, or more rarely,proton emission.Radioactive decay is astochastic(i.e. random) process at the level of single atoms. According toquantum theory, it is impossible to predict when a particular atom will decay,[1][2][3]regardless of how long the atom has existed. However, for a collection of atoms, the collection's expected decay rate is characterized in terms of their measureddecay constantsorhalf-lives. This is the basis ofradiometric dating. The half-lives of radioactive atoms have no known upper limit, spanning a time range of over 55orders of magnitude, from nearly instantaneous to far longer than theage of the universe.A radioactive nucleus with zerospincan have no defined orientation, and hence emits the totalmomentumof its decay productsisotropically(all directions and without bias). If there are multiple particles produced during a single decay, as inbeta decay, theirrelativeangular distribution, or spin directions may not be isotropic. Decay products from a nucleus with spin may be distributed non-isotropically with respect to that spin direction, either because of an external influence such as anelectromagnetic field, or because the nucleus was produced in a dynamic process that constrained the direction of its spin. Such a parent process could be a previous decay, or anuclear reaction.[4][5][6][note 1]The decaying nucleus is called theparentradionuclide(orparent radioisotope[note 2]), and the process produces at least onedaughter nuclide. Except for gamma decay or internal conversion from a nuclearexcited state, the decay is anuclear transmutationresulting in a daughter containing a different number ofprotonsorneutrons(or both). When the number of protons changes, an atom of a differentchemical elementis created.The first decay processes to be discovered were alpha decay, beta decay, and gamma decay.Alpha decayoccurs when the nucleus ejects an alpha particle (helium nucleus). This is the most common process of emittingnucleons, but highly excited nuclei can eject single nucleons, or in the case ofcluster decay, specific light nuclei of other elements.Beta decayoccurs in two ways: (i) beta-minus decay, when the nucleus emits an electron and an antineutrino in a process that changes a neutron to a proton, or (ii) beta-plus decay, when the nucleus emits apositronand a neutrino in a process that changes a proton to a neutron. Highly excited neutron-rich nuclei, formed as the product of other types of decay, occasionally lose energy by way of neutron emission, resulting in a change from oneisotopeto another of the same element. The nucleus may capture an orbiting electron, causing a proton to convert into a neutron in a process called electron capture. All of these processes result in a well-defined nuclear transmutation.By contrast, there are radioactive decay processes that do not result in a nuclear transmutation. The energy of an excited nucleus may be emitted as a gamma ray in a process calledgamma decay, or that energy may be lost when the nucleus interacts with an orbital electron causing its ejection from the atom, in a process calledinternal conversion.Another type of radioactive decay results in products that vary, appearing as two or more "fragments" of the original nucleus with a range of possible masses. This decay, called spontaneousfission, happens when a large unstable nucleus spontaneously splits into two (or occasionally three) smaller daughter nuclei, and generally leads to the emission of gamma rays, neutrons, or other particles from those products.For a summary table showing the number of stable and radioactive nuclides in each category, seeradionuclide. There are 28 naturally occurring chemical elements on Earth that are radioactive, consisting of 33 radionuclides (5 elements have 2 different radionuclides) that date before the time of formation of the solar system. These 33 are known asprimordial nuclides. Well-known examples areuraniumandthorium, but also included are naturally occurring long-lived radioisotopes, such aspotassium-40. Another 50 or so shorter-lived radionuclides, such asradiumandradon, found on Earth, are the products ofdecay chainsthat began with the primordial nuclides, or are the product of ongoingcosmogenicprocesses, such as the production ofcarbon-14fromnitrogen-14in the atmosphere bycosmic rays. Radionuclides may also beproduced artificiallyinparticle acceleratorsornuclear reactors, resulting in 650 of these with half-lives of over an hour, and several thousand more with even shorter half-lives. (SeeList of nuclidesfor a list of these sorted by half-life.)