<Dd> This is a list of the chemical elements and their isotopes, listed in terms of stability . </Dd> <P> Atomic nuclei consist of protons and neutrons, which attract each other through the nuclear force, while protons repel each other via the electric force due to their positive charge . These two forces compete, leading to some combinations of neutrons and protons being more stable than others . Neutrons stabilize the nucleus, because they attract protons, which helps offset the electrical repulsion between protons . As a result, as the number of protons increases, an increasing ratio of neutrons to protons is needed to form a stable nucleus; if too many or too few neutrons are present with regard to the optimum ratio, the nucleus becomes unstable and subject to certain types of nuclear decay . Unstable isotopes decay through various radioactive decay pathways, most commonly alpha decay, beta decay, or electron capture . Many other rare types of decay, such as spontaneous fission or cluster decay are known . (See radioactive decay for details .) </P> <P> Of the first 82 elements in the periodic table, 80 have isotopes considered to be stable . The 83rd element, bismuth, was traditionally regarded as having the heaviest stable isotope, bismuth - 209, but in 2003 researchers in Orsay, France, measured the half - life of Bi to be 7026599594400000000 ♠ 1.9 × 10 years . Technetium, promethium (atomic numbers 43 and 61, respectively) and all the elements with an atomic number over 82 only have isotopes that are known to decompose through radioactive decay . No undiscovered elements are expected to be stable; therefore, lead is considered the heaviest stable element . However, it is possible that some isotopes that are now considered stable will be revealed to decay with extremely long half - lives (as with Bi). This list depicts what is agreed upon by the consensus of the scientific community as of 2016 . </P> <P> For each of the 80 stable elements, the number of the stable isotopes is given . Only 90 isotopes are expected to be perfectly stable, and an additional 163 are energetically unstable, but have never been observed to decay . Thus, 253 isotopes (nuclides) are stable by definition (including tantalum - 180m, for which no decay has yet been observed). Those that may in the future be found to be radioactive are expected to have half - lives longer than 10 years (for example, xenon - 134). </P>

Where are most of the radioactive elements found on the periodic table
find me the text answering this question