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 each other and 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.There is only one stable isotope of Tantalum. However, 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.)Isotope half-lives. Note that the darker more stable isotope region departs from the line of protons (Z) = neutrons (N), as the element number Z becomes largerOf the first 82 elements in the periodic table, 80 have isotopes considered to be stable. Technetium, promethium (atomic numbers 43 and 61, respectively[a]) and all the elements with an atomic number over 82 only have isotopes that are known to decompose throughradioactive 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 presently considered stable will be revealed to decay with extremely long half-lives (as was the case in 2003 with bismuth-209, which had previously been considered to be stable). This list depicts what is agreed upon by the consensus of the scientific community as of 2008.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, 254 isotopes (nuclides) are stable by definition (including Ta-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 1022 years (for example, xenon-134).Of the chemical elements, only one element (tin) has 10 such stable isotopes, one (xenon) has eight isotopes, four have seven isotopes, nine have six isotopes, nine have five isotopes, nine have four isotopes, five have three stable isotopes, 16 have two stable isotopes, and 26 have a single stable isotope.Additionally, about 29 nuclides of the naturally occurring elements have unstable isotopes with a half-life larger than the age of the Solar System (~109 years or more).[b] An additional six nuclides have half-lives longer than 80 million years, which is far less than the age of the solar system, but long enough for some of them to have survived. These 35 radioactive naturally occurring nuclides comprise theradioactive primordial nuclides. The total number of primordial nuclides is then 253 (the stable nuclides) plus the 35 radioactive primordial nuclides, for a total of 288 primordial nuclides. This number is subject to change if new shorter-lived primordials are identified on Earth.One of the primordial nuclides is Ta-180m, which is predicted to have a half-life in excess of 1015 years, but has never been observed to decay. The even longer half-life of 7.7 x 1024 years of tellurium-128 was measured by a unique method of detecting its radiogenic daughter xenon-128 and is presently the longest known experimentally measured half-life. Another notable example is the only naturally occurring isotope of bismuth, which has been predicted to be unstable with a very long half-life, but has only recently been observed to decay. Because of their long half-lives, such isotopes are still found on Earth in various quantities, and together with the stable isotopes they are called primordial isotopes. All the primordial isotopes are given in order of their decreasingabundance on Earth.[c]. For a list of primordial nuclides in order of half-life, see list of nuclides.