The Mattauch isobar rule, formulated by Josef Mattauch in 1934, states that if two adjacent elements on the periodic table have isotopes of the same mass number, one of these isotopes must be radioactive.[1] Two nuclides that have the same mass number (isobars) can both be stable only if their atomic numbers differ by more than one.[1] However, this rule cannot make predictions on the half-lives of these radioisotopes.[1]

A consequence of this rule is that technetium and promethium both have no stable isotopes as each of the elements just before and just after these two unstable elements on the periodic table have a stable isotope for each mass number for the range in which the isotopes of the unstable elements usually would be stable to beta decay.[1] These ranges can be calculated using the liquid drop model: see Isotopes of technetium#Stability of technetium isotopes.[citation needed]

^ a b c d Thyssen, Pieter; Binnemans, Koen; Shinohara, Hisanori; Saito, Yahachi; Gulay, Lubomir D.; Daszkiewicz, Marek; Yan, Chun-Hua; Yan, Zheng-Guan et al (2011). Gschneider, Karl A., Jr.; B├╝nzli, Jean-Claude; Pecharsky, Vitalij K.. eds. Handbook on the Physics and Chemistry of Rare Earths. Amsterdam, The Netherlands: Elsevier. p. 66. ISBN 978-0-444-53590-0. Retrieved January 14, 2012.

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