According to the Mattauch isobar rule (1934), if two adjacent elements in the periodic table have isotopes with the same mass number, then one of them will be radioactive. The mass number (or isobar) is the total of protons and neutrons in an atomic nucleus.
Since technetium (atomic number 43) has the same mass number as its neighbouring elements Mo and Ru (for a range of isotopes) there are no stable nuclides for 43 protons under this rule.
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