Chemical element isotopes: specifically the elements without any stable isotopes are technetium (atomic number 43), promethium (atomic number 61), and all observed elements with atomic numbers greater than 82. Of the 80 elements with at least one stable isotope, 26 have only one single stable isotope. The mean number of stable isotopes for the 80 stable elements is 3.1 stable isotopes per element. The largest number of stable isotopes that occur for a single element is 10 (for tin, element 50). The mass number of an element, "A", is the number of nucleons (protons and neutrons) in the atomic nucleus. Different isotopes
SN 2014J ± 0.3 megaparsecs), SN 2014J was one of the closest supernovae seen for decades. It was the closest type Ia supernova since SN 1972E, and the closest supernova of any type since 2004. Some sources initially stated that SN 2014J was the closest supernova of any type since SN 1987A, but this claim is erroneous. The last supernova that was unambiguously closer to Earth than SN 2014J was SN 2004dj, a type II-P supernova in the galaxy NGC 2403, 8 million light-years from Earth. SN 1993J was a type IIb supernova at almost the same distance as SN 2014J, because