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 1054 and SN 1181) reveals a proximity in the dates of discovery of a guest star in China and Japan, although clearly based on different sources. Fujiwara no Teika's interest in the guest star seems to have come accidentally whilst observing a comet in December 1230, which prompted him to search for evidence of past guest stars, among those SN 1054 (as well as SN 1006 and SN 1181, the two other historic supernovas from the early second millennium). The entry relating to SN 1054 can be translated as: The source used by Fujiwara no Teika is the records of Yasutoshi