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 2002cx suggesting that SN 2002cx is similar to SN 1991T which also lacked such lines. This spectrum for SN 2002cx has a low expansion velocity measuring only ~6400 km s. This marked one way in which it was different from SN 1997br, as SN 1997br's expansion velocity was ~10,400 km s at the same point relative to its own B-band maximum. At the time of measurement SN 2002cx's expansion velocity was the lowest measured for an early time type Ia, Another spectrum taken on 2002 May 20, 1 day before B-band maximum light, showed little evolution from the one taken on