Complex sentence: Astatine-211, however, has a significantly higher energy than the previous isotope, because it has a nucleus with 126 neutrons, and 126 is a magic number corresponding to a filled neutron shell.
Split into simpler sentences: Astatine-211, however, has a significantly higher energy than the previous isotope. It has a nucleus with 126 neutrons, and 126 is a magic number, corresponding to a filled neutron shell.