If one wants to precisely determine the energy exchanged in a nuclear reaction, one needs to determine the atomic mass very precisely. Since the isotopes involved in a supernova are generated in very unusual environments of extreme density and temperature, they exhibit unusual properties, like their neutron-to-proton ratio (N/Z). Unlike typical light nuclei where N/Z is ~ 1 (e.g., C-13 with N/Z = 7/6), those participating in supernova reactions can have N/Z ratios of 2/1 or even 3/1 (e.g, Li-11). As a result they undergo radioactive decay very quickly (usually by beta-decay but in some cases even by nucleon emission) with half-lives (t1/2) that are often only a fraction of second (e.g, Li-11 has a t1/2 of 8 ms.) A section of the ''Chart of the Nuclides" for light nuclei is shown in Figure 6 below.