Of the nine primordial odd–odd nuclides (five stable and four radioactive with long half lives), only 7 N is the most common isotope of a common element. This is the case because proton capture on 7 N is the rate-limiting step of the CNO-I cycle. The nuclides 3 Li and 5 B are minority isotopes of elements that are themselves rare compared to other light elements, while the other six isotopes make up only a tiny percentage of the natural abundance of their elements. For example, 73 Ta is thought to be the rarest of the 251 stable nuclides.