High Quality Content by WIKIPEDIA articles! Naturally occurring niobium (Nb), element 41, is composed of one stable isotope (93Nb). 93Nb is the lightest nuclide theoretically susceptible to spontaneous fission, and although this has never been observed, it makes niobium theoretically the lightest element with no stable isotope. The first 40 elements (to zirconium) all have at least one stable nuclide which is susceptible in theory only to proton decay.