Naturally occurring zirconium (₄₀Zr) is composed of four stable isotopes (of which one may in the future be found radioactive), and one very long-lived radioisotope (⁹⁶Zr), a primordial nuclide that decays via double beta decay with an observed half-life of 2.0×10¹⁹ years; it can also undergo single beta decay, which is not yet observed, but the theoretically predicted value of t_1/2 is 2.4×10²⁰ years. The second most stable radioisotope is ⁹³Zr, which has a half-life of 1.53 million years. Thirty other radioisotopes have been observed. All have half-lives less than a day except for ⁹⁵Zr (64.02 days), ⁸⁸Zr (83.4 days), and ⁸⁹Zr (78.41 hours). The primary decay mode is electron capture for isotopes lighter than ⁹²Zr, and the primary mode for heavier isotopes is beta decay.