The alkaline earth metalstrontium has four stable, naturally occurring isotopes: 84Sr, 86Sr, 87Sr and 88Sr. Its standard atomic weight is 87.62. Only 87Sr is radiogenic; it is produced by decay from the radioactivealkali metal87Rb, which has a half-life of 4.88 × 1010 years. Thus, there are two sources of 87Sr in any material: primordial, formed during nucleosynthesis along with 84Sr, 86Sr and 88Sr; and that formed by radioactive decay of 87Rb. The ratio 87Sr/86Sr is the parameter typically reported in geologic investigations; ratios in minerals and rocks have values ranging from about 0.7 to greater than 4.0. Because strontium has an electron configuration similar to that of calcium, it readily substitutes for Ca in minerals. In addition to the four stable isotopes, thirty-one unstable isotopes of strontium are known to exist. Strontium decay mostly in its neighbours: yttrium and rubidium. The longest-lived of these isotopes, and the most relevantly studied are 90Sr with a half-life of 28.9 years and 85Sr with a half-life of 64.853 days. Of importance are also strontium-89 with a half-life of 50.57 days. They decay with: 89Sr is an artificial radioisotope used in treatment of bone cancer. In circumstances where cancer patients have widespread and painful bony metastases, the administration of 89Sr results in the delivery of beta particles directly to the area of bony problem, where calcium turnover is greatest. 90Sr is a by-product of nuclear fission, present in nuclear fallout. The 1986 Chernobyl nuclear accident contaminated a vast area with 90Sr. It causes health problems, as it substitutes for calcium in bone, preventing expulsion from the body. Because it is a long-lived high-energy beta emitter, it is used in SNAP devices. These devices hold promise for use in spacecraft, remoteweather stations, navigational buoys, etc., where a lightweight, long-lived, nuclear-electric power source is required. The lightest known isotope is 73Sr and the heaviest is 107Sr. All other strontium isotopes have half-lives shorter than 55 days, most under 100 minutes.
