stylistic:tone
Isotopes

At least 40 isotopes of hafnium have been observed, ranging in mass number from 153 to 192. The five stable isotopes have mass numbers ranging from 176 to 180 inclusive. The radioactive isotopes' half-lives range from 400 ms for Hf to 7.0 × 10 years for the most stable one, the primordial Hf.

The extinct radionuclide Hf has a half-life of 8.9 ± 0.1 million years, and is an important tracker isotope for the formation of planetary cores. The nuclear isomer Hf was at the center of a controversy for several years regarding its potential use as a weapon.