Answer to Question #681 Submitted to "Ask the Experts"

Q

Is hafnium a radioactive element?

A

Hafnium is an element that has both stable and radioactive isotopes—and quite a few of each. Isotopes of an element contain the same number of protons in the nucleus but different numbers of neutrons. The number of protons is referred to as the atomic number, and the combined number of protons plus neutrons is called the mass number. Hafnium has an atomic number of 72 and a total number of isotopes of 31. Mass numbers of the hafnium isotopes range from 154 to 184, inclusive. Of these 31 isotopes, six are considered stable (not radioactive) and have mass numbers of 174, 176, 177, 178, 179, and 180. The remainder of the isotopes are radioactive. All of these radioactive isotopes are man-made. The radioactive isotopes have different properties: half-lives (the half-life is the time it takes for an initial population of radioactive atoms to be reduced by a factor of two due to radioactive decay) that range from one second to about two years, and various modes of radioactive decay (the mode of decay has to do with the fashion that the nucleus transforms itself in an attempt to achieve a more stable configuration). The decay modes exhibited by the hafnium isotopes encompass all major decay modes including electron capture, beta-plus emission, beta-minus emission, and alpha decay. You can find a more detailed description of these decay modes in any basic physics text.