# Why are all isotopes of tungsten considered (theoretically) unstable?

Naturally occurring tungsten consists of four stable isotopes ($$^{182}\mathrm W$$, $$^{183}\mathrm W$$, $$^{184}\mathrm W$$, and $$^{186}\mathrm W$$) and one very long-lived radioisotope, $$^{180}\mathrm W$$. Theoretically, all five can decay into isotopes of element $$72$$ (hafnium) by alpha emission, but only $$^{180}\mathrm W$$ has been observed to do so, with a half-life of $$1.8 \pm 0.2 \times 10^{18}$$ years; on average, this yields about two alpha decays of $$^{180}\mathrm W$$ per gram of natural tungsten per year. The other naturally occurring isotopes have not been observed to decay, constraining their half-lives to be at least $$4 \times 10^{21}$$ years.

So all isotopes of tungsten should, theoretically, release the occasional alpha particle but I can find no explanation of why.

But I can find nothing about elements 75 through 82 that says they should decay, slowly, via alpha decay....

So what is it about tungsten that makes theoretical scientists believe no isotope of tungsten should be completely stable? But only tungsten isotopes, not rhenium, osmium.... to lead?

(Important note: Experimentally, no one has seen this happen, except for $$^{180}\mathrm W$$)

• Please provide a link to the material where you read that and/or a reference. Thanks. – StephenG Aug 4 '19 at 7:05
• en.m.wikipedia.org/wiki/Isotopes_of_tungsten mentions this information without reference. – my2cts Aug 4 '19 at 9:44
• Sorry if I screwed up the link(s), or forgot about them.... – Kurt Hikes Aug 4 '19 at 19:03

One can calculate the Q-value (which is the maximal alpha kinetic energy) from the mass difference. When that is positive, decay can theoretically happen.

Alpha decay occurs by tunneling through the barrier around the nucleus. I think it was Gamow who deduced that the relation between kinetic energy and decay rate was exponential.

Uranium-238 has a Q-value of 4.27 MeV and its half-life is still quite long.

Table 1 in Danevich et al shows a Q-values for W-180 of 2.5 MeV. The other tungsten isotopes have Qs below 2 MeV.

• Maybe you can update the wiki page. – my2cts Aug 4 '19 at 11:32
• Aha, physics can be simple! Assuming the barrier for the alpha is the same it should be possible to estimate the half-life of the other isotopes. – my2cts Aug 4 '19 at 11:40
• @my2cts The link to this article was on en.wikipedia.org/wiki/Tungsten#Isotopes – Pieter Aug 4 '19 at 11:53
• Thank you so much, Pieter! BUT, I still think it is weird that the next few elements, up to lead, AREN'T expected to ever emit alphas.... – Kurt Hikes Aug 4 '19 at 19:05
• @KurtHikes You could calculate Q-values. Or try to find them, as I am sure it has been done. But I only found graphs of half lives, Fig. 11.13 or 6:13 here: sciencedirect.com/topics/physics-and-astronomy/alpha-decay – Pieter Aug 4 '19 at 19:58