[thedoc (OP)]

David Spence asked the Naked Scientists:
   This may seem to be a rather odd question, but here goes.....

Does a single atom (say hydrogen (either as part of matter or in space)) have a life span?
What do you think?

[RD]

The nucleus of a hydrogen atom is a proton ... https://en.wikipedia.org/wiki/Proton_decay

[PmbPhy]

Does a single atom (say hydrogen (either as part of matter or in space)) have a life span?

Yes. For a single atom it has the following meaning: There is a specific probability that the nucleus will decay within a specific amount of time. That tells you something about its life span. But in reality either it will decay during that time period or it won't. Life span usually refers to a large number of atoms in a sample and one speaks of the half life. The half life is the amount of time it takes for the number of atoms in the sample to reduce to one half its value.

I have some time this morning so I'll explain some details for you. Note: The presuperscript is the atomic mass number, A, which is the sum of the number of protons and the number of neutrons. Some people will say that all atoms have a life span and that those atoms that don't decay have an infinite life span. I myself choose not to use that terminology so I won't use it in this post.

It's not enough to ask whether a particular kind of atom of an element has a life span or not. One has to specify which isotope of the element you have in mind. Different isotopes of an element differ by the number of neutrons in the nuclei. And sometimes the half-life is so long that it compares to the age of the Earth! But even in those cases they're still considered radioactive and are dangerous to be around if the concentration is too high.

Consider naturally occurring uranium. It's composed of the following three major isotopes; ²³⁸U (99.2739–99.2752% natural abundance), ²³⁵U (0.7198–0.7202%), ²³⁴U (0.0050–0.0059%).[6] All three isotopes are radioactive, creating radioisotopes, with the most abundant and stable being uranium-238 with a half-life of 4.4683×109 years. As I said above, the half life of ²³⁸U is close to the age of the Earth!