Is there something teleological about the half-life of an isotope?
How about defining it as an instance of functional complexity? And treating the list of properties as following necessarily from this definition? Okay, this is very diamond-in-the-rough (or perhaps zirconium...) but here goes my thoughts on the matter: (Complexity) Many diverse parts (functional) acting for the sake of the whole I think I need to add something like "not as the instrument of another" or something like that. Given that they act together as one whole and given entropy, etc., the organism will need to posses more order than its surroundings (homeostasis). It will therefore need to take in energy (nutrition), which it will use to sustain its readiness to interact with its environment so as to preserve its own being (homeostasis again?), but which will eventually break down (death), so that in order for that life form to continue it will need to duplicate itself (reproduction), which, upon occurring, will involve both development (growth) to maturity and...
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I would think of isotopic composition as more of a quality of a thing than as a thing itself with a purpose.
But that same decay is the source of an interesting philosophical problem that I'm seriously trying to address. To give you a sense of that problem, I will first tell a story: before quantum theory was formulated, some folks said there's no need for God because we live in a deterministic universe, where every physical event is determined entirely by previous physical events. After quantum theory became known some folks (probably the same folks) said there's no need to posit God because, if we look at radioactive decay, we find events that have no cause.
An alternative to these two positions is to say that there are causes that are NOT deterministic. As long as physics takes math as its paradigm of knowledge, then such a proposal will seem ludicrous, for causes, to be intelligible, must exhibit necessity. If an event is not necessitated, then it's uncaused.
The alternative position would be that radioactive decay has some kind of cause, but it's not the type that guarantees that the event will happen here and now.
That description overlaps somewhat with a final cause: it's almost as if atoms were trying to decay and eventually do so.
So, in response to your question, I'm talking about the eventual decay rather than the initial composition. But yes, I am not sure that it makes sense to talk about purposes here. The truth is that, for Aristotle, it might not make sense, when considering non-living things, to talk about the purpose of this or that part. Perhaps he is principally committed to regarding nature as a whole as teleological: parts (such as particles) are telic only in a secondary manner.