Suppose I made a fork out of Uranium, and let it sit (completely undisturbed) until all of the uranium had become lead. I realize that would be a real long time, but would I now have a lead fork or would the process of radioactive decay cause the shape of the object to change?
It would maintain it’s shape, as long as the decay was spontaneous. If you built a really big uranium fork, from uranium 235, then you would not be able to see the fork ever again. Or anything else, for that matter.
Think of it like this: the uranium atoms in the fork are “replaced” by lead atoms. So… it’s mass would change, and it wouldn’t have the same physical of chemical properties, but barring some atomic disarray caused by the collision of nuclei within the fork it would remain in the same fork-ish shape.
(I haven’t been able to find a citation yet, but I’m pretty sure the above is accurate. I’ll keep looking, though.)
You shouldn’t put it in the dishwasher with the other forks.
I just thought you ought to know that.
It probably will change size: Different elements will have more or less atoms per volume, and you’ll have the same number of atoms. I don’t have numbers handy for uranium and lead, so I’m not sure how much of an effect this is. It’ll be happening more or less uniformly throughout the fork, though, so the shape shouldn’t change.
Best laugh all week so far. 
Lead has an Atomic Wieght of ~207. Uranium’s is ~238.
The difference between Lead and Uranium is negligible for this purpose. There may be some extremely slight change in physical dimensions and weight, but nothing you’d notice on anything outside of precision lab instrumentation. It’s chemical properties would change, as likely would it’s mechanical strength. Of course, we’re talking about a long time, so record you initial observations very carefully. It takes ~5 half-lives for any radioactive element to decay away to >minimum detectable levels, so, we’re talking about roughly 22,501,588,063 years for our fork to go through five half-lives of each element in it’s 14-step chain of decay to become pure lead. I think by that time, no one will remember it, much less care.
Actually, the weight change would be noticable, just that it takes d*mn-near forever to happen.
I have to agree with Astro!
I missed something kinda important when I was running throught the decay chain. Our fork vanishes completely. The fifth decay produces Radon-222, which is a gas.
SO - In roughly 22,501,587,956 years, the very last bit of our fork wafts-away on the breeze. Bye-Bye fork! Some 113 years later, the last bits of lead particles will be formed, God knows where.
Okay, fine. But if we store the fork below -71 degrees C, THEN it will eventually turn into a lead fork, with little or no change in outward appearance.
The point being, re: the OP, that the process of radioactive decay itself won’t change the shape of the object.
Not necessarily. Radon is a noble gas, which means that in solid form it will have a vary different crystal structure than uranium. This would likely cause the fork to decay into a pile of dust. Even without the Radon peccadillo, the changes in crystal structure during the decay process might be enough to powder the fork.
If said fork were made of isotopically pure c[sup]14[/sup]pvc the decay process would result in the fork turning into a puddle of liquid before evaporating. Radioactive decay destroys the covalent structure of organic molecules.
Good point, Squink. I hadn’t considered changes in crystal structure. You are right about that being a big problem.
So…
Looks as though I shall have to reverse myself.
Here goes: the decay process from uranium to lead would (under any circumstances) probably destroy the fork.