>>5
Technically, the cry should be
"Bismuth-Thallium alloy, FUCK YEAH!". The Bismuth atoms don't disappear; they transform into Thallium, and then the mass of Bismuth is evenly distributed with Thallium atoms over time.
If we just said
"Mass of atoms whose atomic numbers change, FUCK YEAH!", we'd be covered ... well, not FOREVER, but it would also cover
proton decay and that might take a half-life of 10
35 years.
In fact, if the universe contains about 10
80 particles, and almost all of those are protons or will become protons from neutron decay, then a 10
35-year half-life will mean that by year 10
38, all the protons will be gone, and the 2nd statement will no longer apply.