An atomic nucleus is made up of protons and neutrons bound together, and some particular combinations of protons and neutrons are stable indefinitely while others are unstable, prone to radioactive decay, spontaneously transforming into a different nucleus while releasing radiation in the process. Each unstable, radioactive isotope decays at a fixed, statistically predictable rate, described by its characteristic half-life, the time it takes for half of a given sample to decay — and unlike most physical processes, this decay rate is essentially untouched by ordinary external conditions like temperature, pressure, or the isotope's specific chemical environment.
Half-life describes a statistical rate, not a fixed lifespan for any single atom
A radioactive isotope's half-life doesn't mean every individual atom of that isotope decays at exactly that specific age; radioactive decay is a fundamentally statistical, probabilistic process at the level of any single atom, with no way to predict precisely when one particular atom will decay. What half-life actually describes is the statistical behaviour of a large population of atoms: given a large enough sample, very reliably, about half of it will have decayed after one half-life has passed, regardless of how old the sample or its individual atoms already were when that clock effectively started.
Ordinary physical and chemical conditions leave the decay rate essentially unchanged
Most physical and chemical processes speed up considerably under higher temperature or pressure, since those conditions typically increase the rate at which atoms or molecules interact and react. Radioactive decay is a striking exception: because it originates from instability within the nucleus itself, deep inside the atom and largely insulated from the atom's outer electron structure, ordinary changes in temperature, pressure, or even which chemical compound the radioactive atom happens to be part of have essentially no measurable effect on its decay rate. A given radioactive isotope decays at very close to the same characteristic rate whether it's sitting in a hot furnace, a cold vacuum, or bonded into an entirely different chemical compound.
What we're still unsure about
The statistical nature of radioactive decay, and its remarkable insensitivity to ordinary temperature, pressure and chemical environment, are extremely well established, precisely measured nuclear physics, confirmed across an enormous range of studied isotopes. What's more genuinely a matter of specialised ongoing physics research is a small number of unusual, quite extreme exceptions where decay rates have been measured to shift very slightly under highly specific, non-ordinary conditions, such as certain electron-capture decay processes in atoms stripped of their electrons entirely — physicists continue to study exactly how, and how significantly, decay rates can shift under such extreme, atypical conditions, without this changing the well-established rule that decay rates are effectively fixed under the ordinary conditions found in everyday physical and chemical environments.
This sits inside Nuclear Structure & Radioactive Decay, one of seven topics in Modern Physics, one of five domains in Physics, one of seventeen subjects the app can quiz you on.