Radioactive decay is random. Every unstable nucleus has the same chance of decaying in any given time, and nothing we can do changes that chance. Heating the source, squeezing it or reacting it chemically makes no difference. This means it cannot be predicted when a particular nucleus will decay. It might be in the next second or after millions of years.
A nucleus does not get closer to decaying as time passes. A nucleus that has survived for one half-life is no more likely to decay than a brand new one.
Although one nucleus cannot be predicted, a very large number of nuclei behave in a predictable way. Whatever the size of the sample, about half of the undecayed nuclei decay in each half-life. So the half-life lets us predict the activity of a source during the decay process.
With only a small number of nuclei the pattern is less predictable, because chance plays a bigger part and the readings jump about. A source with billions of nuclei is much larger in number and decays in a very predictable way.