The half life is a statistical quantity.
The half life is a statistical quantity, normally defined over a large set of atoms (think large sample size). Wikipedia explains:
Instead, the half-life is defined in terms of probability: "Half-life is the time required for exactly half of the entities to decay on average". In other words, the probability of a radioactive atom decaying within its half-life is 50%.
Because whilst we cannot say with any certainty what one particle will do, we can use statistics to draw conclusions about the population as a whole.
Let’s use popcorn as an example - you cannot say when any of the hundreds of kernels will pop, but you can have confidence in the statement that after 2 minutes in the hot oil, half of them will have popped.
This is because the statistics do not require any knowledge of the particles themselves, just knowledge of the mathematical laws determining the shape of the probability distribution.
As an aside, radioactive decay is one of the most random processes we know of, because the number of particles in even a speck of material is truly enormous.