Half-life depends on probability because the atoms decay at a random time. Half-life is the expected time when half the number of atoms have decayed, on average. Carbon-14 has a half-life of 5,730 years. Taking one atom of carbon-14, this will either have decayed after 5,730 years, or it will not. But if this experiment is repeated again and again, it will be seen that the atom decays within the half life 50% of the time.