 Here's a measured sample of uranium-235. The rate at which unstable radioactive nuclei decay in a sample is called the activity of the sample. The greater the activity, the more nuclear decays per second. This is rather easily measured with devices like a Geiger counter. Here's a 5 second run illustration. Our uranium-235 sample is decaying almost 19 million nuclei per second. Given the number of radiating molecules in a sample and measuring the activity, we can calculate the probability for any one molecule to decay in a second. This is called the decay constant. We find that the decay constant is always a small number, constant over time, and different for different materials. Here we have the decay constant for uranium-235. Both the activity rate and the number of radioactive nuclei vary over time. As a sample decays, the number of radioactive nuclei decreases. With fewer radioactive nuclei, the activity rate also decreases. From this we get the exponential law of radioactive decay. It tells us how the number of radioactive decay in a sample decreases with time. The half-life is the time it takes for the material and activity to be reduced by half. For uranium-235 we get a half-life of 704 million years. But the decay rate we need is not uranium to thorium, but the decay rate of uranium to lead. The two uranium to thorium decays we examined earlier become a pair of complex decay chains with some happening serially and some happening in parallel. But overall decay constants and half-lives have been measured and the fact that there are two paths give us the opportunity to cross-check when we find rocks with both types of uranium present.