Hence the formula to calculate the half-life of a substance is. Some radionuclides have half. Radioactive Decay And Half Life Nuclear Medicine Chemistry Lessons Chemistry Classroom For example in the radioactive decay case the half-life is the length of time after there is a 50 chance that an atom would have undergone nuclear decay. . Each substance has a different half-life. Half-Life stylized as HλLF-LIFE is a science fiction first-person shooter developed and published by Valve. It was originally used to describe the decay of radioactive elements like uranium or plutonium but it can be used for any substance which undergoes decay along a set or exponential rate. Half-life is defined as the time required for half of the unstable nuclei to undergo their decay process. Decay Chain of Isotope Pa-231. Where t 12 is the half-life of the particle t is the elapsed time N 0 is the quantity in the beginning and N t is the quantit