Why do atoms have half lives?

what basically happens is helium molecules break off of the nucleus of the unstable higher order molecules. if there are more molecules, more are going to break off, so the rate is going to be faster as a whole, even if the rate of single atoms is the same.

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One way of looking at it is that it's all probability : the individual atoms are unstable but only have a small probability of spontaneously decaying in a given interval, put a lot of atoms in a lump and the probability is that , during the period called 'half life' about half the atoms will spontaneously decay on average. This means that there will only be about half as many unstable atoms left as you started with - each of these individual atoms still has exactly the same probability to decay as it had during the first period (when it didn't decay - but it could have done) now at the end of your second half life period, on average about half of the unstable atoms that were still around will still be unstable atoms - the rest will have decayed to stable atoms and released a few products in the process.

In decay, a certain statistical % of the atoms in any sample decay each second. Ergo, it is clear that the larger the sample, the more atoms per second will decay. It is, however, still the same %, so half the sample will have left in the time called the half life.

Only radioactive atoms have half lives. They have too much energy in their nuclei, so they must undergo decay in order to reach a stable state. The half life is the amount of time it takes for half of the original sample to reach a stable state (decay). After another half life, half of this amount will have decayed, and so on

radioactive elements are by nature unstable

helium 4 is very stable and does not decay

uranium is a very large element and is unstable, partially because of its size and atomic arrangement of protons, neutrons and electrons

each element that is radioactive - carbon 14, potassium 40, etc decays at a specific rate based on the structure of the atom