What is the lifespan of a hydrogen atom?
That depends on what you identify as life span, and it depends on where….
What sets a hydrogen atom apart from other atoms is its nucleus: a single proton. And a proton lives very, very long. But a hydrogen atom also contains an electron. Do you call it the end of life if the two are separated? Or only when the two can no longer be combined to make a hydrogen atom?
If you keep an electron and a proton together as hydrogen atom in isolation somewhere in the deep vacuum of space, nothing much will happen to it. Until it hits another atom or a photon: then the electron and the proton could be separated. Does that count as end of life? Here on earth, a separate hydrogen atom will encounter other atoms all the time, and it will be quickly take part in chemical reactions. Does that mean its end of life? A hydrogen atom in a molecule is sharing electrons. You can not even distinguish or follow the individual electrons according to quantum theory. Does that mean its end of life? A hydrogen nucleus in the plasma of the sun is swimming in a sea of other protons and electrons. Do you still call it a hydrogen atom? Then its true end of life may be near, because in the fusion reactor that we call our sun hydrogen is combined into helium nuclei.