About the Stability of hydrogen Isotopes Shouldn't tritium be more stable than a hydrogen atom with no neutrons? Why does tritium do alpha decay but not hydrogen atom with no neutrons? I suppose that when the number of neutrons increases the strong nuclear force also increases. So why is tritium unstable?
 A: 
Shouldn't tritium be more stable than a hydrogen atom with no neutrons. Why does tritium do alpha decay but not hydrogen atom with no neutrons?

The tritium nucleus, or triton, is unstable simply because it is more
massive than its decay products which are a helium-3 nucleus, an
electron, and an electron anti-neutrino. Its decay products are more stable. This is called beta decay.

More on this decay:
Tritium is the heaviest isotope of hydrogen with nuclear charge Z = 1.Tritium is radioactive and decays with a half-life of 12.3 years by β decay into helium isotope 3, with charge Z = 2. The mean kinetic energy release in the β-electron is ≈ 5.6 keV, the maximum at 18.59 keV. The β-electron is the electron from the decay of one of the neutrons in the nucleus. 

I suppose that when the number of neutrons increases the strong nuclear force also increases.

As Wikipedia says,
The nuclear force is a residual effect of the more fundamental strong force or strong interaction. The strong interaction is the attractive force that binds the elementary particles called quarks together to form the nucleons (protons and neutrons) themselves. This more powerful force is mediated by particles called gluons. Gluons hold quarks together with a force like that of electric charge, but of far greater strength. Quarks, gluons, and their dynamics are mostly confined within nucleons, but residual influences extend slightly beyond nucleon boundaries to give rise to the nuclear force.

For more : see this Wikipedia page . It's about the semi-empirical mass formula and how it is used to predict the stability of different nuclei. 
A: As far as I know (and I know really little about this topic) science don't really know WHY a nucleus is stable and nother is not.
Yes, the first answer is correct: He-3 is more stable that H-3, but in fact, although we know a lot about the strong interaction, we lack full comprehension on how it works and how atomic nuclei are built.
As answer #2 said, the law to predict nuclei stability is semi-empirical!
If I were young and I studied physics I would try to understand all this, exactly on tritium, deuterium and He-3, because these are the simplest cases to study. I would try to understand WHY strong interaction (and the weak one and the EM one) make tritium and an isolated neutron unstable, while deuterium and He-3 are stable. Most probably this would lead to substantial changes in the quark model we use today.
