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I read something about that the Q-Value hast to be greater then $0$.
But there must be certainly more conditions otherwise there would be nuclei that could't decay.

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  • $\begingroup$ And you have not been able to think of a nucleus that does not decay? $\endgroup$ – Pieter Jul 13 at 10:13
  • $\begingroup$ A positive Q-value is mandatory if you want energy to be conserved, so no matter if other conditions (such as angular momentum, parity, barionic number and charge conservation) are met, nuclei are stable under decays with negative Q. $\endgroup$ – Francesco Arnaudo Jul 13 at 10:50
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The main condition for nuclear decay is that the nucleus should be unstable. There are nuclei that can't decay in the normal course of events, although they can be split by bombarding them with other particles. One of the things which make nuclei unstable is an overabundance of neutrons. Helium 4 is very stable and has 2 protons and 2 neutrons, but helium 5 has an extra neutron and is very unstable, decaying in a fraction of a second. Carbon 12 is very stable and never decays, because it is a low mass element and has an even number of protons (6) and neutrons (6),though exactly even numbers of protons and neutrons are not usually necessary for a nucleus to be stable. Isotopes with an excessive number of neutrons tend to be unstable, and the heavy elements like uranium (92 protons and 143 neutrons) and elements beyond all have excessive numbers of neutrons and are therefore unstable. An insufficient number of neutrons rather than an excess can also make a nucleus unstable.

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    $\begingroup$ PS It is not necessary for protons and neutrons to be exactly equal for a nucleus to be stable but in light elements it helps. In the very heavy elements it's impossible; they have to have a superabundance of neutrons even to be almost stable.. $\endgroup$ – Michael Walsby Jul 13 at 12:17
  • $\begingroup$ Well, Be-8 is an obvious counter example... $\endgroup$ – Jon Custer Jul 14 at 1:43

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