<P> At times much earlier than 1 sec , these reactions were fast and maintained the n / p ratio close to 1 : 1 . As the temperature dropped , the equilibrium shifted in favour of protons due to their slightly lower mass , and the n / p ratio smoothly decreased . These reactions continued until the decreasing temperature and density caused the reactions to become too slow , which occurred at about T = 0.7 MeV ( time around 1 second ) and is called the freeze out temperature . At freeze out , the neutron - proton ratio was about 1 / 6 . However , free neutrons are unstable with a mean life of 880 sec ; some neutrons decayed in the next few minutes before fusing into any nucleus , so the ratio of total neutrons to protons after nucleosynthesis ends is about 1 / 7 . Almost all neutrons that fused instead of decaying ended up combined into helium - 4 , due to the fact that helium - 4 has the highest binding energy per nucleon among light elements . This predicts that about 8 % of all atoms should be helium - 4 , leading to a mass fraction of helium - 4 of about 25 % , which is in line with observations . Small traces of deuterium and helium - 3 remained as there was insufficient time and density for them to react and form helium - 4 . </P>
<P> At times much earlier than 1 sec , these reactions were fast and maintained the n / p ratio close to 1 : 1 . As the temperature dropped , the equilibrium shifted in favour of protons due to their slightly lower mass , and the n / p ratio smoothly decreased . These reactions continued until the decreasing temperature and density caused the reactions to become too slow , which occurred at about T = 0.7 MeV ( time around 1 second ) and is called the freeze out temperature . At freeze out , the neutron - proton ratio was about 1 / 6 . However , free neutrons are unstable with a mean life of 880 sec ; some neutrons decayed in the next few minutes before fusing into any nucleus , so the ratio of total neutrons to protons after nucleosynthesis ends is about 1 / 7 . Almost all neutrons that fused instead of decaying ended up combined into helium - 4 , due to the fact that helium - 4 has the highest binding energy per nucleon among light elements . This predicts that about 8 % of all atoms should be helium - 4 , leading to a mass fraction of helium - 4 of about 25 % , which is in line with observations . Small traces of deuterium and helium - 3 remained as there was insufficient time and density for them to react and form helium - 4 . </P>
for heavy elements what is the ratio of neutrons to protons that predicts a stable nucleus
for heavy elements what is the ratio of neutrons to protons that predicts a stable nucleus
1 : 1
1 : 1