<Dl> <Dd> n + e + ↽ − − ⇀ ν _̄ e + p (\ displaystyle (\ ce (n + e + <=> (\ overline (\ nu)) _ (e) + p))) </Dd> <Dd> n + ν e ↽ − − ⇀ p + e − (\ displaystyle (\ ce (n + \ nu_ (e) <=> p + e -))) </Dd> </Dl> <Dd> n + e + ↽ − − ⇀ ν _̄ e + p (\ displaystyle (\ ce (n + e + <=> (\ overline (\ nu)) _ (e) + p))) </Dd> <Dd> n + ν e ↽ − − ⇀ p + e − (\ displaystyle (\ ce (n + \ nu_ (e) <=> p + e -))) </Dd> <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>

What did the big bang nucleosynthesis take place