Question

In a nuclear fusion reaction hydrogen atoms combine to create a helium atom. But in a hydrogen atom there are no neutrons, then where does the neutrons in helium nucleus come from ?

Answer 1

There is more than one answer to this question. We can look
first about how hydrogen (with no neutrons, just one proton)
becomes helium (two protons, two neutrons) in a star like our
sun. As shown by the figure, it begins with pairs of protons
which interact and fuse in a reaction which results in
one deuteron (heavy hydrogen, one proton, and one neutron
), one positron (a positively charged electron), and
one neutrino. The deuterons thus created now fuse with
another proton to create a light isotope of helium, 3He
(one neutron, two protons), and also a
gamma ray. Two of these 3He now fuse, throwing
off two protons in the process and one alpha particle (4He,
two protons, and two neutrons). The two thrown-off protons are
now free to continue the process, so this chain, called the
proton-proton cycle is self-sustaining until the star runs
out of hydrogen which ends the life of the star. For more earth-bound hydrogen fusion, like fusion reactors or

hydrogen bombs, single-step reactions are sought. The

most frequently used is the deuterium-tritium reaction,

involving the fusion of one deuteron and one triton