Isotopes in Nuclear Fusion

When two atoms of hdrogen having one proton and one electron each without neutron fuse to make helium atom from where did the two neutrons in nucleus of helium came from ?
- Anshul (age 15)
Ghaziabad, uttar pradesh, India

You're right to wonder--if you create helium using the nuclear fusion reaction you describe, there won't be any neutrons in the nucleus of the new helium atom. To get a helium atom with two neutrons, a few other things have to happen.

But first of all, does a helium atom have to contain two neutrons? Each element has a characteristic number of protons in its nucleus. For example, hydrogen has one proton, helium has two, carbon has six, uranium has 92. An element like helium, which always has two protons, can have different numbers of neutrons and still be the same element. Atoms of the same element with different numbers of neutrons are called isotopes. The number of neutrons affects the properties of the isotope, including its stability (whether it will decay into something else) and its mass. You can find more information about isotopes in .

A helium nucleus with no neutrons is very unstable and will decay almost immediately. Most of the time, it decays by emitting a proton, which is just the reverse of your original reaction. But sometimes, it will decay when one of its protons turns into a neutron and emits a positron and an electron neutrino. This is called beta-plus decay or positron emission. The resulting nucleus is called deuterium, it has one proton and one neutron, and it is stable. The creation of deuterium in this way is a first step in the , one of the nuclear fusion processes that power stars like the sun. The final product of the proton-proton chain is a stable helium nucleus with two protons and two neutrons (plus some electrons, positrons, neutrinos, and/or gamma rays, depending on the details of the reaction).

A final note: you mentioned that the two atoms of hydrogen each have one electron, but it's more likely that they are ionized and don't have electrons. If nuclear fusion is happening, the temperature is probably extremely high, and any electrons bound to a nucleus will quickly get enough energy to escape.

Rebecca Holmes

(published on 09/03/2014)