Question

The helium atom has two neutrons and two protons in its nucleus, and it has two electrons outside its nucleus. Which force keeps a helium atom from disintegrating into two neutrons and two hydrogen atoms?

A. electromagnetic
B. gravity
C. strong nuclear
D. friction

Answer 1

Helium nuclei are held together by the residual strong nuclear force.

Explanation:

Protons are positively charged and repel each other due to the electromagnetic force. The electromagnetic force is long ranged so every proton repel every other proton in a nucleus.

Neutrons act as a kind of glue which keep the nucleus stable if in the right numbers. The residual strong nuclear force is very short ranged but strong enough to overcome electromagnetic repulsion.

In fact the first stage of the fusion reaction which fuses Hydrogen into Helium involves the residual strong nuclear force binding two protons together.

It was once thought that the strong nuclear force was mediated by `pi` mesons. When it was discovered that protons and neutrons are composite and consist of three quarks, it was also found that `pi` mesons are also composite consisting of two quarks.

Now that protons and neutrons are known to be made of quarks, the colour force theory was required. This means that quarks are bound together inside mesons and hadrons by gluons.

Now gluons bind the quarks in protons and neutrons together. They also bind the quarks in a proton or neutron to quarks in an adjacent proton or neutron. This residual effect is actually the strong nuclear force.

So, the strong nuclear force actually doesn't exist. It is a residual effect of the colour force. This is why it is now referred to as the residual strong nuclear force.