|8. The Four Forces|
The weak nuclear force, mentioned in the last paragraph, is one of the four elementary forces.
By far the weakest of the four is the Gravitational force. Next comes the Weak (nuclear) force, then the Electromagnetic force and finally the strongest of all, the Strong (nuclear) force.
Why is it then that in our daily life we feel the gravity so strong? Well, the two nuclear forces act only when the distance between objects is extremely small, in the order of 10-15 m.
The other two forces are "long-range", take for example two magnets. But matter is in general electrically neutral, with equal amounts of positive and negative charge,so the electric force cancels out to a large extent. The gravitational force may be very weak, but as there does not exist negative mass, there are no cancellations, and for large bodies like the sun and the earth, the gravitational force dominates.
We can now explain why large numbers of protons and neutrons can no longer form stable elements. The repulsive electromagnetic force between the protons keeps building up, whereas the strong nuclear force only works between nucleons that are neighbours. In the case of the unstable heavy elements, like uranium, the nucleus will decay by emitting two protons and two neutrons together, an alpha particle, becoming lighter in the process. This is called alpha-decay. For some isotopes, like U-235 there is another possibility. When a neutron enters the nucleus, it may split in two parts + a few free neutrons + energy. When these neutrons enter other uranium nuclei, a chain reaction can start. This process, called fission, is used in atomic energy production.
It can also happen that there is no balance between the number of protons and neutrons. Then the Weak Nuclear Force comes into play. Take for example Carbon. The isotope Carbon-14 has too many neutrons. The Weak Nuclear force transforms one neutron into a proton and an electron, with Nitrogen-14 as a result. The electron escapes and the process is called beta-decay
In the case of Carbon-14, after ~ 5730 year half of the carbon atoms have been transformed in Nitrogen-14. We say that the half-life of C-14 is 5730 year.
The radioactive dating method is based on this decay process.