|Hi Cindi, |
This is a great question. There are two competing effects at work which determine how many neutrons and protons will be present in any nucleus. The first of these is caused by the fact that neutrons and protons are in many ways very similar, but are not identical. This similarity, which scientists call "symmetry", would, if it were the only consideration, cause a nucleus of a certain mass to prefer having the same number of neutrons and protons. You can think of this as two buckets of water connected at the bottom by a hose. The water level in one bucket represents the number of protons, and in the other bucket the number of neutrons. The "hose" plays the role of the "symmetry", and causes the water level in both buckets to be the same.
Neutrons outnumber protons in heavier nuclei due to a second effect, caused by the protons each having a charge of +1 while neutrons are neutral. Since the protons all have the same charge, they want to get as far from each other as they can. (This is the same effect you see if you rub two balloons in your hair to charge them up and then place them close to each other: they will tend to separate due to the electrostatic force). A good way for nature to keep protons separate is to add more neutrons to a nucleus, which is what happens in heavy nuclei. You canít add too many more, though, because of the "symmetry force" described in the first paragraph.
So (although there is a bit more to it) a pretty good way to think about it is that the number of protons and neutrons in a given nucleus is a determined by a balance of the symmetry force (which wants to make the numbers the same) and the electrostatic force (which wants extra neutrons).
(published on 10/22/2007)