Why does Atomic Mass Differ from # of Protons & Neutrons?

[Myslius]

Why atomic mass doesn't match the number of protons and neutrons in it?
for example mercury mass is 200 gmol but it has only 80 protons and 80 neutrons. From where does 40 gmol come from?

 

[phyzguy]

The stable isotopes of Mercury have between 116 and 124 neutrons, not 80. The atomic weight is the weighted average mass of all of the stable isotopes.

 

[Myslius]

that makes sense, thanks, i just though that the number of protons and neutrons in nucleus are quite the same

 

[Myslius]

Is there any equation to determine how many neutrons there should be to keep an atom stable? I don't mean mass - number of protons. I mean the equation that can determine an interval of neutrons based on atomic number only and the forces.

 

[phyzguy]

There is a semi-empirical relation that says that light nuclei are most stable when n=p, but that as p increases, the ratio of n/p needs to increase so that the strong force can compensate for the increased electrical repulsion. But I don't think there is a simple equation. Here's a graph showing where the stable nuclei lie:

http://en.wikipedia.org/wiki/Isotope#Nuclear_properties_and_stability

 

[Amok]

Is there any equation to determine how many neutrons there should be to keep an atom stable? I don't mean mass - number of protons. I mean the equation that can determine an interval of neutrons based on atomic number only and the forces.

Try posting that question in the Nuclear physics forum, you might get some interesting answers.