Atomic Weight: What Governs the Number of Neutrons?

[MrEd]

Atomic wieght...

I know, or at least think the wieght of an atom is the number of neutrons...but what I want to know is what governs this number? Is it a gravitational force...I mean all this spinning going on...how can they be so small, so close together and not bump into each other?--what keeps things in order? Please forgive me!

 

[Nabeshin]

The atomic weight is the average mass of the atom in atomic mass units, (basically a proton is 1 and a neutron is 1).
By average I mean that the various isotopes and their relative abundances are taken into consideration (which is why the mass of H = 1.0008 or whatever)

Gravity is far too weak to hold them together, what ultimately binds the neutrons and protons in the nucleus is the strong nuclear force. The electrons orbit the nucleus by virtue of the electromagnetic force.

 

[mathman]

Atomic mass unit is defined so that C12 has an atomic weight of 12 (exactly). Protons and neutrons have at. wt. slightly greater than 1.