Please explain binding energy?

[alexgmcm]

Can some kind person please explain why He-4 is so high and Li-6 is so low on the graph of binding energy per nucleon.
Copy of the graph here http://en.wikipedia.org/wiki/File:Binding_energy_curve_-_common_isotopes.svg"
It means that He-4 must be very stable right? Because binding energy is indicative of stability for reasons I can't quite recall at the moment.. but why is there such a spike as it goes up to He-4 and then down to Li-6 and Li-7 etc.

I remember in something about astrophysics it said there were like resonances in C-12 for example that made it more stable than one might expect so is that the case for He-4? But otherwise why is it so stable, or is Lithium just very unstable?

Argh, this stuff gets pretty confusing.. any help would be greatly appreciated.

 

[malawi_glenn]

Yes He-4 is very stable.

But explaning WHY He-4 is high and WHY Li-6 low BE/nucleon is that He-4 has closed proton- and neutron shells.

http://hyperphysics.phy-astr.gsu.edu/hbase/Nuclear/shell.html

http://hyperphysics.phy-astr.gsu.edu/hbase/Nuclear/shell2.html#c4

The nuclei which has filled shells have significant higher BE/nucleon that its neigbours, this is a consequence of quantum physics really, you have the same thing in atoms with their electron shells.