2 protons have less mass than combined

[Cbray]

I was reading an article about how two protons have less mass combined than by themselves to make helium, which gives off left over energy around 0.7% of their mass.

How is this possible? Why do they have less mass combined?

Thanks!

 

[Pengwuino]

You have it backwards in your first line. Combined they have less mass. The missing energy is due to the binding energy that keeps them together in say, Helium.

 

[Cbray]

You have it backwards in your first line. Combined they have less mass. The missing energy is due to the binding energy that keeps them together in say, Helium.

So I'm guessing the amount of mass lost due to bindings is small? - And the energy left is for explosions ? :P

 

[Pengwuino]

Yes, the mass lost is small (although the energy associated with that mass loss is huge by E = mc^2 compared to chemical reactions). That's what basically happens in nuclear reactions.