Originally posted by sonhouse
So the 'job' of neutrons is to shield proton charges from one another.

Can there be stable atoms with just two protons? no neutrons?

Why do the charges need to be shielded from each other in the first place? They are like charges so they would be in no danger of crashing into each other, self repelling. Why do they need neutrons then?

Protons in an atomic nucleus are very close to each other, so this repulsive interaction is strong. If it is too strong it becomes energetically favourable for the protons to separate from each other, i.e. you get decay.

Originally posted by KazetNagorra
Protons in an atomic nucleus are very close to each other, so this repulsive interaction is strong. If it is too strong it becomes energetically favourable for the protons to separate from each other, i.e. you get decay.

So the neutrons are what allows protons to stick together?

Originally posted by sonhouse
So the neutrons are what allows protons to stick together?

It is actually quite complicated and to really understand it you would need to take a course in the relevant nuclear physics. And even then, I believe a lot of it is not completely understood.
There is a lot of quantum mechanics going on. For example neutrons on their own are unstable and decay relatively quickly. If you had a nucleus with just neutrons many of them would instantly decay into protons. So the balance is not just about forces but also about the fact that the particles involved can decay.

Originally posted by twhitehead
It is actually quite complicated and to really understand it you would need to take a course in the relevant nuclear physics. And even then, I believe a lot of it is not completely understood.
There is a lot of quantum mechanics going on. For example neutrons on their own are unstable and decay relatively quickly. If you had a nucleus with just neutrons ...[text shortened]... balance is not just about forces but also about the fact that the particles involved can decay.

So during the BB, when protons and neutrons were being smelted they must have been made together or there would be no neutrons now, right? What a weird situation. Neutrons keep protons apart while protons keep neutrons from self destructing.

Originally posted by sonhouse
So during the BB, when protons and neutrons were being smelted they must have been made together or there would be no neutrons now, right? What a weird situation. Neutrons keep protons apart while protons keep neutrons from self destructing.

According to the big bang theory, in the very early stages of the big bang there was a quark-gluon plasma as the universe expanded that condensed into nucleons with protons and neutrons in more or less equal quantities. The neutrons and protons could be transmuted into each other because of the huge densities and temperatures, essentially collisions created new neutrons as the old ones decayed. After the end of primordial nucleosynthesis (early fusion into heavier elements) about 75% of the matter in the universe was hydrogen and about 25% helium, with small amounts of lithium. Any free neutrons knocking around after that era would have decayed into protons and electrons within the first couple of hours and new neutrons not produced because the average density of the universe had dropped too much, so not enough collisions were happening. The reason for the existence of elements heavier than helium is stellar nucleosynthesis. Even if no neutrons had survived the first couple of hours (*) so that the early universe contained only protons we would still see heavy elements containing neutrons because of stellar nucleosynthesis.

(*) This sentence is problematic as if primordial nucleosynthesis couldn't happen it's unlikely that stellar nucleosynthesis could. I'd need some fundamental constants to change after the first couple of hours. However the important point is that a star made solely of hydrogen 1 (with no neutrons in the proto-star at all) would still turn itself into helium, and then heavier elements.

Originally posted by DeepThought
According to the big bang theory, in the very early stages of the big bang there was a quark-gluon plasma as the universe expanded that condensed into nucleons with protons and neutrons in more or less equal quantities. The neutrons and protons could be transmuted into each other because of the huge densities and temperatures, essentially collisions cre ...[text shortened]... utrons in the proto-star at all) would still turn itself into helium, and then heavier elements.

Yes, that is what I read also. Heavy elements made in stars up to iron. No more energy available from fusing elements heavier than iron, all that stuff came from novae and supernovae.

I wonder if there is much heavy element creation from the reactions of stuff spit out of black holes? The massive amount of energy given off perhaps could do that.

Just found this bit:

http://phys.org/news/2015-12-puzzle-elements-universe.html