(Original post by sahil19)
Any essential things I need to know about these two processes? Thanks x
The other answer was pretty comprehensive. The only thing I'd mention that was missed out is the iron peak. You only need to understand this pretty superficially at GCSE.
As stated before, fission is the act of splitting a nucleus and fusion the act of combining nuclei. But the processes don't always release energy. Put simply, there are two forces at play. You have the nuclear binding forces, which hold the nucleus together and is very very short range, and the electrostatic force, which pushes them apart but has an affect over a much larger distance. You can think of it like magnetism, it's the force that causes protons to repel each other, cause they're both positive. Now as the nuclear binding force holds the nucleus together, this force helps fusion. On the other hand, the electrostatic force hinders fusion (you don't need to be able to explain this bit, it's just to help you understand the next part).
In small nuclei, the nuclear binding force is much more powerful than the electrostatic force (because there aren't many protons to repel each other). This means that fusion is 'easy', and so releases energy. We call this exothermic, and it means that fusion of lighter nuclei will release energy. This is what generates energy in a star.
But as the nucleus gets bigger, we have more protons. These protons repel each other, which means that the electrostatic force gets bigger, making fusion harder and harder (less exothermic). Eventually, we reach a point where the electrostatic force is great enough that fusion now requires us to put more energy in to make it happen, than we get out of it. At this point, the fusion becomes endothermic. The point where this happens is when we try to fuse iron-56. This is why stars cannot create elements heavier than iron during their natural lifespan - they form only during the death of the star. This is also the point where fission becomes exothermic, for the opposite reasons to those given above.
I'm not sure how much of that you'll be required to know, but for my syllabus we had to know that:
a) Fusion up to iron-56 is exothermic and fission endothermic; fusion beyond iron-56 is endothermic and fission exothermic
b) The reason for this is because of the balance of the nuclear binding force and electrostatic force
c) This dictates what elements stars can fuse, and why even the biggest stars eventually die (because it's impossible to sustain fusion beyond iron-56, no matter how much energy you have)