Explain why you can't put all the electrons in one place?

Answer 1

Because it's a fermion. Bosons can occupy the same state, though. It has to do with valid solutions to the Schrodinger equation for spinors. Heady stuff.

Answer 2

First off an electron is both a particle and a wave, it is not a point but an area. These take up positions in what are called orbitals. Only two electrons can fit in the first level around an atom then eight on the second level and so on.

Secondly the electrons all have the same charge so they will always repel each other so you would need an extremely large amount of energy to keep them in one place.

If you do first year physics or chemistry at uni you will learn about this in more depth.