Why can't electrons leave negatively charged objects in a vaccum? If an object is negatively charged, and electrons repel each other since they have like charges, why doesn't that negatively charged object "kick away" electrons?
Why doesn't diffusion occur? Why does there need to be a conductor for the electrons to go to?
 A: Matter is full of positively charged protons, and those positively charged protons attract the negatively charged electrons. In general, if you take a chunk of neutral matter and add one extra electron then the attractive force due to the protons in the matter outweighs the repulsive force due to the electrons in the matter, and the extra electron is bound into the matter. That is, you have to put in energy to get it out again. This binding energy is known as the work function.
If you now add a second extra electron then the attractive force due to the protons is roughly the same, however the repulsive force due to the electrons is slightly increased because now the first extra electron is adding to the repulsion. So the second electron is less tightly bound into the matter.
And so on as you keep adding more and more electrons. Eventually you'll reach the point where you can't add any more electrons because the electrons you've already added repel them so strongly. At this point electrons will indeed spontaneously leave the matter in a process known as field emission.
There are a couple of other points worth noting. If you heat matter you increase the energy of the electrons within it, and eventually you increase the energy enough for the electrons to jump out of the matter. This process is known as thermionic emission. And finally if you shine light on the matter the light can excite electrons enough for them to jump out, which is known as the photoelectric effect.
