Quantum mechanics! It's a problem of statistical mechanics really. Individual particles don't move in a deterministic fashion, but with probabilities.
The reason there is still an interference pattern is that each particle has a probability of going on some particluar path. Since the particle...
Well, there is some interaction due to the edges, which is why one can obtain diffraction from a single slit. The electrons usually have low enough energy that they elastically scatter. Actually to clarify, Young's double slit experiment refers to interference, due to the two slits. It also has...
A single electon will create a single spot on a flourescence screen, hardly a diffraction pattern.
But, quantum mechanics mantains that each electron will still follow the rules of feynmann paths. Hence, it is in a superposition of all pathes and will on average create a diffraction pattern...