Why can't excess charge built up in a conductor escape the object?

Question

When you add electrons to an insulator the electrons stay where you place them and are unable to move whereas in a conductor they repel each other and move to the edges. Why don't the charges just transfer to the ground?
My second question is when you add excess electrons to an insulator they stay where they are and don't repel from each other. Why does this occur when like charges repel?

my2cts · Answer

In a model of a metal with electrons residing in a volume of positive background charge, indeed, an extra electron would not be bound by a conductor. Real conductors bind electrons by the so called work function, a potential of a few volt. The reason is that the conductor contains highly charged nuclei and these charges are not perfectly shielded. An extra electron uses this to be bound to the conductor.
I ignore the second question. Please ask only one question per post.

Why can't excess charge built up in a conductor escape the object?

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