How does solar electric propulsion (ion propulsion) work?

Why do ion engines need a vacuum to work?

Ion propulsion engines work because you can make a plasma in the chamber. This means that electricity has to flow through the chamber. Air is an insulator, which means that it keeps electricity from flowing. Ion propulsion can only work when there is little or no air pressure in the chamber. The amount of pressure is important because a gas can only ionize, or become electrically charged, at low pressures. This is because the more gas or air there is in the chamber, the more the electrons that are suppose to ionize the atoms in the chamber bump into the air and not the atoms. A vaccuum is a totally empty space--there is no air pressure to stop the electrons if there is no air.

What is an ion?
Could ion propulsion move something on Earth?
Could NASA use ion propulsion to put a ship into space?
What is the difference between an ion engine and a conventional one?
What is a propellant?
What new technologies are being tested on DS1?

What does electrically charged mean?
How are atoms charged?