How does solar electric propulsion (ion propulsion) work?

What is an ion?

An ion is a charged atom or molecule. It is charged because the number of electrons do not equal the number of protons in the atom or molecule. An atom can acquire a positive charge or a negative charge depending on whether the number of electrons in an atom is greater or less then the number of protons in the atom.

When an atom is attracted to another atom because it has an unequal number of electrons and protons, the atom is called an ION. If the atom has more electrons than protons, it is a negative ion, or ANION. If it has more protons than electrons,it is a positive ion.

What is an atom?
How could something as small as an atom move a space craft?
What are other ion using technologies?

Are ion engines radioactive?
How are atoms charged?
Why is an ion plume blue?