Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Channel proteins provide an open channel or passageway through the cell membrane for molecules to move across. Many channel proteins allow the diffusion of ions. Ions are charged atoms. The charge makes it difficult to cross the cell membrane without assistance. Channel proteins are specific for the molecule they transport. For example a sodium ion crosses the membrane through a channel protein specific for sodium ions. Answer: atoms
Ions are charged what?