"(1) The high concentration of K⁺ ions inside the cell causes some of the K+ ions to move (diffuse) out of the cell through the K⁺ ion channels."

The inside of the cell has a high K⁺ concentration compared to the outside extracellular fluid. Thus, there is a significant force, the K⁺ concentration gradient, acting to move K⁺ out of the cell.

(2) However the negatively charged anions can not exit the cell with the positive charged K⁺ ions because they are too large.

(3) This creates a separation of charge across the cell membrane. The inside of the membrane has a lower potential (negative) and the outside has a higher potential (positive)."

courtesy of Robert H. Parsons, Ph.D., Rensselaer Polytechnic Institute, used with permission