Final answer:
An ion is at equilibrium when no net force exists to move it across a membrane, contributing to the resting membrane potential due to the selective permeability of the cell membrane and the action of sodium-potassium pumps.
Step-by-step explanation:
An ion is at equilibrium when there is no net force to move it across a membrane. This state occurs when the forces driving the ion in one direction are balanced by counteracting forces, preventing its net movement. In the context of a neuron's cell membrane, this equilibrium is essential for maintaining the resting membrane potential.
The resting membrane potential is a result of the selective permeability of the cell membrane, which allows some ions to pass while blocking others. Sodium-potassium pumps help to maintain different concentrations of Na+ and K+ ions inside and outside the cell, which contributes to this potential. The permeability to K+ and Cl- ions while being impermeable to Na+ ions, coupled with the diffusion process balanced by the Coulomb force, ensures that ions are at equilibrium and that a voltage is created across the membrane.