Activity Coefficient
The factor by which a species' concentration must be multiplied to give its thermodynamic activity.
Formula / equation
γ = a / c
Unit
Dimensionless
In depth
The coefficient is one in an ideal solution and falls below one for ions in dilute electrolyte solutions, where electrostatic shielding reduces effective concentration. It depends on ionic charge and on the ionic strength of the whole solution, not on the identity of the other ions.
Examples in the real world
A doubly charged ion has a much smaller activity coefficient than a singly charged one at the same ionic strength.