Final answer:
The activity coefficient, explained by the Debye-Hückel Theory, accounts for the non-ideal behavior of ions in a solution, adjusting for the difference between actual and effective ion concentration due to residual interionic attractions.
Step-by-step explanation:
The activity coefficient is a factor used in thermodynamics and physical chemistry to account for the non-ideal behavior of solutes in a solution. According to the Debye-Hückel Theory, proposed by chemists Peter Debye and Erich Hückel in 1923, the activity coefficient helps to explain why ions in an electrolyte solution do not always behave as independently as predicted by ideal models. The theory acknowledges that while the attraction between ions is significantly reduced by solvation and the insulating properties of a polar solvent, some residual attraction still exists. This residual attraction means that the actual ion concentration in the solution is different from its effective concentration or activity. The activity coefficient allows for adjustment of this disparity, especially as the solution becomes diluted and the ions are more widely separated, reducing the interionic attractions.