In thin-layer chromatography, which expression defines the retention factor (Rf) and what does it indicate?

• A. Rf = distance traveled by solvent front / distance traveled by solute.
• B. Rf = distance traveled by solute divided by distance traveled by solvent front; indicates polarity.
• C. Rf = distance traveled by solute / distance traveled by solvent front; indicates relative polarity and interactions with the stationary phase.
• D. Rf = distance traveled by solute / distance traveled by solvent front; indicates molecular weight.

Answer: (C)

In thin-layer chromatography, the retention factor shows how far a compound moves relative to how far the solvent front travels. It’s defined as the distance the solute travels divided by the distance the solvent front travels, giving a unitless value between 0 and 1. This ratio reflects how strongly the compound interacts with the stationary phase: stronger interactions keep the compound closer to the origin, producing a smaller Rf, while weaker interactions with the stationary phase allow it to move farther with the solvent, giving a larger Rf. Because the stationary phase is typically polar (like silica) and the mobile phase is less polar, polarity plays a big role: more polar substances tend to have smaller Rf values, while less polar (more nonpolar) substances move farther and have larger Rf values. The expression correctly captures this relationship; using the inverse ratio would misrepresent how far the solute moves, and tying Rf to molecular weight isn’t appropriate for TLC behavior.