Final answer:
The smallest and lightest hydrocarbons are captured at the top of the petroleum distillation tower during fractional distillation, as they have lower boiling points.
Step-by-step explanation:
The smallest, lightest hydrocarbons in a petroleum distillation tower are captured at the top of the tower. This is because during fractional distillation, crude oil is heated to high temperatures, causing it to vaporize. The vaporized hydrocarbons then rise through the series of trays in the tower. As they rise, they cool down, and because lighter hydrocarbons have lower boiling points, they condense at the top of the tower where it is cooler, separating from heavier fractions that condense lower down.