Question

earths third atmosphere (atmosphere iii) was a thick atmosphere of primarily water vapor and carbon dioxide.
what occurred during the third atmosphere on earth?
heat energy was retained, and water vapor condensed.
ultraviolet light split water vapor into hydrogen and oxygen.
increased oxygen allowed reptiles to thrive.
helium and hydrogen gases escaped earths gravity.

Explanation:

During Earth's third atmosphere (atmosphere III), which was thick and composed mainly of water vapor and carbon dioxide, the greenhouse - like effect of these gases (especially carbon dioxide) retained heat energy. As the Earth cooled over time, the water vapor in the atmosphere condensed.
Ultraviolet light splitting water vapor into hydrogen and oxygen is more relevant to earlier atmospheric stages. The increase in oxygen allowing reptiles to thrive is related to a later oxygen - rich atmosphere. Helium and hydrogen gases escaping Earth's gravity was more characteristic of the first atmosphere.

Answer:

Heat energy was retained, and water vapor condensed.