Venus atmospheric composition percentages
Carbon dioxide (CO2, 96,5 %) and nitrogen (N2, less than 3,5%) already represent more than 99,9% of the composition.
Scientists also discovered minor molecular constituents such as H2, O2, Kr, H2O, H2S and COS in Venus' atmosphere.
Water vapour is present in extremely low quantities (about 30 parts per million or ppm, 1 ppm = 0.001 %), making Venus the driest planet of the solar system.
More details in Venus atmospheric composition percentages table
How is Venus atmosphere different from Earth and Mars?
The troposphere of Venus does not receive sunlight at wavelengths below 400 nanometres, since the UV is absorbed by SO2 and the aerosols in the cloud layer. Even at visible wavelengths, only 5% of the sunlight reaches the surface. This is a major difference between the atmosphere of Venus and those of the Earth and Mars, which are quasi-transparent to sunlight.
Since ultraviolet rays do not reach the troposphere, photochemical processes only play a secondary role there.
More details about Historical discoveries in Venus’ atmosphere