Why is Venus the hottest planet though Mercury is closest to sun?

As we all studied in our lower classes that Venus is the hootest planet of our solar system.But did you ever thought why Venus and not Mercury?

 

The degree of hotness of a planet does not depend on as much on the closeness to the Sun as on its atmosphere.Carbon Dioxide has the tendency to absorb heat which in turn increases the temperature

Mercury does not contain carbon dioxide(because of which all heat is returned to space).Venus contains a high percentage of carbon dioxide due to which it is the hottest planet