Mercury is the closest to the Sun so you might think it’s Mercury. Mercury orbits at a distance of only 58 million kilometers, travelling in a blast-furnace of scorching radiation. Its temperature can skyrocket to 700 Kelvin, or 426 degrees Celsius on the sunward side. In the shadows, temperatures plunge down to 80 Kelvin, which is -173 degrees Celsius. So Mercury sure is hot, but Venus is hotter. How Let's see.
Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. Average temperature there is a hellish 735 Kelvin, or 462 degrees Celsius – hot enough to melt lead. The main reason why Mercury is colder that it has not atmosphere. Mercury's gaseous molecules reached escape velocity long ago and the atmosphere was 'burned off' leaving a vacant wasteland of rock with a similar surface to the earth's moon. There is nothing to hold in solar radiation, thus is flies out into space. In compare to it, Venus, has a very thick atmosphere of CO2, which adds incredible pressure, and traps in the heat. Venus also shows us what happens when carbon dioxide levels just keep on rising. Radiation from the Sun is absorbed by the planet, and the infrared heat emitted is trapped by the carbon dioxide, which creates a runaway greenhouse effect. The solar heat enters, but it cannot leave. Thus, an oven is created. This is why Venus has no water. Pictures suggest that Venus once had an aqueous environment, but because of climatic changes and intense heat.... it evaporated.