Mercury is only 57.9 million kilometers away from the Sun, which makes it the closet planet to it. However this does not make Mercury the hottest planet in the solar system. The average surface temperature of Mercury is about 167^oC, where Venus, the second closet planet to the sun, has an average surface temperature of 464^oC. This is because the atmosphere of Venus consists in 96% of carbon dioxide, which makes it a super green house.
The temperature on Mercury changes rapidly between day and night. It can go up to 450^oC during the day, and cools down to -180^oC at night due to little atmosphere. In 2004, the Messenger Spacecraft was launched to go around Mercury, and found that the surface of Mercury is covered by carters.