mercury is the closest planet to the sun,but not the hottest because it dosen't has a atmosphere cause of which the light gets out of it as easily as it enters it and the atmosphere of it was worned out by the the soler storms.That's why the side of mercury facing the sun gets hottest (449 degree celcius)and the facing in the oppasite direction gets really cold(-170 degree celcius).
Despite its close proximity to the Sun, unlike any other planet, Mercury has no real atmosphere to scatter the Sun's rays, and so its sky appears black.
Astronomers have not been able to detect much surface pressure on Mercury, if any. This is due to its very thin atmosphere, meaning there is minimal air pressure on Mercury.So if you ever go there you weight will decrease a lot but your mass will remain the same.