The lowest surface temperature in the solar system was recorded on Uranus (-224 degrees Celsius). The temperature of a planet does not only depend on the amount of solar radiation that it receives but also on the amount of heat that it gives off. Because of Uranus' orientation it absorbs little radiation which makes it colder than Neptune although Neptune is further away from the Sun. <span />
The atomic mass is the weighted average of the atomic masses of the isotopes. Simply multiply each isotopes's atomic mass by its relative abundance, then sum the results.
