Mercury is element number 80 and is symbolized Hg. It appears on the periodic table in the far-right column of transition metals, right next to the poor metals. With only two electrons in its outer shell, it is incredibly reactive and volatile. There are also several radioactive isotopes of mercury, several of which are highly unstable: HG 197 has a half-life that is measured in hours, and HG 206 has a half life of less than nine minutes. The most unique thing about mercury is that it is the only metal that is liquid at room temperature. it's melting -- or freezing -- point is a chilly -38.9 degrees Celsius (roughly -38.02 degrees Fahrenheit). It is also a very poor conductor of heat, which is one of the reasons it is so useful in thermometers -- it expands and contracts over a wide range in response to temperature changes.

Mercury is also very good at conducting electricity, and is often used in batteries. It is also very reactive with other metals, and can be used to make amalgams (alloys) with other metals. It is so good at this that it is often used to extract gold and other precious metal from raw ore. There are many compounds that can be made out of mercury that are both useful and dangerous. Currently, mercury poisoning of waterways and thereby fish -- and thereby creatures like humans that eat the fish -- is a very big issue in ecology and the human impact on the environment. It is also a highly volatile element, and vapors can accumulate rapidly from any sizeable amount of mercury, so it is best to work with it in a very well ventilated area and only after proper training.

In nature, mercury is usually found in cinnabar ore, which is made up of sulfur and mercury. Demonstrations of mercury forming used to be done by heating this ore, but due to the release of vapors this is no longer recommended. It was named mercury after the quick Roman messenger god, and is also called quicksilver because of the way it flows as a liquid.

Sources

Mercury." WebElements.com. http://webelements.com/mercury/

Mercury." Chemical Element.com. http://www.chemicalelement.com/element/mercury.html