Why is mecury unlike any other metal? Why does it have a low melting point of -40 degrees (either fahrenheit or celsius)?
Maybe some chemists and physicists will be along later to talk about atomic weights and so on, but on the face of it, your question is a bit of an odd one; mercury is like that because that’s what mercury is like; it’s like asking “why is water wet”, metals have a variety of different melting points, mercury’s happens to be below room temperature; if we happened to be creatures that lived on a much hotter planet (if life is possible under such conditions), you’d be asking “Why are lead and tin unlike other metals?”
According to http://www.webelements.com “The melting point is the temperature at which the vapour pressure of the solid and the liquid are the same and the presssure totals one atmosphere.”
So now all you need is someone to explain that because I’m still kind of in the dark, even though I know that -40 C is the temperature at which the vapour pressure of the solid and the liquid are the same and the presssure totals one atmosphere, at least as far as mercury is concerned.
I knew I shoulda stayed awake in class…
First let’s get some things straight:
It’s mercury.
And it’s melting point is -38.83 °C (-37.89 °F) (according to WebElements)
I’ve always thought that it was because of the crystal structure. (SC - simple cubic, whereas most metals are fcc - face centered cubic). This means that the atoms are generally rather far from each other, so the electron cloud gets dispersed, and the average attraction decreases.
However, according to the World Chemistry website, there’s more to it:
See the link above for details
And let’s face it. It’s not that far ahead of sodium.
Sodium doesn’t tend to stick around very long as a pure element though.
Also, don’t forget about Gallium. The metal the melts in your hand, but not in the room. 
(Melting point of 85.57 degrees F)
Jman