To answer this question, we have to talk a little about atoms. All elements have negatively charged electrons bound in states around the positively charged nucleus. These states are wave patterns, like the patterns a string or a drum make as they vibrate. Only certain energies of states are possible, sort of like how a musical instrument vibrates at only special frequencies. It turns out that it's impossible for more than one electron to be in the same state. Electrons fill up the lowest energy states closest to the nucleus of the atom first, and then gradually fill up the higher energy states.
There's a pattern to how the energies increase as more states fill up. Some of the states have about the same energy, then there's a jump up to the next level. Atoms are most stable when adding another electron would mean having to jump up to the next energy level. The noble gases are the atoms like that. The electrons they have are tightly bound but the next electron to come along wouldn't be. So they tend to just stay as they are, not giving up or gaining electrons, or sharing electrons with other atoms. That means that the atoms don't stick together into molecules.
On the other hand, the most reactive elements are those that have one extra electron or are missing one, compared to a noble gas. Sodium (Na), for example, has one more electron than neon. That last electron can come off and stick to another atom that has an empty state at low energy, such as fluorine (F), which has one less electron than neon. Since the Na is left with a positive charge and the F is left with a negative charge, they attract and tend to stick together as HF molecules.
Interestingly, the "noble" in noble gas refers to a meaning of the word noble that we don't often think of: this noble means "having admirable properties." It was given this name because they are not very common on earth, and so their inertness was very rare compared to other elements.
Thanks for your question!
Ben M. and Mike W.
(published on 01/04/2011)