What's a covalent compound?

I'm sure glad you asked!  A covalent compound is a compound in which the atoms that are bonded share electrons rather than transfer electrons from one to the other.  While ionic compounds are usually formed when metals bond to nonmetals, covalent compounds are formed when two nonmetals bond to each other.

The big question that students frequently have is, "Why do elements share electrons?  After all, wouldn't electrons rather grab electrons outright?  That's what happens when ionic compounds are formed."

The reason that nonmetals have to share electrons with each other has to do with electronegativity.  Recall that electronegativity is a measure of how much an element pulls electrons away from other elements it is bonded to.  Metals generally have very low electronegativities (they don't much want to grab electrons) while nonmetals have high electronegativities (they really want to grab electrons).  The reason for this trend is the octet rule, which says that all elements want to have the same number of electrons as the nearest noble gas, because noble gases are unusually stable.  When metals bond to nonmetals, ionic compounds are formed because the metal atoms don't want electrons and easily give them to nonmetals that do want electrons.

It's a different story when two nonmetals bond with each other.  Instead of having one element give electrons to another, we run into a case where we have two elements that have roughly the sameelectronegativity.  As a result, neither element can steal electrons from the other.  As a result, if either of them are going to be like the nearest noble gas, they'll have to share electrons rather than transfer them.

What are the properties of covalent compounds?

Covalent compounds have the following properties (keep in mind that these are only general properties, and that there are always exceptions to every rule):

1)  Covalent compounds generally have much lower melting and boiling points than ionic compounds. 

As you may recall, ionic compounds have very high melting and boiling points because it takes a lot of energy for all of the + and - charges which make up the crystal to get pulled apart from each other.  Essentially, when we have an ionic compound, we need to break all of the ionic bonds in order to make it melt.

On the other hand, when we have covalent compounds we don't need to break any bonds at all.  This is because covalent compounds form distinct molecules, in which the atoms are bound tightly to one another.  Unlike in ionic compounds, these molecules don't interact with each other much (except through relatively weak forces called "intermolecular forces"), making them very easy to pull apart from each other.  Since they're easy to separate, covalent compounds have low melting and boiling points.

2)  Covalent compounds are soft and squishy (compared to ionic compounds, anyway).

The reason for this is similar to the reason that covalent compounds have low melting and boiling points.  When you hit an ionic compound with something, it feels very hard.  The reason for this is that all of the ionic bonds which hold together the crystal tend to make it very inflexible and hard to move.  On the other hand, covalent compounds have these molecules which can very easily move around each other, because there are no bonds between them.  As a result, covalent compounds are frequently flexible rather than hard.

Think of it like this:  Ionic compounds are like giant Lego sculptures.  If you hit a Lego sculpture with your fist, it feels hard because all of the Legos are stuck very tightly to one another.  Covalent compounds are more like those plastic ball pits they have at fast food playgrounds for little kids.  While the balls themselves are held together very tightly (just like covalent molecules are held together tightly), the balls aren't really stuck to each other at all.  As a result, when little kids jump into the ball pits they sink in rather than bouncing off.  

3)  Covalent compounds tend to be more flammable than ionic compounds.

The main reason that things burn is because they contain carbon and hydrogen atoms that can react to form carbon dioxide and water when heated with oxygen gas (that's the definition of a combustion reaction).  Since carbon and hydrogen have very similar electronegativities, they are mostly found together in covalent compounds.  As a result, more covalent compounds than ionic compounds are flammable.

There are a couple of exceptions to this rule.  The first is with covalent compounds that contain neither carbon nor hydrogen.  These tend not to burn, and if they do, they burn by mechanisms other than the classic combustion reaction.  The other exception comes with ionic compounds referred to as "organic salts".  These organic salts are ionic compounds in which the anion is basically a big covalent molecule containing carbon and hydrogen with just a very small ionic section.  As a result, they burn even though they're technically ionic compounds.

4)  Covalent compounds don't conduct electricity in water.

Electricity is conducted in water from the movement of ions from one place to the other.  These ions are the charge carriers which allow water to conduct electricity.  Since there are no ions in a covalent compound, they don't conduct electricity in water.

5)  Covalent compounds aren't usually very soluble in water.

There's a saying that, "Like dissolves like".  This means that compounds tend to dissolve in other compounds that have similar properties (particularly polarity).  Since water is a polar solvent and most covalent compounds are fairly nonpolar, many covalent compounds don't dissolve in water.  Of course, this is a generalization and not set in stone - there are many covalent compounds that dissolve quite well in water.  However, the majority of them don't because of this rule.