What is 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 same electronegativity. 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.
What are ionic compounds?
Ionic compounds are basically defined as being compounds where two or more ions are held next to each other
by electrical attraction. One of the ions has a positive charge (called a "cation") and the other has a negative
charge ("anion"). Cations are usually metal atoms and anions are either nonmetals or polyatomic ions (ions with
more than one atom). Think back to grade school: The same thing that makes the positive and negative ends of
a magnet stick to each other is what makes cations and anions stick to each other.
Ions are atoms that have satisfied the octet rule (which for those of you who've been sleeping the last couple of
months, states that every atom wants to have eight valence electrons, just like the nearest noble gas). If you have
two neutral elements, and one wants to gain electrons to be like the nearest noble gas and the other wants to lose
electrons to be like the nearest noble gas, chances are that they will react with each other and make an ionic
compound.
Usually, when we have ionic compounds, they form large crystals that you can see with the naked eye. Table
salt is one example of this - if you look at a crystal of salt, chances are you'll be able to see that it looks like a
little cube. This is because salt likes to stack in little cube-shaped blocks.
Sometimes when you see a salt, it looks like a powder instead of a cube. This doesn't mean that the salt is not a
crystal - it means that the crystals are just so small that you can't see them with the naked eye. If you were to put
the powder under a microscope, chances are that you would see little geometric blocks.
So, what are the main properties of ionic compounds (also called salts)? Well, I'm sure glad you asked...

All ionic compounds form crystals. So far as I know, there are no exceptions to this. Again, salts like
to form crystals because when you have a whole bunch of little electrical positive and negative charges
all stuck together, they seem to like to bunch into little stacking groups. The arrangement that these ions
like to stack into is different, and is referred to as the "unit cell". There are ten or so different general
shapes of unit cells. When you get to graduate school, ask me about them. For high school classes, it's
really not all that important.

Ionic compounds tend to have high melting and boiling points. When I say "high", what I mean is
"very, very high." Most of the time, when you work with ionic compounds in a chemistry class, the
melting point is hot enough that you can't melt them with a Bunsen burner. So, why are these
temperatures so high? Well, it has to do with the way that ionic materials are held together. Remember
how we said above that ionic compounds form crystals? These crystals are basically just great big blocks
of positive and negative charges all stuck together. To break the positive and negative charges apart, it
takes a huge amount of energy. This means that if we heat up the compound to add energy, it takes a
huge amount of energy to break it apart.

Ionic compounds are very hard and very brittle. Again, this is because of the way that they're held
together. Above, we said that it takes a lot of energy to break the positive and negative charges apart
from each other. This is the reason that ionic compounds are so hard - they simply don't want to move
around much, so they don't bend at all. This also explains the brittleness of ionic compounds. It takes a
lot of energy to pull ionic charges apart from each other. However, if we give a big crystal a strong
enough whack with a hammer, we usually end up using so much energy to break the crystal that the
crystal doesn't break in just one spot, but in a whole bunch of places. Instead of a clean break, it
shatters.

Ionic compounds conduct electricity when they dissolve in water. If we take a salt and dissolve it in
water, the water molecules pull the positive and negative ions apart from each other. (This is because of
the unusual properties of water, but that's a different story for a different time). Instead of the ions being
right next to each other, they wander all over the water. Now, think back to what electricity is hopefully, you remember that electricity is just the movement of electrons through metals (or anything
else). Now, electrons are just negatively-charged particles, and metals have the property that they're
good at letting them wander around. Dissolved salts are the same way. When you dissolve the salt in the
water, the positive and negative ions in the water allow electrons to flow much better than if you just had
water by itself. Voila! Salt water conducts! A question you might have is "Does electricity travel
through salt crystals?" Nope. It doesn't. Because the ions are stuck in one place due to the structure of
the crystal, the electricity doesn't move around very well. Another good question: "Does water without
salt in it conduct electricity?" The answer: Not very well. Water by itself is a lousy conductor. The
reason that boneheads who put the hairdryer in the bathtub with them turn into human fritters is that
when they wash themselves, all the crud on them gets dissolved in the water. Some of the crud is ionic,
so when the dryer hits the water, they get zapped. An interesting "thought experiment" would be to wash
all the salt off yourself and then drop a hairdryer in the bathtub with you. In theory, you would be fine.
In real life, you'd still become a crispy critter because tap water by itself has ionic compounds dissolved
in it anyway. Thus, my warning:
If you put a hairdryer in the bathtub with you under ANY conditions, you will fry yourself!
Covalent Compound Questions
1. What is a covalent compound? What two things usually bond together to form these
compounds?
2. What is electronegativity?
3. ________________ do not want to grab electrons very much while, ________________ do
want to grab electrons.
4. The octet rule states that all elements want to have the same number of electrons as the
nearest ________ _______ because they are unusually _____________.
5. Because in a covalent compound both elements have the same electronegativity, one element
does NOT take electrons from the other, instead they _______________ electrons.
List the Properties of Covalent Compounds and Give the reason for that Property.
Property 1ExplanationProperty 2ExplanationProperty 3ExplanationProperty 4ExplanationProperty 5Explanation-
Ionic Properties Questions
1. An ionic compound is when two or more ions are held next to each other by
___________________ attraction.
2. An ion is:
3. A cation has a ________________ charge and is usually a _________.
4. An anion has a _______________ charge and is usually a ______________.
5. Usually ionic compounds form large _______________.
List the Properties of Ionic Compounds and Give the reason for that Property.
Property 1ExplanationProperty 2ExplanationProperty 3ExplanationProperty 4Explanation-