Covalent compounds are characterized by the fact that they may be gases, liquids, or solids. So the melting point of covalent compounds is low. We recognize the formula of a compound as that of a covalent substance because the formula contains only nonmetallic elements.

Covalent compounds are not composed of ions. Ionic compounds are composed of ions. Metals like to lose electrons, and in the presences of metals nonmetallic elements will gain electrons. However, when nonmetals are together they do not like to lose electrons. Yet they still have the tendency to want to acquire 8 electrons. So how do nonmetals deal with the tendency to gain electrons to get to the magic number of eight? They do it by sharing electrons with other nonmetals. Lets look at a familiar example, water.

The formula for water is H2O. Can we understand this formula in terms of the nonmetallic elemetns trying to get 8 electrons. Lets look at the two elemetns in this compound, oxygen and hydrogen. Oxygen has 6 valence electrons and hydrogen has 1 valence electron. Oxygen would like to acquire two more electrons to have 8 valence electrons. Hydrogen on the other hand would be like helium (its nearest nobel gas) if it acquired one more electron. So lets draw a picture of what an oxygen atom and a hydrogen atom look like with their valence electrons.

Using this kind of a picture of the valence electrons on the oxygen atom and a hydrogen atom we can see that if the oxygen atom shared electrons with two hydrogen atoms, the oxygen atom would have 8 electrons and each hydrogen atom would have 2 electrons.

When a covalent compound is formed the atoms are attracted together by the sharing of electrons (more about this later). Chemists have found that almost all covalent compounds contain only nonmetallic elements.

So when trying to come up with formulas for compounds containing only nonmetallic elements we try to share electrons to get 8 electrons around the atom or 2 electrons for hydrogen. Lets try a few other examples to get the hang of this...

Example #1:

Write the formula of a compound which contains hydrogen and chlorine.

To do this we first draw the structure showing the number of valence electrons the element has. Then we try to determine how many of each atom we need to obtain 8 electrons around the chlorine and 2 electrons around the hydrogen atom.

In this case we see that chlorine only needs one electron as does the hydrogen atom.

So the formula is HCl.

Example #2:

Write the formula of a compound which contains hydrogen and nitrogen.

To do this we first draw the structure showing the number of valence electrons the element has. Then we try to determine how many of each atom we need to obtain 8 electrons around the nitrogen and 2 electrons around the hydrogen atom.

In this case we see that nitrogen needs three electrons to get eight electrons around it. If we use three hydrogen atoms, each donating one electron we have...

The formula is NH3.

Example #3:

Write the formula of a compound which contains hydrogen and carbon.

To do this we first draw the structure showing the number of valence electrons the element has. Then we try to determine how many of each atom we need to obtain 8 electrons around the carbon and 2 electrons around the hydrogen atom.

In this case we see that nitrogen needs three electrons to get eight electrons around it. If we use three hydrogen atoms, each donating one electron we have...

The formula is CH4.

One interesting question relates to what these molecules look like at the atom level. That is what is their shape? Interestingly the shape of all of these compounds HCl, H2O, NH3, CH4 and are related. All of the compounds havean atom with four pairs of electrons. It turns out that electrons prefer to be paired when in compounds. There are two kinds of pairs, one is bonding (the pair between two atoms) and one pair is non-bonding. It is these combinations, of bonding pairs and non-bonding pairs which define the geometry of molecules. Lets look at the molecuels we've discussed so far in terms of the bonding pairs and non-bonding pairs of electrons.

Molecule

Bonding pairs
of electrons

Nonbonding pairs
of electrons

Geometry/Shape

HCl

1

3

linear

H2O

2

2

bent

NH3

3

1

pyramidal

CH4

4

0

tetrahedral

Here is a link with a different kind of software. You will need a special plug-in to view these animations.

In class we explored these shapes using balloons.

 

So draw the structure of each of the following compounds, determine the number of bonding pairs and nonbonding pairs of electrons on the central atom and identify the geometry/shape of the molecule.

a) CCl4

This compound contains a carbon atom and four chlorine atoms. Drawing the electron structure of the individual atoms,

The carbon needs four electrons to have eight and each chlorine requires one electron to have eight. So the structure (four chlorines around the carbon) would look like,

The central carbon atom has 4 bonding pairs of electrons and no nonbonding pairs. So the geometry/shape of the molecule is tetrahedral.

b) NF3

This compound contains a nitrogen atom and three fluorine atoms. Drawing the electron structure of the individual atoms,

The nitrogen needs three electrons to have eight and each fluorine requires one electron to have eight. So the structure (four fluorine around the nitrogen) would look like,

The central nitrogen atom has 3 bonding pairs of electrons and one nonbonding pairs. So the geometry/shape of the molecule is pryramidal.

c) H2CO

This compound contains a carbon atom, an oxygen atom and two hydrogen atoms. Drawing the electron structure of the individual atoms,

Now this is starting to look a little complicated. But not really. The hydrogens in the formula are next to the carbon so we'll have them bond to the carbon, and well have the oxygen bond to the carbon also. So the structure would look like,

But this looks funky. Carbon does not have eight electrons, but seven and the oxygen also has seven electrons. But both need eight! The hydrogen atoms are happy. But how do we handle the carbon and the oxygen. Since they each have an unpaired electron, why don't we pair those electrons. But where do we put the pair? Well we'll put it in the same place as the other pair. Since one of the electrons is carbon's and one is oxygens and they both need one more electron...why not!?

The central carbon atom has eight electrons around it as does the oxygen atom.

The two pairs of electrons between the carbon and oxygen atom are called a double bond. So the central carbon atom still has four pairs of electrons around it. But not like the four pairs of electrons in CH4. In this case it has two bonding pairs and one double bonding pair and no nonbonding pairs. The geometry for this shape is called trigonal planar.

d) C2H6

This compound contains two carbon atoms, and six hydrogen atoms. Drawing the electron structure of the individual atoms,

This is looking complicated also. Lts try to reason this one through. We know carbon likes to form four bonds. So how do we distribute the hydrogens, and what do the carbons do. The fact that the formula is C2H6 means the molecule contains two carbon atoms and six hydrogen atoms bonded together in some way. If we try to put four of the hydrogens around one of the carbons that would use up all the bonds to that carbon and we would still have a carbon atom and two hydrogen atoms left over wondering what should be done with them?! It turns out that carbon like to bond to itself and since we have two carbon atoms lets bond them together,

Notice there are six unpaired electrons around the two carbon atoms, and we have six hydrogen atoms that we need to include. So it seems obvious to bond each electron ina hydrogen atom with the unpaired electron on the carbon atoms.

Each carbon atom has four bonding pairs of electrons around it so each carbon atom has a geometry/shape of tetrahedral. What does that look like!?

 

Now we are ready to start taking about organic chemistry.