Lewis Diagrams and The Octet Rule
Lewis diagrams are a mental construct for atoms and molecules that is based on the
localized electron model of bonding. These diagrams help us to envision how atoms
interact with each other and how molecules interact with their environment.
We know from previous work that atoms are made up of three major subatomic particles:
protons, neutrons, and electrons. Largely, the chemistry and interaction between atoms is
due to the electrons. So in any atom, where are the electrons, and which ones are the
ones that matter?
Quantum theory tells us that we can never really know the exact location of an electron,
but we can use an orbital to describe where they might be. This does not change,
however, the number of electrons that an atom has. Furthermore we know that any
particular atom can have electrons in an unfilled and a filled shell. Those in the unfilled
shell are called the valence electrons, and are responsible for the chemistry of the atom.
Let us now look at the simplest atom, hydrogen.
Hi
In its neutral state, hydrogen has one proton and one electron. It is symbolized as shown
above. Now look at helium, which has 2 electrons.
He:
Here we have they symbol for helium and two dots signifying two electrons. We know
that these electrons are filling the 1s orbital. These are the valance electrons for those
two atoms.
The table below shows Lewis diagrams for a number of elements. Notice that they are
only from the s and p blocks of elements.
Question:
Give the electron configuration and the Lewis diagram for the following atoms. In the
electron configuration, circle the electrons that are the valence electrons.
Example: C
•
C = 1s22s22p2
iC i
•
1. Be
2. S
3. Cl4. Ca+
5. Br
Knowing how to draw Lewis structures for individual atoms is a useful skill. However,
there is a variety of matter outside of atoms. Atoms, making use of their valence
electrons, bind together to form chemical bonds, and by extension, molecules. There are
two basic types of bonds:
•
Ionic: These bonds involved the exchange of electrons from one species to
another. A positive ion is attracted to a negative ion, and a bond is formed
between them. By giving up an electron, one species may be gaining stability and
by gaining an electron, another species may be gaining stability.
•
Covalent: Instead of electrons being completely transferred from one atom to
another, they are shared between atoms. As a result, two atoms are becoming
more stable by sharing a pair of electrons.
The interesting things about bonds, is that they don’t really exist. They aren’t something
that can be pointed at the same way that an atom can. Bonds represent forces of
attraction between atoms. Though some may disagree, they are really just forces of
attraction, but its easier to think of them as little sticks that hold atoms together.
In ionic bonds we have two oppositely charged species that are attracted to one another.
In covalent bonds we have two atoms that are near enough to one another to have a
shared area of electron density between them. In any sort of bond, ionic or covalent,
atoms are trying to achieve a lower state of energy.
In doing so, they generally achieve a noble gas configuration where their outermost shell
of electrons is filled. This is referred to as the octet rule.
According to the octet rule, atoms attempt to have a total of electrons in their valence
shell when they undergo bonding.
Ionic
Sodium Chloride is made through the combination of sodium and chlorine. The electron
configurations for both are
Na = 1s22s22p63s1
Cl = 1s22s22p63s23p5
When neutral sodium gives an electron to neutral chlorine they both achieve noble gas
configurations. Notice, in their valence shells there are a total of 8 electrons, thus an
octet.
Na+ = 1s22s22p6
Cl = 1s22s22p63s23p6
-
The Lewis diagram for this would be
i i
Na :Cl:
i i
As it can be seen, there is a bonding pair of electrons between sodium and chlorine and
three lone pairs of electrons around chlorine. Also, it can be seen that all of the valence
electrons from sodium (1) and chlorine (7) are accounted for.
Question:
Draw the Lewis diagrams for the following ionic compounds:
1. MgO
2. LiBr
3. CaBr2
4. K2O
5. SrO
Covalent compounds
Not all bonds between compounds are ionic in nature. Some are covalent, that is,
electrons are shared between atoms as opposed to exchanged like in ionic bonds.
There are some basic rules for drawing Lewis diagrams for molecules with covalent
bonds.
1. Add up all of the valence electrons for each atom in the molecule. For example
CH4 = 4(carbon) + 1(hydrogen) + 1(hydrogen) + 1(hydrogen) + 1(hydrogen) = 8
2. Write out the structure. One atom will be the central atom in most cases. This is
generally the atom that there is only one of. Again with CH4.
H
H
C
H
H
3. Write in the bonds between each of the atoms. The sticks that represent bonds
also represent a pair of electrons. This means that there is a pair of electrons
being shared between carbon and hydrogen. The total number of electrons that
appear in the Lewis diagram must be equal to the total number of valence
electrons.
H
H
C
H
H
This means that there are two 4 sets of two electrons around carbon making eight.
That’s an octet. Carbon has a filled shell or a filled octet. The hydrogen atoms all
have two electrons around them. Hydrogen has a filled 1s orbital at this point,
making it stable. As opposed to an octet like carbon, hydrogen only has to fill a
duet.
Question:
Why does hydrogen only need to fill a duet, while an element like carbon has to fill an
octet?
Another example. Carbon Dioxide – CO2
1. Count up all of the valence electrons.
CO2 = 4(carbon) + 6(oxygen) + 6(oxygen) = 16
2. Arrange the atoms.
O
C
O
3. Draw in the bonds to fill octets.
O
C
O
4. Here we can see that we have 16 electrons to work with and we only have
accounted for four of them. This is not necessarily what we want, especially since
none of the electrons have a filled octet. At this point we may try to draw in
double bonds.
O
C
O
5. This is getting better. Here we can see that carbon now has a filled octet, but we
have only used ½ of the electrons that have been allotted. There must be another
8 electrons that can be added in there. Since carbon already has a filled octet,
they must go around the oxygen atoms.
6. Take a look at what we have now.
Are there eight electrons around every atom – Yes
Are all valence electrons being used – Yes
Is this a valid Lewis structure – Yes
Question: Draw the Lewis structure for the following molecules
1. NH3 (ammonia)
2. CCl4 (Carbon tetrachloride)
3. CH2Cl2 (Methylene dichloride)
4. CH3OH (Methanol, carbon is the central atom)
5. BeH2 (Beryllium dihydride)
6. NO3- (Nitrate ion)
Exceptions to The Octet Rule
Earlier it was said that atoms look to have a full outer shell. In most cases this turned out
to a total of eight electrons. Where did that number come from?
An s shaped orbital can hold up to two electrons and a p shaped orbital can hold up to six.
Adding them we get a total of eight.
This is true for the first two rows of the periodic table. After that the octet rule is not
necessarily followed. You didn’t think chemistry would be that easy did you?
At the first energy level (n=1) there is only an s shaped orbital available for elements like
hydrogen and helium. At the second energy level (n=2) there are an s and a p shaped
orbital available. This applies from Li to Ne on the periodic table. At the third energy
level (n=3) there are s, p, and d shaped orbitals. Therefore elements from Na to Ar have
the 3s, 3p, and 3d orbitals available to them.
For example sulfur has 6 valence electrons. It forms a complex with fluorine called
sulfur hexafluoride (SF6)
Question:
Draw the Lewis diagram for sulfur hexafluoride. (Hint there will be more than 8
electrons around sulfur)
Since sulfur has the d-orbitals in its energy level, extra electrons can be put there. If the
diagram for SF6 has been drawn correctly, there should be 12 electrons around sulfur.
Since sulfur and any element in the third energy level or higher, has the d-orbitals to
expand into, they can violate the octet rule.
Question:
Draw the Lewis diagrams for the following atoms.
1. SO2Cl2 (sulfur is the central atom)
2. XeO4
3. NO434. ClO35. ClF5