The
Covalent Bond
Lewis
Structures
A covalent bond represents
a shared electron pair between nuclei. The Stability of covalent bonds is due
to the build-up of electron density between the nuclei. Using Coulomb's law
(discussed in Ionic Bonding),
you should note that it is more stable for electrons to be shared between
nuclei than to be near only one nucleus. Also, by sharing electron pairs nuclei
can achieve octets of electrons in their valence shells, which leads to greater
stability.
To keep track of the number
and location of valence electrons in an atom or molecule, G. N. Lewis developed
Lewis structures. A Lewis structure only counts valence electrons because these
are the only ones involved in bonding. To calculate the number of valence
electrons, write out the electron configuration of the atom and count up the
number of electrons in the highest principle quantum number. The number of
valence electrons for neutral atoms equals the group number from the periodic
table. Each valence electron is represented by a dot next to the symbol for the
atom. Because atoms strive to achieve a full octet of electrons, we place two
electrons on each of the four sides of the atomic symbol. Some examples of
Lewis structures for atoms are shown in .
We can create bonds by
having two atoms come together to share an electron pair. A bonding pair of
electrons is distinguished from a non-bonding pair by using a line between the
two atoms to represent a bond, as in the figure below. A lone pair is what we
call two non-bonding electrons localized on a particular atom.
You should note that each
atom in the H-Br molecule has a full valence shell. Both the hydrogen and the
bromine can count the two electrons in the bond as its own because the
electrons are shared between both atoms. Hydrogen needs only two electrons to
fill its valence, which it gets through the covalent bond. The bromine has an
octet because it has two electrons from the H-Br bond and six more electrons,
two in each lone pair on Br.
The deadly gas carbon
monoxide, CO, provides an interesting example of how to draw Lewis structures.
Carbon has four electrons and oxygen has six. If only one bond were to be
formed between C and O, carbon would have five electrons and oxygen 7.
A single bond here does not
lead to an octet on either atom. Therefore, we propose that more than one bond
can be formed between carbon and oxygen so that we can give each atom an octet
of electrons. To complete the carbon and oxygen octets in CO, we must employ a
triple bond, denoted by three lines joining the C and O atoms as shown in . A
triple bond means that there are six electrons shared between carbon and
oxygen. Such multiple bonds must be employed to explain the bonding in many
molecules. However, only single, double, and triple bonds are commonly
encountered.
So far we have only dealt
with very simple, uncharged molecules. For more complex molecules and molecular
ions, it becomes important to keep an accurate count of the number of electrons
in the molecule. For example, let us make a Lewis structure for NO2-.
We have five electrons from N, twelve from the oxygen (six from each O), and
one extra electron because the molecule has a negative charge. Therefore, NO2 - has
a total of eighteen electrons and we should draw the following Lewis structure:
If we had tried to draw the
above structure without taking the charge of the ion into account, we could not
have produced a full octet around at least one atom. If the ion had been
positively charged, as in NO2 +, we would count the
electrons as follows: five from N, twelve from O, and minus one due to the
charge. The total number of electrons is sixteen for NO2 +,
and the molecule will have a Lewis structure different from that of NO2 - because
it has a different number of electrons.
To improve your skills in writing
Lewis structures, you should draw as many molecules as possible until you feel
confident in your ability to draw Lewis structures.
Formal
Charge
When trying to draw the
Lewis structures of charged molecules like NO2 - ,
we encounter the problem of trying to tell where the negative charge is
located. Is it on nitrogen or on one of the oxygens? To combat these troubles,
chemists have devised the notion of formal charge. Using the Lewis structure
and the rules for assigning formal charges, we can assign a formal charge to
each atom in a Lewis structure to determine where the charges are located.
Using NO2 - as
an example, let's discuss how to determine the formal charges on atoms in
molecules. First, we must draw the correct Lewis structure. Then, we break all
bonds around each atom giving half the electrons in the bond to each bonded
atom. All lone pairs remain on the atom to which they belong in the molecule.
This process serves to count the number of electrons each atom has in the
molecule and is shown in the figure below.
Once we have counted the
number of electrons assigned to each atom, we compare the number to the number
of valence electrons in the free atom. For example, oxygen has six electrons in
the free atom, and it has six electrons in the right-hand oxygen in the .
Therefore, the right-hand oxygen has no formal charge because it has the same
number of electrons in the NO2 - molecule as it
does as an atom. The left-hand, singly bonded oxygen has seven electrons-- one
more electron than has the free atom. Therefore, this oxygen has a -1 formal
charge because it has one more electron in the molecule than oxygen has as a
free atom. The nitrogen has five electrons around it and five valence electrons
in the free atom, so the N has no formal charge. In general, formal charge
equals the difference between the number of valence electrons of the atom and
the number of electrons around the atom in a molecule as assigned by the rules
for drawing Lewis structures.
A complete Lewis structure
should include both bonding and formal charge information. Therefore, the
structure of NO2 - should be drawn as shown in
.
Resonance
Structures
While drawing the , you may
have noticed that the two oxygens appear to be different. One bears a negative
charge and has only one bond to N while the other is neutral and doubly bonded
to N. Why should these two oxygens be different? There is absolutely no reason
why they should be. It is impossible to make a single Lewis structure that
depicts the equivalency of the two oxygens. Instead, we can represent NO2 - as
a hybrid of two resonance structures as shown in .
It is important to note
that NO2 - is neither one nor the other
resonance structure but is the average of the two. A good analogy for resonance
structures is found in mixing colors. Green is neither yellow nor blue but is a
mixture of the two colors, just as NO2 - is
neither of the resonance forms but is a mixture of the two structures.
When more than one
reasonable Lewis structure can be drawn for a molecule, the actual structure of
the molecule will be a resonance hybrid of the structures.
Exceptions
to the Octet Rule
Although the octet rule has
allowed us to draw almost every conceivable Lewis structure, there are certain
molecules that do not obey the octet rule. In this section, we will point out
the most common exceptions.
Boron and aluminum
compounds commonly place only six electrons around the metal center. For
example, AlH3 has only six electrons on Al. Compounds with less
than an octet (or duet for H) of electrons around each atom are called electron
deficient. Boron and aluminum compounds are frequently electron deficient while
compounds involving most other elements are not. The reason why boron and
aluminum can form electron deficient compounds has to do with their low
electronegativities. Because both atoms are not very electronegative, they are
not terribly unhappy when they have fewer electrons than they require for full
octets.
While boron and aluminum
can have less than a full octet, some atoms like phosphorous and atoms in
period three or below on the periodic table (greater period numbers) can exceed
their octets. Try to draw a reasonable Lewis structure for either PF5 or
SF6. You should not find it possible to obey the octet rule on
either phosphorous or sulfur. Frequently textbooks say that atoms like P and S
are able to expand their octets by letting the extra electrons fill their empty
3d orbitals. Your chemistry course may even require you to memorize this
"fact". However, this description of the bonding in such compounds is
completely false. After you have read Molecular Orbital Theory you
should be able to come up with a better reason. The explanation of the expanded
octet must wait until then due to is complexity. For now, realize that atoms
below period two may expand their octets to accommodate more than eight
electrons.
Valence
Shell Electron Pair Repulsion Theory
When drawing Lewis
structures, only bonding and charge information is available. Such structures
tell us absolutely nothing about the real three-dimensional shapes that
molecules have. To determine molecular geometry, chemists use Valence Shell
Electron Pair Repulsion theory-- abbreviated VSEPR. The VSEPR model makes the
reasonable assumption that electron pairs repel each other. Therefore, electron
pairs in bonds and lone pairs will want to be oriented as far away from each
other as possible. By analyzing all possible combinations of lone pairs and
bonding pairs we can predict the structure of any covalent molecule. , , , and
show the results of such an analysis. (The tables are broken down into four
parts due to the sizes of the images and not because there are any fundamental
differences between the tables.) For each, A stands for the central atom, B
stands for any other atoms bonded to A, and e stands for the lone pairs on
the central atom.
By comparing a Lewis
structure to the examples provided in the above figures, you can predict the
geometries of many molecules accurately. Note in the tables that the lone pair
groups e are placed at positions to minimize interactions with other groups e
or B; the lone pairs take up these positions preferentially when you must choose
to place either e or B there. For example, for the molecule AB3e2 in
, the e's are placed in the equatorial positions where they are at 90 and 120
degree angles from other groups, rather than at the axial positions where they
would be restricted to 90 degree angle interactions. We can understand this
trend by visualizing the condensed electron density of a lone pair near an
atomic center, in comparison with a bonded electron pair in which electron
density is distributed between two atoms.
VSEPR theory does not work
well for transition metals. To predict their geometries you will need a more
advanced treatment of bonding which will not be presented in this SparkNote.
To predict the geometries
of polycentric molecules (those with A greater than one), simply use the above
tables of geometries to predict the geometry of each center independently of
others. For example, to predict the geometry of HOCH2NH2,
you need only predict the geometry at oxygen, carbon, and nitrogen. To do so,
first draw the Lewis structure as shown in :
Next, classify the VSEPR
type for each atom bonded to more than one other atom using A, B, and e. Oxygen
is AB2e2, so it is bent. Carbon is AB4, so it
is tetrahedral. Nitrogen is AB3e, so it is pyramidal. Now you can
draw the structure of HOCH2NH2in three dimensions.
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