Pirate Chemistry
Lewis Dot Structures and the Octet Rule
Atoms can now be combined together in such a way as they share enough electrons so that all
the atoms in the compound have a stable number. Remember that atoms are going to be stable
when their valence shells are filled. For most atoms, this means they need to have 8 electrons
in their valence shell. This is called the octet rule– most atoms are stable when they have 8
valence electrons. Hydrogen is a notable exception to this rule as it is only at the first energy
level and thus is stable when it gets two electrons.
Combining all of these ideas, we can begin to put together what are called Lewis Dot Structures
– a way of representing atoms or compounds by showing valence electrons as dots and lines
to satisfy the octet rule.
Let’s examine the compound hydrogen chloride, HCl. First let’s draw the Lewis Dot structure
of hydrogen:
H
Hydrogen is in the first column so gets one dot to
represent its one valence electron.
Now let’s draw the Lewis Dot structure of chlorine:
Cl
Chlorine is in the seventh column so gets seven
valence electrons.
We must now remember that most atoms will be stable when they have 8 valence electrons; a
full octet. Hydrogen will be stable with only 2 electrons. We can now combine these two atoms together to form a covalent bond; sharing their electrons:
H Cl
H Cl
Hydrogen now has two valence electrons and is stable.
Chlorine now has 8 valence electrons and is stable. By
sharing their electrons, the atoms have managed to satisfy each others’ valence shell to make a stable compound.
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Pirate Chemistry
We can clean this picture up by showing that the two electrons that are being shared have made
a covalent bond– a bond formed when 2 electrons from different atoms are being shared by
those two atoms. We represent this by replacing the two electrons in question with a line. The
line means the two electrons in the bond are being shared.
The line here represents two
electrons, one from each atom,
that are being shared and so
count for both atoms’ octets.
H Cl
H Cl
H Cl
The line of two electrons satisfies the electron needs of the
hydrogen, who only needs 2
electrons.
H Cl
Note that the bond counts fully
for each of the two atoms it is
bonding.
The line of two electrons also
counts for the chlorine.
Counted with the other 6 electrons makes 8 total electrons
and satisfies the chlorine’s
octet.
Note that what has been created here is a molecule– the smallest particle of a substance that
retains the properties of the substance and is comprised of two or more different atoms
chemically bonded. Molecules of covalent compounds differ from ionic compounds because
covalent compounds have a definite beginning and a definite end. Ionic compounds make repeating three-dimensional crystal lattices. This will cause a major difference in their properties
and will be discussed in more detail later.
This is a repeating 3D crystal of the
ionic compound sodium chloride
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org, and
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http://z.about.com/d/chemistry/1/0/q/p/Sodium-chloride-3D-ionic.jpg
http://en.academic.ru/pictures/enwiki/72/Hydrogen-chloride-3D-vdW-labelled.png
This is a molecule of the covalent
compound hydrogen chloride.
Pirate Chemistry
Let’s look at some other covalent molecules we can make using this method; like water, H2O,
for instance. Water consists of two hydrogen atoms and one oxygen atom. Let’s see why:
H
H
O
Oxygen has 6 valence electrons.
Each hydrogen needs a total of 2 electrons to be stable
and the oxygen needs 8.
Each hydrogen has only 1 valence electron.
H O
H
Each of the hydrogen atoms attaches to a
free electron in the oxygen atom. This creates bonds between the atoms.
H O
H
This is the Lewis Dot structure of water. Note that each hydrogen atom is bonded to the central
oxygen atom; thus each hydrogen has 2 electrons and is satisfied. The oxygen atom has two
bonds (2 electrons each) and 2 lone pairs of electrons– electrons that are on an atom but not
shared and so only count towards that atoms’ octet. If you add up the electrons on the central
oxygen, you get 8 electrons and so it is satisfied.
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org, and
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Pirate Chemistry
Draw the Lewis Dot structure for ammonia, NH3
Atom Valence Electrons
H
H N
H
H
1
N
5
H N H H N H
H
H
Example 2:
Draw the Lewis Dot structure for Methanol, H3COH
Atom Valence Electrons
H
H C O
H H
http://upload.wikimedia.org/wikipedia/commons/8/8f/Methanolalternative-3D-balls.png
http://z.about.com/d/chemistry/1/0/Y/a/ammonia.jpg
Example 1:
H
1
C
4
O
6
H
H C O
H H
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org, and
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Pirate Chemistry
Example 3:
Atom Valence Electrons
Draw the Lewis Dot structure for carbon dioxide, CO2
O C O
4
O
6
O C O
Each atom still has 8 electrons; making
it stable. Each O has 2 lone pairs and 2
bonds to make 8 while the C has 4
bonds making 8 electrons.
http://www.tennoji-h.oku.ed.jp/tennoji/oka/2004/carbon%
20dioxide.gif
Note that each oxygen can
bond with the carbon atom
twice; making a double
bond.
C
Example 4:
Atom Valence Electrons
Draw the Lewis Dot structure for Hydrogen cyanide, HCN
H C N
Note that the carbon here
can bond 3 times with the
nitrogen making a triple
bond.
C
4
N
5
H
1
H C N
Each atom has a filled valence. H has 2
electrons from the bond. C has 8 from
the 4 bonds. N has 8 from the 3 bonds
and the lone pair.
http://mls.jpl.nasa.gov/images/HCN.jpg
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org, and
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Pirate Chemistry
Questions
1.
2.
3.
4.
How many valence electrons do most atoms need to become stable?
How many valence electrons does Hydrogen need to become stable?
What is a covalent bond?
How many electrons are represented by each of the areas circled below?
Cl
I
Cl Br O O
5. Draw the Lewis Dot structure for each of the molecules below:
A. H2
B. HI
C. OCl2
D. N2
E. H2CO
F. H3CCH3
G. H2CCH2
H. HCCH
I. H3COCH3
J. CCl4
http://www.chemheritage.org/classroom/chemach/images/lgfotos/06synthesis/lewislangmuir3.jpg
Gilbert N. Lewis (1875-1946) was an American Chemist
who deduced the concept of covalent bonding in a 1916
paper. His works were far ranging and his ideas are
some of the most important for modern chemistry. He
was nominated for a Nobel Prize 35 times but never
won.
All text copyright Chris Smith 2009. All pictures obtained from internet and are copyright of their owners but assumed to be public accessible. If you are the owner of a picture and want it removed, email csmith@d211.org, and
it will be.