Practice Sheet
Electron Dot (Lewis) Structures
A Lewis or Electron Dot Structure is a convenient representation of the valence electrons in an atom.
An electron dot structure for an atom is simply the symbol for the element, surrounded by a number of dots
equal to the numbe r of valence electrons.
Avoid a common mistake: the dots represent valence electrons only, so make sure you use only the number
of dots corresponding to the number of valence electrons.
Examples:
.
.
.
Al
.
.
.
.
C
.
.
P
.
.
.
.
.
.
.
.
.
.
.
Kr
.
.
Ca
.
Na
• Draw lewis dot structures for an atom of each of following elements:
1. K
2. Si
3. Ar
4. As
Covalent Bonds
An atom can also achieve a noble gas configuration by sharing electrons with other atoms. When two non-
metals combine, they typically share electrons in covalent bonds and form what are know as covalent compounds.
We can draw Lewis Electron Dot Diagrams for covalent molecules to determine what type of bonds the
molecules form.
For example, consider two hydrogen atoms. Each one has one valence electron. However, both hydrogens
would like to have two electrons, to obtain the same configuration as [He]. To do this, each H atom shares
an electron with the other, forming a covalent bond. In H
2
, each H atom has 2 electrons.
each H has 1 e−
and wants 1 more
shared pairs can
be represented as
lines between atoms
a hydrogen
molecule
(diatomic = 2 atoms)
H H H
2
+
H H H H
H’s share e−’s
forming a covalent
bond
. .
.
.
Here’s another example, this time for chlorine. A Cl atom has 7 valence electrons, so it needs 1 more to
achieve an octet (8 electrons).
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
forming a covalent
2
molecule
and wants 1 more
each Cl has 7 e−
Cl
a chlorine
+
Cl Cl Cl Cl ClCl
. .
each Cl shares 1 e−’s
bond consisting of
2 electrons
the shared electron pair
is represented as a line
Each chlorine now has 8 electrons, be cause you can count the shared ones:
Lewis Dot Structures Practice Sheet, page 2
.
.
.
.
.
.
.
.
.
.
.
.
.
.
ClCl
this Cl atom also has an octet
this Cl now has an
octet of 8 electrons
Sometimes you need to share more than one pair of electrons to achieve an octet:
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
2
molecule
+
O O
O
O O O
O
each O has 6 e−
and wants 2 more
forming a double
covalent bond
an oxygen
each O shares 2e−’s
the four shared electrons
.
.
. .
two lines between the atoms
are represented as
We can check to see that each oxygen has the appropriate number of electrons:
.
.
.
.
.
.
.
.
.
.
.
.
O O
each oxygen atom
now has an octet
total # valence
electrons = 12
(note the 12 dots)
For the follow ing problems, calculate the total number of valence electrons in each of the following molecules.
Then, draw lewis dot structures for each:
• Br
2
# valence e
−
• HCl # valence e
−
• HBr # valence e
−
• N
2
# valence e
−
• NO # valence e
−
This one is tricky, there is no way to achieve an octet. So, get each atom as close to an octet as possible
without going over.
.
Lewis Dot Structures Practice Sheet, page 3
Steps for Drawing Lewis Dot Structures for Larger Molecules
1. First, determine the central atom:
(a) Hydrogens (H) and halogens (F,Cl,Br,I) are almost always outer atoms. They only want to form
one bond to get to a noble gas configuration.
(b) If the choice is still ambiguous, the atom further to the right on the periodic table is generally an
outer atom, the one further to the left is often the central atom.
For example: COH
2
The H’s are outer atoms, leaving C and O as candidates.
C is further to the left than O, so C is the central atom.
2. Arrange the outer atoms around the central atom:
3. Count up the valence electrons: C (4) + O (6) + 2H (2x1) = 12 electrons
4. Draw a bond between each outer atom and the central atom. Count the electrons you have used in
these bonds and subtract them from your total valence electrons.
.
.
.
.
.
.
C HH
O
6 electrons are use in making these bonds
so 12 − 6 = 6 electrons remain
5. Use the remaining electrons to fill o c tets around the outer atoms. Put any remaining ones on the
central atom.
.
.
.
.
.
.
C HH
O
.
.
.
.
.
.
the H’s can’t accept any more electrons
so the remaining six are place around the oxygen
6. If all the atoms don’t have an octet, move a non-bonding electron pair from 1 atom into a sharing
position.
.
.
.
.
.
.
C HH
O
.
.
.
.
.
.
.
.
.
.
.
.
C HH
O
.
.
.
.
.
.
O has an octet, but C does not
the H’s have 2 electrons
move one of the pairs on oxygen
in to a sharing position with carbon
.
.
.
.
.
.
.
.
.
.
C HH
O
.
.
C HH
O
.
.
.
.
and H has 2 electrons
both C and O have octets
or
Try drawing lewis dot structures for these molecules:
Lewis Dot Structures Practice Sheet, page 4
• SiH
4
# valence e
−
• CCl
4
# valence e
−
• PCl
3
# valence e
−
• NCl
3
# valence e
−
• CS
2
# valence e
−
• CH
2
Cl
2
# valence e
−