Model Kits – Lewis Structures
Complete this table regarding the atoms in your molecular model kit:
Model Kit
Color
Atom
white
hydrogen, H
black
carbon, C
red
oxygen, O
blue
Green, orange
or purple
nitrogen, N
Halogen,
F, Cl, I
Number of
Valence
Electrons
Lewis
Structure
Number of
electrons
needed to fill
valence shell
Number of
single bonds
that can be
formed
Covalent Compounds: Single Covalent Bonds
Chlorine, with 7 valence electrons, needs one more electron to fill its valence shell and be
more stable. Iodine, with 7 valence electrons, also needs one more electron to fill its
valence shell and be more stable.
..
..
: Cl .
¨
.I:
¨
If chlorine and iodine SHARE AN ELECTRON pair, each will be more stable. The
Lewis structure of such a molecule would look like this: .
.. ..
: Cl : I :
¨ ¨
A pair of electrons shared between two atoms is called a COVALENT BOND. It may
also be represented by a dash like this:
Cl−I
Sometimes the shared electron pairs are shown as dashes and the unshared electrons are
shown as dots:
.. ..
: Cl − I :
¨
¨
A group of atoms held together by covalent bonds is called a COVALENT
COMPOUND.
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Name__________________________________________________________
Your model kit may be used to construct covalent compounds. Use the short gray sticks
to represent shared electrons holding two atoms together. To construct ICl, link a green
and purple or orange ball together with a short gray stick. (Notice that the model does
not show the unshared pair of electrons.)
Complete the table below. Use scratch paper to first figure out the structure, then draw
the Lewis structure below. The first one is done for you.
Molecular
Formula
ICl
Lewis Dot Structure
(with dots)
Lewis Structure
(with dashes to
show bonds)
.. ..
: Cl : I :
¨ ¨
.. ..
: Cl─ I :
¨ ¨
Br2
H2
HCl
H2O
CH4
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Check here
when you have
completed the
model.
NH3
CH3Cl
N2H4
CH2Cl2
C2H6
C3H8
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Covalent Compounds - Multiple Bonds
Sometimes a Lewis dot structure cannot be drawn so that all atoms have eight electrons
in their outer shell. There appears to be a “shortage” of electrons. An example of this is
O2. Consider this incorrect structure:
.. ..
:O:O:
˙ ˙
In such cases, octet rule structures can often be made in which two atoms are bonded by
two pairs of electrons rather than by one pair of electrons. The two pairs of electrons
form a double bond. The Lewis dot structure for O2 looks like this:
.. ..
:O::O:
When two pairs of electrons are shared between two atoms, the bond is still called a
covalent bond, but now it is a DOUBLE BOND. A double bond may be represented by
two dashes like this:
O=O
Sometimes the shared electron pairs are shown as dashes and the unshared electrons are
shown as dots:
.. ..
:O=O:
Your model kit may be used to construct covalent compounds with double bonds. Use
two of the longer flexible gray sticks to represent the pairs of shared electrons holding
two atoms together. To construct O2, link two white balls together with two flexible grey
sticks.
Complete the table on the following page:
:
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Molecular
Formula
Lewis Structure
(with dashes to
show bonds)
Lewis Dot Structure
(with dots)
O2
N2 (triple
bond!)
H2CO
C2H4
C2H2
HCN
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Check here
when you have
completed the
model.
Geometry of Molecules
There are several kinds of shapes that covalent compounds take. We will be concerned
with these five. Your instructor will show you these five shapes.
tetrahedral
Trigonal
planar
Trigonal
pyramidal
linear
bent
Construct these molecules with your molecular model kit and figure out which shape each
represents:
CO2, H2O, CH2O, CH4, NH3,
Complete the table with your instructor:
Molecule
Draw shape using
element symbols, dots for
electron pairs and dashes
for bonded electrons.
Identify
the
Central
Atom
Number of
atoms
bonded to
central atom
Number of nonbonding electron
pairs around the
central atom
Number
of
VESPR
Groups*
CO2
H2O
CH2O
NH3
CH4
* Number VESPR Groups is equal to the number of atoms bonded to the central atom
plus the number of non-bonding pairs.
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Name
Shape
Name__________________________________________________________
Draw Lewis dot structures and PREDICT the shape of the following molecules. Figure
them out first on scratch paper before drawing them here in the correct shape.
SiO2
PBr3
CH3F
BF3
BeCl2
NF3
H2S
Cl2O
CS2
CH2Cl2
SiF4
PH3
note: boron is an exception to the rule and will be stable with 6 electrons in its outer
shell
note: beryllium is another exception to the rule and will be stable with 4 electrons in
its outer shell.
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