Shapes and Polarities of Molecules
ANSWER KEY
In this exercise you will construct models of molecules. The shapes of these molecules will be correlated
with the bond types, bonding orbitals, bond polarities, and molecular polarities.
In the lab are small plastic pieces with varying numbers of “prongs”. Each piece represents an
atom; each prong represents a valence shell electron pair. Plastic soda-straw-like tubes can connect a
prong of one atom with a prong of another – a covalent bond; a shared valence shell electron pair.
Flexible connector tubes allow connecting more than one prong on each atom to another atom – multiple
bonds can be made.
Each atom’s prongs are arranged at the proper angle for the particular atom and bonding style
represented, as given by VSEPR theory. Pieces (atoms) with from 1 prong to 6 prongs are available; one
electron pair, as with H, up to 6 electron pairs, for expanded octet cases like S or Xe. Examples:
1-prong connector; 1 bond. H.
4-prong; 1 bond, linear. F.
2-prong; linear. 2 bonds. Be.
5-prong; trigonal bipyramid. 5 bonds. P.
3-prong; flat triangle. 3 bonds. B.
5-prong; 4 bonds. “See-saw” S.
4-prong; tetrahedral. 4 bonds. C.
6-prong; octahedron. 6 bonds. S.
4-prong; 3 bonds, trig. Pyramid. N.
6-prong; 5 bonds, sqr-based pyramid
4-prong; 2 bonds, bent. S.
6-prong; 4 bonds, flat square. Xe.
For the following molecules, draw a Lewis electron dot structure and construct a model. Sketch
and name the shape. From your knowledge of electronegativity predict whether the bond(s) in the
molecule will be polar or non-polar covalent. Finally, from the shape and bond polarity, predict whether
or not the molecule as a whole will have a dipole moment, will be a polar molecule or not. Each model,
with its data all filled in, will be checked by the teacher and initialed when approved.
You may find it useful to draw orbital box overlap diagrams to indicate which of several possible
shapes the central atom is assuming. For example, sulfur may call for a 4, 5, or 6-prong connector, with
2, 4, or 6 prongs used for bonding.
There is a shortage of some pieces – flexible bonds, 5- and 6-prong connectors especially. Do not
keep these for long; use them and return them quickly so others may use them. There are not enough
pieces for everyone in the class to do the same model at once; you don’t have to work in any order. Don’t
lose any pieces, don’t mix up the various types. Penalties will be assessed on the whole class for pieces
left around the room or improperly sorted.
Special Cases. The last two models must be shown to the teacher at the same time; they must have the
same color pieces for each atom. The three molecules before them, all with the formula C2H2Cl2, will
need only one model; but you will then change that model in front of the teacher to make the other two
molecules. All data for all three molecules must be done before you do this.
FORMULA
OF2
DOT
STRUCTURE
SKETCH
of shape
NAME
of shape
bent
BOND(S)
POLAR?
MOLECULE
POLAR?
yes
yes
tetrahedral
yes
no
triagonal
pyramid
yes
yes
CCl4
PH3
OK by
Teacher
BCl3
FORMULA
DOT
STRUCTURE
SKETCH
of shape
flat
triangle
NAME
of shape
yes
BOND(S)
POLAR?
no
MOLECULE
POLAR?
N2
linear
no
no
CO2
linear
yes
no
BeBr2
linear
yes
no
SF6
octahedron
yes
no
IF3
“T”
yes
yes
yes
yes
IF5
irregular
square base
pyramid
XeF4
flat
square
yes
no
PCl5
trigonal
bipyramid
yes
no
“see-saw”
yes
yes
tetrahedral
yes
yes
SF4
CH2F2
OK by
Teacher
FORMULA
HCN
DOT
STRUCTURE
SKETCH
of shape
NAME
of shape
BOND(S)
POLAR?
linear
H3CCH3
MOLECULE
POLAR?
yes
yes
yes
yes
no
2 tetrahedral
H2CCH2
2 flat
triangle
HCCH
2 trigonal
bipyramid
H3COH
H2CO
(C-H) yes
(C-C) no
no
yes
yes
no
tetrahedral
yes
yes
bent
yes
yes
flat
triangle
yes
yes
ClHCCHCl
2 flat
triangle
ClHCCClH
2 flat
triangle
Cl2CCH2
2 flat
triangle
(C-H) yes
(C-C) no
(C-Cl) yes
(C-H) yes
(C-C) no
(C-Cl) yes
(C-H) yes
(C-C) no
(C-Cl) yes
yes
no
yes
OK by
Teacher
CHClFBr
CHClBrF
tetrahedral
yes
yes
tetrahedral
yes
yes