The Geometry of Atoms in a Molecule
In the specified atom’s uncharged state, it has been observed that carbon makes only four
bonds, nitrogen makes only three bonds, oxygen makes only two bonds, and hydrogen and the
halogens make only one bond with row two elements. It has also been observed that if four
atoms of two different types are bonded with a single carbon, there is only one type of compound
formed. Thus dichloromethane has only one structure. When drawing the Lewis structure of
CH2Cl2, it looks like two are possible. It fact, if carbon bonded in a planar fashion, there would
be two different structures which would have different physical properties. We have therefore
assigned the geometry to a more three dimensional structure called the tetrahedral. When one
tries to draw a three dimensional structure on a two dimensional surface, wedges and dotted lines
denoting atoms out of the page or back into the page are necessary. Since just drawing molecules
on paper may not convey all the geometry around each individual atom very well, molecular
models lab will help you learn three dimensional perceptions.
Definition:
1) A “group” is either:
a. A single bonded atom
b. A double bonded atom
c. A triple bonded atom
d. Or a pair of electrons (called a lone pair)
2) Tetrahedral geometry stabilizes four groups around an atom.
3) Trigonal planar geometry stabilizes three groups around an atom.
4) Linear geometry stabilizes two groups around an atom.
Model building:
1) Count out the atoms needed from the formula given.
2) Set aside the atoms that only make one bond (these will go on the “outside” of the
molecule).
3) Bond together those atoms which make two, three, or four bonds with a single bond. Here,
there may be more than one possibility.
4) Count the number of remaining bonds on your molecule and compare this number to the
number of “single bonders” you set aside in step #2.
a. SB = Bonds left … then just put them on.
b. SB < Bonds left … then make rings, double, or triple bonds until “a” is satisfied.
5) Determine the stability of the substance (if one cannot have each position for bonding
covered, the species is unstable)
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The Geometry of Atoms in a Molecule
The formula of molecules/ions to do:
CH4
CH2
Cl2
+
H3O
CH2
NH3
H2O2
N2
C2H4
C2H2Br2 CO2
SCN
HNO3
C2H4Cl2 PCl3
HF
CO
SF2
C3H6O
C3H7N
C2H4O2
C2H4O
SiF4
C3H7N
CH4 S
CO3
HCO2
CH4O
+
NH4
C2H2
NO3
H3O
C2H6O
-2
SO4
C3H6O
HNO2
H2O
SO2
Procedure:
1. For each formula given, do the following in some organized way in your lab notebook:
2. Draw the Lewis structure of the model you made with the maximum number of atoms in the
plan of the paper (arrange the model this way before attempting this). Represent the atoms not
in the plane of the paper as wedges or dotted lines.
3. Determine the geometry around each atom that makes two, three, or four bonds.
4. Using electronegativity values found in your book, determine which bonds in your molecule
are polar.
5. Using the ideas of geometry (part 2) and polarity (part 3) determine if the molecule itself is
polar.
Determine if there is another molecule you can make with the same formula … an isomer! If one or
more isomers exist, do steps 1 through 4 for one additional isomer. (Not more than 2 isomers for each
formula!)
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