Download Molecular Models Activity

Chemistry
Name
Period
Molecular Models Activity
Background:
You can represent a molecule with either a molecular formula or a structural
formula. For example, the molecular formula for methane, CH4, tells us that the gas we burn in the lab
has one carbon and four hydrogens.
H
/
The structural formula for methane,
H–C–H
, tells us how the four hydrogens are
\
H
bonded with a central carbon atom. However, neither molecular formulas nor structural formulas
provide information concerning the actual arrangement of atoms in the molecule with their correct
geometry. In other words, not every molecule is as flat as we draw then on paper when we determine
a structural formula. Structural formulas only give some information about the real arrangement of
atoms in the molecule.
Since structural formulas are two-dimensional on paper, we need to understand that the actual
structural molecular shapes are three-dimensional. A molecular model is far superior to a structural
formula when it comes to visualizing atomic arrangement. Compared to molecular formulas and
structural formulas, molecular models show much more information about the true shapes of the
molecules.
The most common type of chemical bond between two non-metal atoms is a covalent bond.
The covalent bond consists of a pair of shared electrons; one from each atom that moves between
atoms holding then together in the bond. If this pair of electrons is shared between two atoms of equal
electronegativities, the bond is called a nonpolar covalent bond. However, in most cases, the pair of
electrons is shared by two atoms of different electronegativities. Here, the pair of electrons is shifted
toward the more electronegative element. A partial negative charge results on one side of the bond
and a partial positive charge on the other side of the bond. This type of covalent bond is called a polar
covalent bond. The key factor for determining the polarity of a molecule is its shape. Molecular
models are helpful for determining the actual three-dimensional shape of molecules. If the polar bonds
(dipoles) are symmetrical around the central atom, they offset each other and the resulting molecules
are nonpolar. However, if the dipoles are not symmetrical around the central atom, the electrons will
be pulled to one end of the molecule. The resulting molecule is polar.
“Ball and stick” models are often used to demonstrate molecular shape. In this exercise you
will build several covalent molecules and predict each molecule’s polarity on the basis of its molecular
shape.
Purpose:
1. To build models of some simple molecular compounds,
2. To determine a molecules three dimensional shape, and
3. To predict each molecule’s polarity on the basis of its symmetry and shape.
Procedure:
1. Using the ball and stick model set, construct models for the following molecules identified with an
asterisk (*).
*Molecular hydrogen
*Diethyl alcohol (CH3CH2OH)
Carbon monoxide
*Hydrogen chloride
Formaldehyde (H2CO)
Hydrogen peroxide
*Water
Acetylene (C2H2)
Sulfate ion
*Methyl amine (CH3NH2)
Hydrazine (N2H4)
Thiocyanide ion (SCN-)
*Methane
*Chloroform (CHCl3)
Hydrogen nitrite (HNO2)
*Hydrogen hypochlorite
Formic acid (HCOOH)
Phosphate ion
*Nitrogen trifluoride
2. For each molecule, determine the electron dot structure, the Lewis dot structure (dot and tick), the
geometry or shape of the molecule and the molecular polarity (polar or nonpolar) for each
molecule. Make a Data Table on you own paper like the one below to record your answers.
Data Table:
Formula
Structural Formulas, Shapes and Polarities of Molecular Compounds.
Geometry or
Molecular
Lewis Structure
Electron Dot Diagram
Shape of
Polarity
(dot and tick)
Molecule
(P/NP)
H2
HCl
Procedure: (continued)
After constructing the molecular models …..
Two different compounds having same molecular formula are called structural isomers. They
have identical molecular formulas but different structural formulas. The molecule and its isomer have
the same molecular formula, but they are different from each other in the way that a left hand if
different than the right hand. They also have different physical and chemical properties. Structural
isomers play a very important role in organic chemistry.
3. a. Construct a model of bromochlorofluoromethane, CHBrClF.
b. Sketch the structural formula for the compound in your table.
c. Construct an isomer of this compound and look at both isomers together to be sure that you
see that they are different molecules.
d. Prove to yourself and your partner that it is not possible to have structural isomers for a
trigonal planar molecule. Explain why!