Most of our studies have focused around the composition and physical properties
of elements and compounds, and the reactions that elements and compounds undergo.
It turns out that a number of the properties of compounds are determined by
the very shape of the molecules comprising the compound. Properties such as
physical state (solid, liquid, and gas), as well as freezing point, boiling
point, viscosity, and many others, are actually related to the shape of molecules.
Indeed, the shape of molecules also determines the shape of the world around
us, in that all macroscopic objects are comprised of molecules of particular
shapes and sizes. Nature is amazing.
In this laboratory you will predict the shape of molecules using a physical
model for molecular structure called the “ball and stick” model.
The ball and stick model represents atoms as hard spheres with holes drilled
in them, and represents bonds by sticks that connect the wooden “atom”
spheres together. We must recognize that all models have limitations. Clearly,
atoms are not hard spheres, and bonds are not sticks, however, the ball and
stick model can still be used very effectively to show approximate bond angles
and bond lengths. Since bond angle and bond length are at the very heart of
molecular structure, we can gain great insight into the shapes of molecules
from this primitive model. In addition, this simple model can predict if a molecule
is polar or non-polar by the evaluation of the electronegativity of the elements
in each of the molecular bonds, coupled with the geometry of the molecule.
The geometry of small molecules can be predicted by examining the number of
electron pairs that surround the central atom using the Valence Shell Electron
Pair Repulsion (VSEPR) rule. Simply stated, this rule tells us that since the
valence shell electron pairs surrounding a central atom repel one another, they
will position themselves as far apart from each other as possible. This rule
yields the shape of the central atom, from which the molecular geometry is based.
The molecular geometry is determined by identifying the number of bonds that
are formed by the central atom, along with and the number of unshared electron
pairs. Unshared electron pairs (called lone pairs) have greater repulsion than
do bonding pairs, thus the lone pairs will reside in the positions around the
central atom that allow for the greatest space. The molecular geometry, however,
is determined only by the atoms that are bonded to the central atom, thus the
shape of the bonding electron pairs dictates the molecular shape.
Once the shape of the molecule has been determined, the hybridization of the
central atom or atoms can be predicted. Also, the covalent bonds of the molecule
can be examined individually to see which, if any, are polar covalent bonds.
If the molecule contains polar covalent bonds, then whether the molecule is
polar or non-polar can be determined from the location of the polar bonds within
the geometry of the molecule.
Provide written responses to the following items before starting the activity.
You may type and print your responses, or write them by hand on a separate piece
of notebook paper. Your responses are due at the start of your laboratory period
when this activity is scheduled.
What is meant by molecular shape?
Name and describe three characteristics of molecules (and the elements comprising
the molecules) that are responsible for the physical dimensions and shape
of the molecule that forms.
Name and describe at least three other models that are used to depict or
describe molecules. Explain how each differs from each other.
For each molecule listed by formula, draw a Lewis dot structure, determine
the geometry of the central atom(s) according to VSEPR, determine the hybridization
of the central atoms, and the overall shape of the molecule. List any polar
bonds, determine if the overall molecule is polar, then draw a reasonable 3-D
depiction of the model you have created. Record the information listed in the
results section for each molecular formula listed either in your laboratory
notebook, or by completing the appropriate row in the Data Table according to
the instructions of your teacher.
Examine each completed structure carefully. Draw a reasonable 3-D representation
of each structure in the space allocated on the Data Table, or in your laboratory
notebook according to the instructions of your teacher.