Notes and Dichotomous Key for Determining “Molecular Polarity”
Name: ____________________________________________
Notes: You already know how to determine the “bond polarity” for a covalent bond; and, you also know that molecules that have more than one
covalent bond may have bonds with different polarities. For example in the molecule “HCN” there are 2 bonds (H—C , and C—N). H—C is a nonpolar
covalent bond and C—N is a polar covalent bond. This means that “H” and “C” share valence electrons fairly equally; but, “C” and “N” do not share
valence electrons equally. When a bond is “polar covalent”, there is a “separation of charge” giving a “partial positive” area and a “partial negative”
area at either sides of the bond. Often we speak of a corresponding “partial positive” atom and a “partial negative” atom.
“Molecular polarity” refers NOT to just one bond, instead, this term refers to “the polarity of the whole molecule”. If the whole molecule is nonpolar,
it means that there is no overall separation of charge. AND, when a nonpolar molecule is put into an electric field, the molecules will not respond at
all. If the whole molecule is polar it means that there is an overall separation of charge (there is ONE area of partial positive….. and ONE area of
partial negative) within the molecule. And, when a polar molecule is put into an electric field, the molecules WILL respond by lining up with their
partial positive area attracted to (pointing toward) the negative of the electric field, and the partial negative area attracted to (pointing toward) the
positive of the electric field.
Please at this time read (textbook) the section of “polar molecules” (pp 239 – 240). Notice the CO2 example! Notice that there are times when polar
bonds cancel…. Depending on both shape and orientation of the polar bonds. Since shape is largely determined by the presence of unshared pairs on
the central atom, the following dichotomous key focuses on the presence or absence of unshared pairs on the central atom.
Dichotomous Key – a tool in which you respond with either “yes” or “no” to a question (and then follow either the “yes arrow” or the “no arrow”),
and continuing to answer questions until the arrow that you follow arrives at a conclusion “molecular polarity = ________”
Start with the molecule H2. Always start on the left; move toward the right. Stop when you hit an answer. Write down the answer on your answer
sheet. For H2 it goes like this: Is every bond in the molecule nonpolar? YES….molecular polarity = nonpolar. Done. Start over and do next molecule.
Molecular polarity = nonpolar
Is every bond in the
molecule nonpolar?
Molecular polarity = polar
Is there exactly 1 polar bond
in the structure?
Molecular polarity = polar
Are there unshared pair(s)
on the central atom?
Molecular polarity = nonpolar
SIDE 2
Notes and Dichotomous Key for Determining “Intermolecular Force of Attraction”
Name: ____________________________________________
Notes: You already know how to determine the “molecular polarity”. The last thing to do with a molecule is to talk about what “force of attraction”
makes one molecule stick to another molecule. Molecular substances that are found in the solid state or in the liquid state contain molecules that
stick together; but, molecular substances that are found in the gaseous state contain molecules that don’t stick together. There are NO forces of
attraction present in gaseous substances. A “dispersion force” is the weakest type of force of attraction that exists, and it is what keeps nonpolar
molecules stuck together. A nonpolar liquid that you know of is gasoline. A stronger type of force of attraction is called a “dipole force”, and it is
what keeps some polar molecules stuck together. The strongest type of force of attraction is called a “hydrogen bond”, and it is what keeps polar
molecules that meet specific criteria stuck together. An example of a macromolecule that you know of that stays stuck together through hydrogen
bonding is DNA….Each half of the double helix stays hooked to the other half by hydrogen bonds.
Please at this time read (textbook) the section of “attraction between molecules” (pp 240-241).
Dichotomous Key – a tool in which you respond with either “yes” or “no” to a question (and then follow either the “yes arrow” or the “no arrow”),
and continuing to answer questions until the arrow that you follow arrives at a conclusion “intermolecular attraction = ________”
Start with the molecule H2. Always start on the left; move toward the right. Stop when you hit an answer. Write down the answer on your answer
sheet. For H2 it goes like this: Is the molecule nonpolar? YES….intermolecular attraction = dispersion. Done. Start over and do next molecule.
Intermolecular attraction = dispersion force
Is the molecule
nonpolar?
intermolecular attraction =
hydrogen bond
Does the Hydrogen atom bond to
either a fluorine, oxygen, or nitrogen atom?
Does the polar molecule
contain a Hydrogen atom?
intermolecular attraction =
dipole force
Intermolecular attraction =
dipole force