Chem 110 - Class Guide: Polarity, Lewis Structures and Covalent Bond Strength (Chapter 8, Sections 4 – 5
and 8)
Chapter 8, Sections 4 -5 & 8 Learning Goals
Upon completion of these sections, you should be able to:
Calculate formal charges for all the elements in a Lewis Structure.
Using formal charges, determine the best Lewis Structure out of a choice of several.
Determine the electronegativity of different elements in a covalent bond, and whether the bond
is polar or nonpolar.
Using Lewis structures, determine the relative bond strength and bond length for covalent compounds.
Chapter Reading Guide: Chapter 8, Sections 4 – 5 & 8
Section 4: BOND POLARITY AND ELECTRONEGATIVITY
The concept of electronegativity is an important one because it leads to polarity. For
example, the reason that oil does not mix with water is because of the polarity differences
of oil and water.
Electronegativity is the ability of an atom in a bond to attract electrons to itself.
ELECTRONEGATIVITY
Watch this movie on electro-negativity:
Periodic Trend ElectroNegativity, then study the trends in the figure below (note that
the trends are not smooth, unlike trends on size, electron affinity and ionization
energy):
Based on electronegativities, we can calculate polarity of bonds.
TIP! In any bond, the atom that is more electronegative will have the electrons spend
more time around it, thus making that element slightly more negative and leaving the
other element slightly more positive.
If there is a difference in electronegativities between atoms in a bond, we say the bond
is polar. We could also say there is a dipole moment (which you do NOT have to
calculate), or that the dipole points towards the most electronegative atom. We can
also show this in pictures!
Try Practice Exercise 8.4.
Section 5: DRAWING LEWIS STRUCTURES
A critical concept in this section is formal charge.
Formal charge is a bookkeeping approach to allow chemists to determine reasonable
Lewis Structures.
The rules for formal charge are listed on the next slide. Learn these rules while you
practice using them.
Formal Charge of an atom = number of valence electrons – number of bonds – non-bonded electrons
To calculate the formal charge on any atom in a Lewis Structure, we assign the electrons to
the atom as follows:
1. All unshared (nonbonding) electrons are assigned to the atom on which they are found.
2. For any bond – single, double, or triple – half of the bonding electrons are assigned to
each atom in the bond.
The formal charge of each atom is then calculated by subtracting the number of electrons
assigned to the atom from the number of valence electrons in the isolated atom.
The diagram below is the solution to Part A of Sample Exercise 8.9.
When using formal charges to decide which structure is better, we have to follow the rules
below. We’ll use Part B of Sample Exercise 8.9 as the example.
We want the formal charges to be as low as possible, while still totaling the charge on the
compound. Thus, the first structure is out, since the second and third structures have lower
formal charges.
To decide between the second and third structures, we go to electronegativity. Since N is
more electronegative than S, it should carry the negative formal charge. The more
electronegative element should carry the negative formal charge if possible. So the second
structure is the best one!
Try Practice Exercise 8.9.
Section 8: STRENGTH OF COVALENT BONDS
You can skip almost all of this section. There are two take-home messages here:
The higher the bond order, the stronger the bond
C≡C > C=C > C-C
The higher the bond order, the shorter the bond
C≡C < C=C < C-C
Learning Resources
Chapter Learning Goals
Chapter 8, Section 4 – 5 & 8 Learning Goals
Additional Chapter Resources
Periodic Trend ElectroNegativity
(http://www.berks.psu.edu/clt/PeriodicTrendElectrngtvty.mov)
Pre Class Assignment: This assignment must be completed prior to the next class. Check your syllabus
for the exact due date and time.
Complete the pre class assignment
(http://berks.psu.edu/clt/chem110/PolarityLewisStructures_HW.doc)
Submit a copy to the dropbox located in ANGEL called “Pre Class Assignment Submission:
Polarity and Lewis Structures”
End of Chapter Problems:
Practice with these problems if you are having difficulty with any of the concepts covered in this
class guide AFTER we have met in class. If you cannot easily complete these problems, seek
help from your instructor, your mentor, or the learning center.
Chapter 8: 39, 41, 85, 90 (a, c, e only)