Answers Questions 16.2
Molecular polarity
1.
Write a dot diagram for the HCl molecule.
Find the difference in electronegativity between the hydrogen and chlorine atoms
Difference in electronegativity = 1.06
Is the bond nonpolar covalent, polar covalent, or ionic?
Polar covalent
What is the percentage ionic character in the bond?
22% ionic character
Which atom is slightly negative and which is slightly positive? Why?
Chlorine is slightly negative because it has a higher attraction for the bond electrons than the hydrogen.
About 22% of the time the electron bond pair is around the chlorine atom only. The element with the
higher electronegativity will always be slightly negative.
Hydrogen is slightly positive because the bond electrons are completely absent part of the time.
2.
Why does increasing the number of pi bonds add to charge density?
Sigma bonds are formed from hybridized orbitals and will have some s orbital and some p orbital
character. Single bonds are always sigma bonds.
Pi bonds are formed from an overlap of unhybridized p orbitals. The pi bonds will overlap the sigma
bonds, creating a volume with a larger charge density because you have more electron bond pairs in the
volume between the atoms bonded together.
Single bond Sigma bond only
Double Bond 1 Sigma bond and 1 Pi bond
The sigma bonds are arranged at 120 degrees in a plane. The pi bond has two parts, one above the plane
and one below the plane. The pi bond overlaps the sigma to form the double bond.
Triple Bond 1 Sigma bond and 2 Pi bonds
The triple bond is formed from one sigma bond and two pi bonds. The sigma bond forms a linear
molecule (180 degrees) while the two pi bonds overlap the sigma to form a "pig in a blanket" tunnel of pi
electrons.
3.
Build models and draw sketches of CH4, CH3Cl, CH2Cl2, CHCl3, and CCl4
All models have 4 single bonds, sp3 hybridized, with a bond angle of 109.5 degrees. No pi bonds are
present.
Is the carbon-hydrogen bond polar?
The C-H bond in nonpolar (difference in electronegativity of 0.45)
Is the carbon-chlorine bond polar?
The C-Cl bond is polar (difference in electronegativity of 0.61)
Which atom is negative and which is positive?
Chlorine is slightly negative since it has the higher electronegativity
Which of the CH3Cl, CH2Cl2, CHCl3, and CCl4 molecules has the biggest distance between the center of
positive and negative charges?
CH3Cl has the largest distance between the center of positive and negative charges.
The dipole moment is a vector from the center of positive charge to the center of negative charge of the
molecule. The larger the dipole moment the more polar the molecule.
Why is the center of negative and positive charges the same in CCl4?
Because the slightly negative chlorines are arranged symmetrically the centers of positive and negative
charges are both in the center of the molecule.
4.
Draw a sketch of BBr3
Is the boron-bromine bond polar or nonpolar?
Difference of electronegativity = 0.92
Polar covalent bond with bromine negative and boron positive.
Locate the center of positive and negative charge in the BBr 3 molecule. Why is the molecule nonpolar?
The slightly negative charges are distributed symmetrically on the molecule
In general molecular polarity can be summed up as follows:
Negative areas of a molecule are caused by more the electronegative atom of polar bonds, with the strength of
the negative charge increasing with higher percentage ionic character. Lone pairs and pi bonds produce weaker
areas of negative charge.
If a molecule has no polar bonds, lone pairs, or pi bonds it cannot be polar.
If a molecule has negative charges distributes symmetrically it is not polar.
If polar bonds are on the opposite end of a molecule from nonpolar bonds the molecules will be polar because
the centers of positive and negative charge will be far apart.
Finding the shapes of molecules from the hybridization
5.
For each molecule below sketch the molecule, tell if each bond is polar or nonpolar, put S- by each
negative part of the molecule and S+ by each positive part of the molecule, and tell if the molecule is polar or
nonpolar.
Na2S
Sulfur has 2 bond pair and 2 lone pair (sp3 hybridized) (distorted tetrahedral)
Difference in electronegativity is 1.65. Polar covalent bond with sulfur negative and sodium positive
AlF3
Aluminum has 3 bond pair no lone pair (sp2 hybridized) (Trigonal planar)
Difference in electronegativity is 2.37. Ionic Bond
SiI4
Silicon has 4 bond pair, no lone pair (sp3 hybridized) (Tetrahedal)
Difference in electronegativity is 0.76 Polar covalent bond
Iodine is negative, Silicon is positive
NH3
Nitrogen has 3 bond pair, 1 lone pair. (sp3 hybridized) (distorted tetrahedral)
Difference in electronegativity = 0.94 Polar Covalent bond
Nitrogen is slightly negative, Hydrogen is slightly positive
H2O
Oxygen has 2 bond pairs and 2 lone pairs (sp3 hybridization) (distorted tetrahedral)
Difference in electronegativity = 1.34 Polar covalent bond
Oxygen is slightly negative, Hydrogen is slightly positive
PCl3
Phosphorus has 3 bond pair and 1 lone pair (sp3 hybridized) (distorted tetrahedral)
Difference in electronegativity is 0.97 Polar covalent bond
Chlorine is slightly negative, phosphorus is slightly positive
AsBr5
Arsenic must have 5 bond pairs and no lone pairs. (sp3d hybridized)
(trigonal bipyramidal)
Difference in electronegativity = 0.78 Polar covalent bond
Bromine is slightly negative, Arsenic is slightly positive
SCl6
Sulfur has 6 bond pairs, no lone pairs (sp3d2 hybridized) (octahedral)
Difference in electronegativity = 0.58 Polar covalent bond
Chlorine slightly negative, sulfur slightly positive