Standard 3.2
Covalent Compounds
Everybody on the left side of the room will be an
atom of sulfur. Everybody on the right side of the
room will be fluorine. Please hold your element’s
valence electrons in your hand.
Everybody on the left side of the room will be an
atom of sulfur. Everybody on the right side of the
room will be fluorine. Please hold your element’s
valence electrons in your hand.
• Walk around the room and exchange
electrons with your classmates in such a
way that both you and your classmates end
up being stable. Stay with your trading
partner(s) when everyone in your trading
group has become stable.
Formation of covalent
compounds
• Covalent compounds form when nonmetals
share electrons.
Formation of covalent
compounds
• Covalent compounds form when nonmetals
share electrons.
• Because all nonmetals tend to gain valence
electrons, the valence electrons must be
shared to complete the octets.
Formation of covalent
compounds
• Covalent compounds form when nonmetals
share electrons.
• Because all nonmetals tend to gain valence
electrons, the valence electrons must be
shared to complete the octets.
• When electrons are shared between atoms,
the bonds are relatively weak.
Weak bonds mean…
• Low melting and boiling points.
Weak bonds mean…
• Low melting and boiling points.
• Generally gases at room temperature.
Weak bonds mean…
• Low melting and boiling points.
• Generally gases at room temperature.
• Some are liquids or solids.
Weak bonds mean…
•
•
•
•
Low melting and boiling points.
Generally gases at room temperature.
Some are liquids or solids.
The solids have soft crystals.
Weak bonds mean…
•
•
•
•
•
Low melting and boiling points.
Generally gases at room temperature.
Some are liquids or solids.
The solids have soft crystals.
Strong odor
Weak bonds mean…
•
•
•
•
•
•
Low melting and boiling points.
Generally gases at room temperature.
Some are liquids or solids.
The solids have soft crystals.
Strong odor
Flammable
Weak bonds mean…
•
•
•
•
•
•
•
Low melting and boiling points.
Generally gases at room temperature.
Some are liquids or solids.
The solids have soft crystals.
Strong odor
Flammable
Non-electrolytes
Polar vs nonpolar covalent
molecules
• The smallest unit of a covalent compound is
called a molecule.
Polar vs nonpolar covalent
molecules
• The smallest unit of a covalent compound is
called a molecule.
• Molecules can be polar (a negative and
positive region) or nonpolar.
Polar vs nonpolar covalent
molecules
• The smallest unit of a covalent compound is
called a molecule.
• Molecules can be polar (a negative and
positive region) or nonpolar.
• The more polar a molecule, the stronger its
bonds are.
Polar vs nonpolar covalent
molecules
• The smallest unit of a covalent compound is
called a molecule.
• Molecules can be polar (a negative and
positive region) or nonpolar.
• The more polar a molecule, the stronger its
bonds are.
• Compounds with similar polarities will
dissolve one another (“like dissolves like”)
Determining Polarity
• To determine whether a compound is polar
or nonopolar, we need to first look at the
difference in electronegativity between the
atoms being bonded (table on p.265).
Determining Polarity
• To determine whether a compound is polar
or nonopolar, we need to first look at the
difference in electronegativity between the
atoms being bonded (table on p.265).
• If the difference is less than 0.40, the
molecule is nonpolar (CH4 as an example).
Determining Polarity
• To determine whether a compound is polar
or nonopolar, we need to first look at the
difference in electronegativity between the
atoms being bonded (table on p.265).
• If the difference is less than 0.40, the
molecule is nonpolar (CH4 as an example).
• If the difference is greater than 0.40, a
Lewis structure is needed (CO2).
Drawing a Lewis structure
• Begin by determining how many bonds the
molecule contains.
•
•
Number of bonds
• Add the total number of valence electrons
needed by the atoms in the molecule.
Number of bonds
• Add the total number of valence electrons
needed by the atoms in the molecule.
Example: CO2
Number of bonds
• Add the total number of valence electrons
needed by the atoms in the molecule.
Example: CO2
carbon needs 4 electrons
Number of bonds
• Add the total number of valence electrons
needed by the atoms in the molecule.
Example: CO2
carbon needs 4 electrons
each oxygen atom needs 2 electrons
Number of bonds
• Add the total number of valence electrons
needed by the atoms in the molecule.
Example: CO2
carbon needs 4 electrons
each oxygen atom needs 2 electrons
4 + 2 + 2 = 8 valence electrons needed
Number of bonds
• Add the total number of valence electrons
needed by the atoms in the molecule.
Example: CO2
carbon needs 4 electrons
each oxygen atom needs 2
4 + 2 + 2 = 8 valence electrons needed
• Divide the total by 2 (8 / 2 = 4 bonds)
Drawing a Lewis structure
• Begin by determining how many bonds the
molecule contains.
• Connect the atoms with lines representing
the bonds. Make it symmetrical if possible.
•
In this case, placing carbon in the
middle with double bonds between
the carbon and each oxygen makes it
symmetrical.
O=C=O
Drawing a Lewis structure
• Begin by determining how many bonds the
molecule contains.
• Connect the atoms with lines representing the
bonds. Make it symmetrical if possible.
• Finally, add any unshared valence electrons
as pairs of dots.
Oxygen: 6 valence electrons, two of
which are shared, 4 unshared electrons.
Oxygen: 6 valence electrons, two of
which are shared, 4 unshared electrons.
Carbon: 4 valence electrons, all of them
are shared, 0 unshared electrons.
Analyzing the structure
• At least one of two things in a Lewis
structure will make it polar…
Analyzing the structure
• At least one of two things in a Lewis
structure will make it polar…
1) the molecule is not symmetrical
Analyzing the structure
• At least one of two things in a Lewis
structure will make it polar…
1) the molecule is not symmetrical
2) unshared electrons on the central atom
Analyzing the structure
• At least one of two things in a Lewis
structure will make it polar…
1) the molecule is not symmetrical
2) unshared electrons on the central atom
• If neither of the above are true, the molecule
is nonpolar.
CO2 is nonpolar, because the
molecule is symmetrical and
there are no unshared electrons
on the central atom.
Is H2O polar or nonpolar?
Is H2O polar or nonpolar?
• First, we must check the difference in
electronegativity between H and O.
Is H2O polar or nonpolar?
• First, we must check the difference in
electronegativity between H and O.
3.44 – 2.20 = 1.24 (a Lewis structure will be
needed)
Is H2O polar or nonpolar?
• First, we must check the difference in
electronegativity between H and O.
3.44 – 2.20 = 1.24 (a Lewis structure will be
needed)
• Calculate the number of bonds.
Is H2O polar or nonpolar?
• First, we must check the difference in
electronegativity between H and O.
3.44 – 2.20 = 1.24 (a Lewis structure will be
needed)
• Calculate the number of bonds.
each H needs 1 electron, O needs 2 electrons
4 / 2 = 2 bonds
Is H2O polar or nonpolar?
• Try to draw a symmetrical molecule
containing 2 bonds.
H–O–H
Is H2O polar or nonpolar?
• Try to draw a symmetrical molecule containing
2 bonds.
• Add dots for any unshared valence electrons.
hydrogen, 1 valence electron, 1 shared electron
oxygen, 6 valence electrons, 2 shared electrons
Is H2O polar or nonpolar?
Even though the molecule is symmetrical, there
are unshared electrons on the central atom (O)
making water a polar molecule.
Is PF3 polar or nonpolar?
Is PF3 polar or nonpolar?
• Electronegativity difference of 1.79, a
Lewis structure is needed.
Is PF3 polar or nonpolar?
• Electronegativity difference of 1.79, a
Lewis structure is needed.
• 3 bonds
Is PF3 polar or nonpolar?
• Electronegativity difference of 1.79, a
Lewis structure is needed.
• 3 bonds
• 2 unshared electrons on P, 6 unshared
electrons on each F.
• Molecule is polar.
Is AsH3 polar or nonpolar?
Is AsH3 polar or nonpolar?
• Electronegativity difference of 0.02,
molecule is nonpolar.
Is SO2 polar or nonpolar?
Is SO2 polar or nonpolar?
• Electronegativity difference is 0.86, a Lewis
structure is needed.
Is SO2 polar or nonpolar?
• Electronegativity difference is 0.86, a Lewis
structure is needed.
• 3 bonds
O–S=O
Is SO2 polar or nonpolar?
• Electronegativity difference is 0.86, a Lewis
structure is needed.
• 3 bonds
O–S=O
Is SO2 polar or nonpolar?
• Electronegativity difference is 0.86, a Lewis
structure is needed.
• 3 bonds
• add the unshared electrons
O–S=O
When two electrons are left unpaired, one of
them must be moved.
Is SO2 polar or nonpolar?
• Electronegativity difference is 0.86, a Lewis
structure is needed.
• 3 bonds
• add the unshared electrons
O–S=O
Always move the electron to the more
electronegative element.
Is PCl5 polar or nonpolar?
Is PCl5 polar or nonpolar?
• Electronegativity difference tells us a Lewis
structure will be needed.
Is PCl5 polar or nonpolar?
• Electronegativity difference tells us a Lewis
structure will be needed.
• With a total of 8 electrons needed, we
calculate 4 bonds are present, but 4 bonds
are not enough.
Is PCl5 polar or nonpolar?
• Electronegativity difference tells us a Lewis
structure will be needed.
• With a total of 8 electrons needed, we
calculate 4 bonds are present, but 4 bonds
are not enough.
• When this happens, you will only have
single bonds and you use the minimum
number of bonds possible (5 in this case).
Structures such as this require an
expanded octet, meaning the central
atom ends up with more than 8 valence
electrons.
Unshared electrons are distributed just
like in a regular Lewis structure.
Unshared electrons are distributed just
like in a regular Lewis structure.
This molecule is nonpolar.
Is BF3 polar or nonpolar?
Is BF3 polar or nonpolar?
• Electronegativity difference indicates that a
Lewis structure is needed.
Is BF3 polar or nonpolar?
• Electronegativity difference indicates that a
Lewis structure is needed.
• With 8 electrons needed, 4 bonds should be
needed.
Is BF3 polar or nonpolar?
• Electronegativity difference indicates that a
Lewis structure is needed.
• With 8 electrons needed, 4 bonds should be
needed.
Is BF3 polar or nonpolar?
• Boron should be involved in all 4 bonds, but
it only has 3 valence electrons to donate.
Only 3 bonds can exist in this molecule.
Is BF3 polar or nonpolar?
• Molecules in which boron is the central
atom will have an incomplete octet for the
boron.
Is BF3 polar or nonpolar?
• Molecules in which boron is the central
atom will have an incomplete octet for the
boron.
• This molecule is nonpolar.