Covalent Bonds and
Structures
Chemistry 11
Ms. McGrath
Covalent Bonds
Because covalent bonds range from
nearly ionic to non-polar covalent, the
shapes of molecular compounds have a
great variety.
We already know that ionic compounds
have a crystal like shape created by the
orderliness of the ions.
Covalent Bonds – Lewis Structures
We will be analyzing and predicting the
structures and properties of molecules. Lewis
structures are going to be very important in
determining these.
Covalent Bonds – Lewis Structures
Draw the Lewis Structure for methanol,
CH2O
Covalent Bonds – Lewis Structures
Step 1
Determine the total number of valence
electrons in all of the atoms in the
molecule
Covalent Bonds – Lewis Structures
Step 1
(1 C atom x 4e-) + (1 O atom x 6e-) +
(2 H atoms x 1e-)
= 4e- + 1e- + 2e= 7e-
Covalent Bonds – Lewis Structures
Step 2
Draw a skeleton structure for the
molecule.
The atom with the largest number of
unpaired electrons will be the central
atom.
Join the atoms with one pair of bonding
electrons.
Covalent Bonds – Lewis Structures
Step 2
O
..
H:C:H
Covalent Bonds – Lewis Structures
Step 3
Place lone pairs of electrons around all
atoms, except the central atom, to form
an octet of electrons.
Hydrogen, of course, has only two
valence electrons.
Covalent Bonds – Lewis Structures
Step 3
..
:O:
..
H:C:H
Covalent Bonds – Lewis Structures
Step 4
(a) If all the valence electrons determined in
Step 1 have not been used, add one or more
lone pairs around the central atom to
complete an octet of electrons
(b) If all of the valence electrons have been
used up but the central atom does not have
an octet, move one or more of the lone pairs
to form double or triple bonds between the
central atom and an adjacent atom
Covalent Bonds – Lewis Structures
Step 4
..
:O:
..
H:C:H
There are 12 electrons in the structure, which is the
same as the total number in Step 1. Carbon, however,
does not have an octet. Therefore, move one of the
lone pairs around the oxygen atom to the position
between the oxygen and carbon atoms to form a
double bond.
Step 4
:O:
..
..
H:C:H
Coordinate Covalent Bonds
A coordinate covalent bond is another
type of covalent bond where a pair of
electrons is shared between two atoms
but both electrons were originally part
of one of the two atoms (i.e. both
electrons of the shared pair come from
one of the bonded atoms).
Coordinate Covalent Bonds
Consider the formation of boron
trifluoride (BF3):
Coordinate Covalent Bonds
One of the boron’s orbitals is empty and
the BF3 is attracted to molecules with
lone pairs of electrons that can be
donated.
Coordinate Covalent Bonds
Consider the reaction of BF3 with NH3
to form NH3BF3:
Coordinate Covalent Bonds
The bond between N and B is a
coordinate covalent bond – it can’t be
experimentally distinguished from other
shared pair covalent bonds.
Resonance Structures
 More than one possible Lewis
Structure
 Draw the Lewis Structure for sulfur
dioxide, SO2
Resonance Structures
The Lewis Structure suggests that SO2
contains a single and a double bond. In
fact, experimental measurements
determine that the bond lengths in SO2
are identical. SO2 contains two “one
and a half” bonds.
Resonance Structures
These are models that give the same
relative position of atoms as in Lewis
Structures, but show different placing of
their bonding pairs and lone pairs.
It is important to know that resonance
structures do not exist in reality.
Resonance Structures
It is important to know that resonance
structures do not exist in reality. SO2
does not shift back and forth from one
structure to another.
An actual SO2 molecule is a combination
of its two resonance structure. You can
imagine one pair of electrons as
resonating across the entire molecule
from one oxygen atom to the other.