Structures and Properties of
Substances
Molecular Shape and Polarity
Molecules and chemical bonds
A molecule is a chemical entity in which atoms are held together by
the electrostatic attractions of bonds.
In a Lewis structure each atoms in the molecule achieves a noble gas configuration
through the sharing of electrons. In some cases there are exceptions to the octet rule.
Bonding theory can be used to predict the shape of the molecules.
From the shapes of these molecules (and the electronegativity difference of their atoms)
it is possible to determine the polarity of the substances that are comprised of these
molecules.
Why is polarity important? It explains why, for example, water is a liquid at
room temperature, rather than a gas. In other words, it helps explain why
Earth is capable of supporting life!
Lewis Structures for a Polyatomic Molecule
The procedure to draw the Lewis structures for molecules and ions that have
a central atom, with other atoms around it, involves 5 main steps.
1.Locate the least electronegative atom in the centre of the molecule. Write the other atoms around
this central atom, with each atom bonded to the central atom by a single bond. Always place a hydrogen
atom or a fluorine atom (if there is any) at an end position in the structure.
3.Determine the total number of valence electrons in the molecule (pay close attention to the
charge). For example, if you are drawing a polyatomic anion such as CO32-, add two electrons to the
total number of valence electrons calculated for the structure CO3. For a polyatomic cation such as
NH4+, subtract one electron from the total number of valence electrons calculated for the structure
NH4.
4.Once you have the total number of valence electrons, determine the total number of electrons
needed for each atom to achieve a noble gas electron configuration.
5.Subtract the first total (number of valence electrons) from the second total (number of electrons
needed to satisfy the octet rule) and get the number of shared electrons. Then divide this number by
2 to get the number of bonds. Double or triple bonds may be needed to account for this number of
bonds.
6.Subtract the number of shared electrons (step 4) from the number of valence electrons (step 2) to get
the number of non-bonding electrons. Add these electrons as lone pairs to the atoms surrounding
the central atom so that you achieve a noble gas electron configuration for each atom.
Lewis Structures for a Polyatomic Molecule
How to draw Lewis Structures for simple molecules and ions with a central atom:
Lets try to draw the Lewis structure for
formaldehyde (methanal)
CH2O
Formaldehyde is an organic compound used as disinfectants
and for preservation of biological samples. It is colorless
and has a characteristic pungent, irritating odor
The molecular formula, CH2O, tells the number of each kind of atom in the molecule.
•1 atom of Carbon
•2 atoms of Hydrogen
•1 atom of Oxygen
Draw the Lewis structure for formaldeh
Lewis Structures for a Polyatomic Molecule
How to draw Lewis Structures for formaldehyde (CH2O)
Solution
Step 1
CH
O,
The
molecular
formula,
tells
yo
2
Position the least electronegative atom in the centre and write the other
ofitatom
inbond.
the molecule. Following ste
atoms bonded around
by a single
Always place H or Foutlined
atoms at an end
of the structure.
above:
Step 1 Since H is always placed at the e
Since H is always placed at the end position and C is less electronegative than O, C should
be the central atom.
Place the other atoms around C, attached by single bonds.
electronegative than O, C should
the other atoms around C, attach
O
H
C
H
Step 2 Determine the total number of v
Lewis Structures for a Polyatomic Molecule
How to draw Lewis Structures for formaldehyde (CH2O)
Step 2
Determine the total number of valence electrons (pay attention to the
charge!).
Determine the total number of valence electrons:
1Catom × 4e-/Catom = 4 e1Oatom × 6e-/Oatom = 6 e2 Hatoms × 1e-/Hatom = 2 eTotal number of valence e-=4 + 6 +2 = 12 e-
Lewis Structures for a Polyatomic Molecule
How to draw Lewis Structures for formaldehyde (CH2O)
Step 3
determine the total number of electrons needed for each atom to achieve
a noble gas electron configuration.
This would be 8 e- for C, 8 e- for O (to fill their respective valence energy levels) and 2 e-for
each H (to fill its 1s orbital).
So the total number for CH2O is 20 e2atoms × 8 e-/atom + 2atoms × 2 e-/atom
Lewis Structures for a Polyatomic Molecule
How to draw Lewis Structures for formaldehyde (CH2O)
Step 4
Subtract the number of valence electrons from the number of electrons needed to
satisfy the octet rule and get the number of shared electrons.
Then divide this number by 2 to get the number of bonds.
Double or triple bonds may be needed.
To find the number of shared electrons, subtract the first total from the second:
20e- − 12e- = 8 shared eNow divide the number of shared electrons by two to obtain the number of bonds.
20 e- − 12e- =8e- ÷ 2 = 4 covalent bonds
Since there are only three atoms surrounding the central atom, one of the bonds is likely a
double bond.
Lewis Structures for a Polyatomic Molecule
How to draw Lewis Structures for formaldehyde (CH2O)
Step 5
Subtract the number of shared electrons from the number of valence electrons to
get the number of non-bonding electrons and add these electrons as lone pairs
Step
3 Determine
thegasnumber
of non-bon
to the surrounding atoms
so that
you achieve a noble
electron configuration
for each atom.
the number of shared electrons fro
electrons:
Determine the number of non-bonding electrons by subtracting the number of shared
electrons from the total number of 12
valence
electrons: electrons − 8 shared el
valence
12 valence e- − 8 shared electrons
= 4 non-bonding
valence
electrons (2 lone pairs)
=4
non-bonding
valence electrons
Since H can only form a single bond,Since
a possibleH
structure
can is:only
• •
•
•
H
O
C
H
form a single bo
Solution
The formula, NH4+ , tells you the
Lewis Structures for a Polyatomic Molecule
Following steps 1 to 3 from the
Sample Problem
Step 1 Since H is always placed
Draw the Lewis structure for the ammonium ion, NH4+
H
•Step 1:
Since H is always placed at the end position, N is the central atom.
H
N
H
Continued ...
H
Number of shared
electrons,
andofthe
resulte
Step 2 Total
number
valence
•Step 2
- = 8eTotal number of valence e-: (1 Natom × 5 e-/Natom) +16
(4 H
−
e−atom
− ×8 1e−e-/H
= atom
8 e)−- 1e
(1
N
atom
×
5
e
/N atom)
-/atom)+(4
-/atom)= 16eTotal number of e- required for noble gas configuration:
(1
×8
e
×2
e
atom
atoms
−
8
e
÷ 2 = 4 covalent
= 8bonds
e−
Number of shared e : 16 e - 8 e = 8e
Number of bonds: 8 ÷ 2 = 4 covalent bonds
Step 3 Number of non-bonding
electrons:
Total number
of electrons
•Step 3
/atom) + (4
× 8 e−electrons
8 valence electrons(1−atom
8 shared
= 16 e− (0 lone pairs)
= 0 non-bonding electrons
Number of non-bonding e-: 8 valence e- - 8 shared e- = 0 non-bonding e-.
A possible structure for NH4+ is:
A possible structure for ammonium ion is
+
H
H
N
H
H
Co-ordinate Covalent Bonds
In some cases, such as the Hydronium ion, H3O+, one atom contributes both of the
electrons to the shared pair. The bond in these cases is called a co-ordinate covalent
bond.
In terms of the quantum mechanical model, a co-ordinate covalent bond forms when a filled
atomic orbital overlaps with an empty atomic orbital.
Once a co-ordinate bond is formed, it behaves in the same way as any other single covalent
bond.
H
O
H
+
↑↓
↓
+
H
↑↓ ↑ ↑
H
↓
O
..
H
Hydronium ion
H
ve
properties
that
are
somewhere
between
a
single
bond
nce
Structures:
More
One
Possible
Resonance:
More Than
OneThan
Possible
Lewis
StructureLewis St
ond. In effect, the SO2 molecule contains two “one-and-a
In some
cases,
moreasked
than one Lewis
structurethe
is suitable
for thestructure
same molecule. for sulfur
that
you
are
to draw
Lewis
ypical
answer
like SO
this:
E.g. To draw
the Lewis would
structure forlook
sulfur dioxide,
2, a typical answer would look like:
mmunicate the bonding in SO2 more accurately, chemis
Lewis structures and insert
arrow betw
O
Sa double-headed
O
ch of these
Lewis
structures
a resonance
structu
Experimental
measurements
indicateis
thatcalled
the bonds between
the S and
each O are identical.
The two bonds
have
properties
that are somewhere
SO
Lewis
structure
suggests
that
contains
single bond
2
ce
structures
are
models
that
give
the
same arelative
posi
between a single bond and a double bond.
bond.
However,
experimental
measurements
offor
bond
len
in
Lewis
structures,
but
show
different
places
their
To represent the bonding in SO2 more accurately, chemists draw two opposite Lewis b
structures
insert a double-headed
that
the and
bonds
between arrow
the between
S andthem:
each O are identical. T
pairs.
ve properties that are somewhere between a single bond
O the
S SO
O2 molecule contains
O StwoO“one-and-a
ond. In effect,
• •
•
•
• •
• •
•
•
• •
• •
•
•
• •
• •
• •
•
•
• •
• •
• •
•
•
•
•
• •
Each of these Lewis structures is called a resonance structure.
Resonance structures are models that give the same relative position of
SOdifferent
ommunicate
bonding
inshow
accurately,
chemis
atoms asthe
in Lewis
structures, but
places
for their
2 more
bonding and lone pairs
o Lewis structures and insert a double-headed arrow betw
ch of these Lewis structures is called a resonance structu
Lewis Structures for a Polyatomic Molecule
Practice Problem
• Draw Lewis structures for each of the following molecules.
a)NH
b)CF
c)BrO
d)H2
e)CH
f) AsH
g)H2
e)ClNO
• Draw Lewis structures for each of the following ions. (Note: Consider resonance structures.)
a)CO
b)NO
c)ClO
d)SO
st explained by a model that shows more than eight electrons in
Central
an Expanded
Valence
Level
nce
energyAtoms
level ofwith
the central
atom. This
central
atom is said to
xpanded
level.
Onebonds
example
a molecule
The octetvalence
rule allowsenergy
a maximum
of four
around of
a central
atom. with a
Anyway
in someenergy
molecules
theiscentral
atoms might
have more thanPCl
eight
nded
valence
level
phosphorus
pentachloride,
5. T
electrons in the valence energy level.
ance
is a pungent, pale-yellow powder that is used in the agricu
This central atom is said to have an expanded valence energy level.
maceutical, and dyeing industries. Organic chemists also use rea
One example
is phosphorus penta-chloride (PCl5), a pungent, pale-yellow powder that is used
PCl
5 to identify compounds that contain hydroxyl groups.
in the agricultural and pharmaceutical fields:
• •
•
•
• •
•
•
Cl
Cl
•
•
• •
•
•
Cl
•
•
•
•
Cl
•
•
P
• •
•
•
Cl
•
•
• •
• •
larger atoms can accommodate additional
valence electrons because of their size.
Chapter 4 Structures
Other molecules with an expanded valence energy level are SF6 and BrF5