Basics of General Chemistry
 The Covalent Molecule
 Electronegativity and covalent bonds
 Naming covalent compounds
 Polyatomic ions
 Writing Lewis structures
 Formal charges
Sharing is organic.
Living without sharing is
living without chemistry.
Chapter 4
1
The Covalent Bond
• Covalent bonds occur between atoms of similar
electronegativity (close to each other in the periodic table)
• Atoms achieve octets by sharing of valence electrons
• Molecules result from this covalent bonding
Which GROUPS of ELEMENTS (i.e. #A with #A) would you expect to
easily form COVALENT BONDS with each other?
The nature of sharing can be different, depending on the
difference between the ELECTRONEGATIVITY of the two elements.
Chapter 4
2
Electronegativity
 Electronegativity is the ability of an atom to attract electrons
 It increases from left to right and from bottom to top in the
periodic table (noble gases excluded)
Fluorine is the most electronegative atom and can stabilize
excess electron density the best
Chapter 4
3
Representing the Covalent Bond
 Valence electrons can be indicated by dots (electron-dot
formula or Lewis structures) but this is time-consuming
 The usual way to indicate the two electrons in a bond is
to use a line (one line = two electrons)
Chapter 4
4
Naming Covalent Compounds
Similar to ionic compounds except prefixes are more often used.
Name the following compounds:
N2O3
CCl4
Dinitrogen trioxide
H2O
Carbon tetrachloride
Dihydrogen oxide
Give the formula for the following compounds:
Dinitrogen pentoxide
N2O5
Iodine pentafluoride
IF5
Chapter 4
5
Polyatomic Ions
What are they???
A group of atoms with a charge.
Table 4.15 (know those names)
3 special ions:
NH4+
ammonium
5 -ate: NO3nitrate
4 -ite:
OH-
CN-
hydroxide
cyanide
ClO3-
CO32-
chlorate
carbonate
SO42sulfate
NO2-
ClO2-
SO32-
nitrite
chlorite
sulfite
Chapter 4
PO43phosphate
PO33phosphite
6
Polyatomic Ions
Adding one “H” takes away one “-” charge.
5 -ate: NO3-
ClO3-
CO32-
SO42-
PO43-
HCO3-
HSO4-
HPO42H2PO4-
4 -ite:
NO2-
ClO2-
SO32-
PO33-
HSO3Chapter 4
7
Polyatomic Ions
Write formulas including the charge for the following
polyatomic ions:
bicarbonate
ammonium
HCO3-
NH4+
hydrogen sulfate
HSO4
phosphate
PO43-
-
hydroxide
sulfite
SO32-
OHChapter 4
8
Common Inorganic Acids
H2O
H+ + OH-
HNO3
H+ + NO3-O
NO2-
HClO3
H+ + ClO3-O
ClO2-
H2CO3
2H+ + CO32+H
HCO3Chapter 4
9
Common Inorganic Acids
HSO42-
+H
H2SO4
2H+ + SO42-O
SO32- + H
+H
H3PO4
HPO42-
3H+ + PO43- + 2H
-O
PO33Chapter 4
HSO3-
+H
H2PO4-
10
Common Inorganic Acid-Base Reactions
OH-
+
H3O+
2H2O
NaOH
+
HNO3
NaNO3 + H2O
H2CO3
CaCO3 + 2H2O
Ca(OH)2 +
bases
acids
Chapter 4
11
In-Class Practice
Write the formula for the polyatomic ion in each and
name the compound:
Na2CO3
CO32-
Sodium carbonate
Li3PO4
PO43-
lithium phosphate
NaHCO3
HCO3-
Sodium hydrogen carbonate
Chapter 4
12
Writing Lewis Structures
Recall: The number of valence electrons is equal to the group
number of the atom
Carbon is in group 4A and has 4 valence electrons
Hydrogen is in group 1A and has 1 valence electron
Oxygen is in group 6A and has 6 valence electrons
Nitrogen is in group 5A and has 5 valence electrons
To construct covalent molecules the atoms are assembled
with the correct number of valence electrons
If the molecule is an ion, electrons are added or subtracted
to give it the proper charge
The structure is written to satisfy the octet rule for each
atom and to give the correct charge
If necessary, multiple bonds can be used to satisfy the
octet rule for each atom
Chapter 4
13
Writing Lewis Structures
 Write the Lewis structure for the chlorate ion (ClO3-)
 The total number of valence electrons including the
electron for the negative charge is calculated
Three pairs of electrons are used to bond
the chlorine to the oxygens
The remaining 20 electrons are
added to give each atom an octet
Central
atom
Chapter 4
14
Draw the Lewis structure of the carbonate ion
 must incorporate a double bond to satisfy the octet
rule for every atom (carbon in this case)
 The element with higher E. N. takes the electrons
first.
Chapter 4
15
Draw the Lewis structures of C2H4 and C2H2
The organic molecules ethene (C2H4) and ethyne (C2H2) must
also use multiple bonds to satisfy the octet rule for each atom
H
H
H
C C
H
C C
H
H
How many electrons are shared in each molecule above?
Chapter 4
16
In-Class Practice
Write Lewis structures for the following molecules.
Cl2
C2H6
H
H
Cl
C C
Cl
SO3
H
H
H
H
HNO2
O
O
S
O
O
Chapter 4
N
O
H
17
Exceptions to the Octet Rule
The octet rule applies only to atoms in the second row of
the periodic table (C, O, N, F) which are limited to valence
electrons in the 2s and 2p orbitals
In second row elements fewer electrons are possible
In higher rows other orbitals are accessible and more than
8 electrons around an atom are possible
Example: BF3, PCl5 and SF6
Chapter 4
18
Formal charge
 A formal charge is a positive or negative
charge on an individual atom
 The formal charge is calculated by subtracting
the assigned electrons on the atom in the
molecule from the electrons in the neutral
atom
 Electrons in bonds are evenly split between
the two atoms; one to each atom
 Lone pair electrons belong to the atom itself
Chapter 4
19
Formal charge
Formal charge = #V.E. – (#Bonds + #Unshared E.)
 Example: Find the formal charge for each atoms
in the following molecules
7-7=0
3-3=0
6-6=0
7-7=0
7-7=0
These are neutral molecules.
Chapter 4
20
Formal charge
Formal charge pairs can be replaced by forming multiple bonds.
This helps to determine the location of the multiple bond.
 Nitrous Acid HNO2
6-7=-1
6-6=0
O
N
O
H
5-4=+1
 Nitrate ion (NO3)-
-1
O
-1
O
N
+2
-1
O
Net charge =-1
Chapter 4
Why not form two double
bonds?
21
Formal charge
The sum of formal charges on individual atoms is the total
charge of the molecule or ion
 Examples
Ammonium ion (NH4)+
Chapter 4
22
A Summary of Formal Charges
An atom will always have the same formal
charge depending on how many bonds and lone
pairs it has regardless of which particular molecule it is in
Chapter 4
23
In-Class Practice
Write the Lewis structure for the polyatomic ion in
each and name the compound:
Na2SO4
-1
Sodium sulfate
-1
-2
O
O
O
S
32 – 8 = 24
6 + 4×6 + 2 = 32
SO42-
O
S
O
-1
O
O
-1
+2
O
Chapter 4
24
In-Class Practice
Write the Lewis structure for the polyatomic ion in
each and name the compound:
PO43-
Li3PO4
Lithium phosphate
-1
-3
O
O
O
P
32 – 8 = 24
5 + 4×6 + 3 = 32
-1
O
P
O
-1
O
O
-1
+1
O
Chapter 4
25
In-Class Practice
Write the Lewis structure for the polyatomic ion in
each and name the compound:
NaHCO3
1 + 4 + 3×6 + 1 = 24
HCO3-
-1
Sodium hydrogen carbonate
-1
-1
O
24 – 8 = 16
O
C
O
H
O
O
C
+1
O
H
Chapter 4
26