Molecular Compounds (a.k.a.
Covalent Compounds)
Ionic Compounds Review
• Ionic compounds are the combination of
ions. E.g. Na+ + Cl-  NaCl.
• This formula represents the Ratio of
sodium ions and chloride ions not the
exact number of them.
• They are composed of a Metal and a
Non-Metal and form a crystal structure
(e.g. NaCl and CaCl2 which are both salts)
Molecular Compounds
• Consist of atoms covalently bonded
together
• The elements involved are all Non-Metals
Some examples
• N2O  Nitrous Oxide= Gas used at the
dentist to relax patients
• NO2  Nitrogen Dioxide= poisonous
toxin emitted from car exhaust
More Examples
• H2O  Water
• CO2  Carbon Dioxide= Gas
exhaled and created during
combustion reactions
• CO  Carbon Monoxide= Lethal gas
created during incomplete combustion
Recall
• In ionic compounds, the non-metal rips
away the electron(s) from the metal
because the metals loosely hold their
electrons
• This creates a positive and negative ion
which attract each other making them stick
together (opposites attract)
Molecular Compounds
• made up of a non-metal
and a non-metal.
• also called Covalent
Compounds
Molecular Compounds
• Share electrons (e-) to form
a stable arrangement.
• Neg. e- are attracted to the
Pos. nuclei of both atoms
Molecular Compounds
Molecular compounds are formed
when non metals share
electrons to fill their outer
electron orbit (shell).
If we draw the Lewis Structure for
Fluorine,
We can see that it needs one more
electron to become stable
It wants to share one electron with
another atom.
If it bonds with another fluorine
atom
We draw the shared pair of
electrons between the two
atoms.
F
F
F
F
Shared electron
pair  both now
have 8 electrons
in outer orbit.
Molecular Compounds
Many molecular compounds are
predictable.
How will Oxygen bond with
Hydrogen to form a molecule.
Draw the Lewis Structure of both
Atoms.
Oxygen wants to share two
electrons and hydrogen wants to
share one.
H
H
O
H
H O
Clearly we need another Hydrogen
atom
Putting the shared electrons
between the atoms.
The molecule is H2O
**The Oxygen atom has 8
valence electrons (full) and
the Hydrogen has 2 valence
electrons (full)
Molecular Compounds
Not all molecular compounds are
as predictable.
How will Oxygen bond with
Carbon?
Draw the Lewis Structures of each
atom.
Carbon wants to share 4.
Oxygen wants to share 2
C
O
C
O
If we add one more Oxygen, each
oxygen can share two.
However, if Oxygen will share two
and donate one of its other
electrons pairs, Oxygen can
bond with just on Carbon atom.
O
Carbon Dioxide
C
Carbon
Monoxide
O
Types of Bonds
H
H O
C
Water – Single
bond because One
pair of electrons are
shared
O
C
O
Carbon Monoxide – Triple
Bond because three pairs
of electrons are shared
O
Carbon Dioxide – Double bond because
two pairs of electrons are shared
Molecular Compounds – Naming
So Oxygen can form CO or CO2
We need a more flexible naming
system for covalent
compounds to reflect the
many different bonding
possibilities.
To name a covalent compound,
1. Starting with the atom that is
to the left in the periodic
table, (or lower)
2. Write the name of the atoms
with the prefix indicating the
number of that atom in the
compound.
3. Change the ending of the last
atom to – ide.
The prefixes are,
1
Mono
2
Di
3
Tri
4
Tetra
5
Penta
6
Hexa
7
Hepta
Note, the mono is omitted on the
first atom.
CO
Carbon Monoxide
CO2
Carbon Dioxide
Memorize me!
Number
Prefix
Number
Prefix
1
mono
6
Hexa
2
Di
7
hepta
3
tri
8
octa
4
Tetra
9
nona
5
penta
10
deca
Naming Example
• P4O10
Phosphorus - use prefix tetra (4)
Oxygen – change ending – Oxide
Oxide – use prefix deca (10)
= Tetraphosphorus Decaoxide
Molecular Compounds - Naming
Name the following compounds
Write the chemical formula for,
CF4
Sulphur Dioxide
Carbon tetrafluoride
SO2
H2O
Dihydrogen Monoxide
DiCarbon Tetrahydride
C2H4
PF5
Phosphorus Pentafluoride
Molecular Compounds – Diatomic
Gases
Molecular compounds show an
incredible variety in structure,
physical and chemical
properties.
There are categories that further
divide covalent compounds
into categories with common
properties.
Diatomic Gases – When two
atoms of the same type form a
covalent molecule, the result is
often a colorless, odorless gas.
The Halogens, Oxygen, Nitrogen
and Hydrogen form diatomic
gases.
H2 is called Hydrogen Gas not
Dihydrogen.
Writing Formulas
left-most element goes 1st
Carbon monoxide
- Carbon (C)  no prefix therefore = C
- Monoxide – Mono = 1 therefore, 1 oxygen
atom = O
= CO
Writing Formulas
• Nitrogen dioxide
- Nitrogen (N)  no prefix therefore = N
- Dioxide – Di = 2 therefore, 2 oxygen
atoms = O2
= NO2
Writing Formulas
DiCarbon Tetrahydride
- Dicarbon (C)  Di = 2 therefore = C2
- Tetrahydride (H)  Tetra = 4 therefore, 4
Hydrogen atoms = H4
= C2H4
Eg. N2 - Nitrogen Gas
O2 - Oxygen Gas
Air
Exceptions that need to be
Memorized!
CH4 – Methane
H2O2 – Hydrogen peroxide
H2O – Water
These Too!
NH3 – Ammonia
O3 – Ozone