SNC2P
Molecular Compounds
We have seen ionic compounds form between metals and non-metals.
They combine because their stable (full valence shell) ions have opposite
charges and attract each other, forming an ionic bond. Ionic compounds
share certain properties:
 They are solid at room temperature.
 They have high melting points.
 They conduct electricity when dissolved in water.
Molecular compounds are groups of non-metal atoms bonded together by
sharing electrons. These compounds have properties distinct from ionic
compounds:
 They have lower melting and boiling points.
 They are found as solids, liquids or gases.
 These compounds usually do not conduct electricity when dissolved.
 Molecular compounds contain covalent bonds.
 Covalent bonds are formed when 2 non-metal atoms share electrons.
Water (H2O) is a molecular compound in which each hydrogen shares one
electron with an oxygen atom.
Oxygen has 6 valence electrons. Therefore it needs to gain 2
electrons by sharing to become stable.
Each hydrogen has 1 valence electron. They need to gain 1 electron
by sharing.
Each atom in water “feels” like it has a full outer shell by sharing.
Diatomic Elements
Hydrogen bonds together by sharing its one electron to produce hydrogen
gas (H2 (g)):
This arrangement allows each hydrogen atom to “feel” as though it has a
full outer shell. Many non-metallic elements exist as covalently-bonded
diatomic molecules (e.g. O2, N2, F2, Cl2, Br2, I2).
Naming Molecular Compounds
Because non-metals can combine in many ways, a Greek prefix used to tell
how many of each atom is in the compound. For example, the greenhouse
gas CO2 is named this way:
Step 1:
Name the first element: carbon
Step 2:
There is only 1 carbon, so the prefix is “mono”. However
mono is skipped for the first element.
Step 3:
Name the second element, changing the ending to “–ide”:
oxygen  oxide
Step 4:
Choose the correct prefix: 2 = di = dioxide
Step 5:
Combine the 2 parts: carbon dioxide
e.g. SO3
__________________________________
e.g. C2H4
_________________________________
Table 1: Greek Prefixes for 1 – 9
#
Prefix
#
Prefix
1
mon(o)3
tri2
di4
tetra-
#
5
6
Prefix
pentahexa
#
7
8
Prefix
heptaocta-
Some molecular compounds are simply known by their common names:
H2O = ___________________
NH3 = __________________
H2O2 = _____________________________________________
Writing Formulas of Molecular Compounds
Reversing the process above, we can solve the chemical formula of a
molecular compound. For example, we can write the formula for
diphosphorus trioxide.
Step 1:
On te periodic table, find and write the element symbols: P O
Step 2:
Using the table of prefixes, determine the number of each
element: di = 2; tri = 3
Step 3:
Write the chemical formula with subscripts: P2O3
e.g.
sulfur dioxide _____________
e.g.
phosphorus pentaiodide ____________________
e.g.
trioxygen _____________________ (also called ozone)
Molecular Compounds Come in a Variety of Forms
The elements nitrogen and oxygen can exist in many different molecular
compounds:
Chemical
Formula
Chemical Name
What it is
Pollutant from car exhaust
NO
Laughing gas, greenhouse gas
dinitrogen monoxide
Pollutant that forms acid rain.
NO2
Deep blue liquid.
dinitrogen trioxide
Rocket fuel.
N2O4
Reacts with water to make acid.
dinitrogen pentoxide
Modeling Molecules
Ball and stick models can be used to represent molecules. Which models
represent these molecules? water, carbon tetrachloride, boron trifluoride
Molecular Model Building Activity
Goal: To build models of several molecular compounds.
What to do:
1. Using your notes, complete columns 1 -4.
2. Using he information, build a model for each.
3. Sketch your model in the fifth column.
Number and
Type of
Element 1
Number and
Type of
Element 2
Chemical
Formula
Chemical
Name
CO
1C
2S
oxygen
difluoride
1N
3 Cl
carbon
tetrahydride
(methane)
phosphorus
trifluoride
Model
Molecular Compounds Practice Questions
1. Referring to the examples of covalent bonding in hydrogen and water, draw diagrams
to represent covalent bonding in these compounds. Circle the pair(s) of shared
electrons.
a) methane (CH4)
c) nitrogen trifluoride (NF3)
b) fluorine gas (F2)
d) hydrogen sulfide (H2S)
2. Write the name for each compound, using the Greek prefic method
a) CS2 ___________________________
f) AsF5 ___________________________
b) N2O3 __________________________
g) N2O ____________________________
c) NO ____________________________
h) SO2 ____________________________
d) CCl4 ___________________________
i) P2O5 ____________________________
e) Si2Br6 _________________________
j) BH3 _____________________________
3. Write the chemical formula for each element or compound.
a) dinitrogen tetroxide ________________
f) carbon monoxide__________________
b) diphosphorus hexoxide ___________
g) oxygen difluoride _________________
c) phosphorus tribromide ___________
h) sulfur hexafluoride _______________
d) sulfur trioxide __________________
i) nitrogen gas ____________________
e) oxygen gas _____________________
j)carbon tetrabromide _______________