Formulas of Molecular Compounds

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Formulas of Molecular Compounds
Many compounds- made up of molecules and not ions
Use molecular formulas to write formulas of m.c.
Compound made u p of molecules are molecular
compounds

• note- the molecular formula may be its empirical
  formula
• Example is CO2

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UNLIKE IONIC COMPOUNDS, THERE IS NO SIMPLE WAY TO
PREDICT AND WRITE FORMULAS FOR MOLECULAR
COMPOUNDS!!!
Nitrogen and Oxygen can form 5 different compounds

• The same elements can form a number of different
  compounds, which have different formulas

1. N2O - dinitrogen monoxide
2. NO- nitrogen monoxide
3. N2O3- dinitrogen trioxide
4. NO2- nitrogen dioxide
5. N2O5- dinitrogen pentoxide
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IN MOLECULAR COMPOUNDS- ELECTRONS ARE SHARED BY
TWO ATOMS NOT TRANSFERRED FROM ONE ATOM TO ANOTHER
In some molecular compounds - electrons are not
shared equally
Oxidation Number-
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Oxygen
N2O-
Other examples NO N2O3 NO2 N2O5
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Sample problem The compound with a formula of
P2O3 is composed of 2 non metals- phosphorus and
oxygen. What is the apparent charge on a
phosphorus atom in this compound.
Oxygen has 2- per atom and there are 3 oxygen
atoms so there is a total negative charge of
6-. Take 6 and divide by number of P atoms (2 in
this case) and you get 3. So P has an apparent
charge of 3
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Naming Molecular Compounds

• In naming binary molecular compounds, element
  with the apparent positive charge comes first.

A system prefixes is used to distinguish between
compounds.
Prefixes are
1. Mono- 4. Tetra 2. Di
5. Penta 3. Tri
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The stock system can also be used to name
molecular compounds.
The roman numeral is used to show the apparent
charge in the atom with the positive oxidation
number.
Example- PbO2
traditional name is
lead dioxide. Stock system name is
lead(IV) oxide.
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Using the Stock system to name a molecular
compound
N2O - dinitrogen monoxide What is the name of
this compound in the stock system?
Nitrogen (I) oxide
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Sample problem Name the compound with the
formula P2O3 using both the traditional and stock
system
Answer- Traditional - Diphosphorus trioxide.
Stock system- phosphorus (III) oxide.