SNC2D1: Notes
Molecular or Covalent Compounds
Molecular compounds are formed when two or more non-metals are bonded
together by covalent bonds. E.g. hydrogen gas (H2) and glucose(C6H12O6.).
A covalent bond is a chemical bond that is formed when atoms share
electrons. These types of bonds tend to form when non-metals atoms
combine with each other. Covalent compounds are also known as molecular
compounds. These compounds have a number of similar properties: they
are often soft and when dissolved in water they form solutions that do
not conduct electricity.
Diatomic Molecules
Many non-metallic elements exist as covalently bonded molecules, however
some of these exist in the form of diatomic molecules. Diatomic molecules
are composed of two identical atoms covalently joined together. For most
elements the formulas is the same as the symbol, however this is not the
case for diatomic molecules therefore their formulas must be memorized.
Listed below are all the diatomic molecules:
H2,
N2,
O2,
F 2,
Cl2,
Br2,
I 2,
For Example:Hydrogen gas consist of two atoms of hydrogen bonded
together by covalent bonds and
chlorine gas is a molecule that consist of two chlorine atoms
also bonded by covalent
bonds. In both case electrons are shared and not exchanged.
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Prefixes
When naming molecular compounds one uses the prefix method
whereby prefixes are used to indicate the number of atoms of the
element to which the prefix is attached. Use the following prefixes
below:
mono
1
di
penta
2
5
hexa
nona
6
9
deca
10
tri
3
hepta
7
tetra
4
octa
8
Naming Molecular Compounds

The first element retains its name.

The suffix “ide” is added to the end of the second element.
 Prefixes are added to indicate the number of each atom.
Note: The prefix mono is not used on the first element. The second
element always has a prefix associated with it.
Note: no prefix is needed if the first element only has one atom.
For example:SO2 is called sulphur dioxide
N2O3 is called dinitrogen trioxide
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CO2 is called carbon dioxide
Write the name for each compound
1. CF4
2. P2O5
3. NH3
4. CH4
5. PBr3
6. SO3
13.
N2O4
14. I2 (gas)
15. H2O2
16. NO2
17. CO
7. BH3
18. SF4
8. H2O
19. SiC
9. F2
10. SiO2
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12. PCl5
20. H2 (gas)
21. O2 (gas)
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11. CS2
22. OF2
Writing Formulas from Names

Simply transform the name into chemical symbols.

The prefix indicates the number of atoms of each element.
Note: molecular formulas are never simplified we never simply formulas
For example: chlorine gas is - Cl2
trinitrogen tetrasulphide is – N3S4
carbon monoxide is - CO
Write the formulas for the following compounds
1. carbon dioxide
2. diphosphorus trioxide
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12. nitrogen tribromide
13. carbon tetrachloride
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3. Oxygen gas
14. phosphorus pentabromide
4. nitrogen monoxide
15. dihydrogen dioxide
5. dihydrogen monoxide
6. chlorine dioxide
17. carbon tetrahydride
7. carbon disulphide
18. silcon carbide
8. hydrogen gas
19. ozone (trioxygen)
9. sulphur trioxide
10. carbon monoxide
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16. dinitrogen tetroxide
20. sulphur dioxide
21. phosphorus trichloride
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11. nitrogen trihydride
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22. oxygen difluoride
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