Magnesium

1
3. Combining Capacity

• The ability of an element to combine with other
  elements is called its combining capacity.

• Different elements have different abilities to
  combine with other elements (see p.234 table
  11.4, 11.5)

e.g.
Name
Symbol
Combining Capacity
Sodium
Na
1
Magnesium
Mg
2
Aluminum
Al
3
Potassium
K
1
Sulphur
S
2
Bromine
Br
1
Chlorine
Cl
1
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4. Writing Formulas
e.g. Magnesium oxide
1. Write both symbols (metal first) with
combining capacities
2
2
Mg
O
2. Criss-cross the combining capacities and
write them as subscripts
Mg O
2
2
MgO
3. Reduce if possible
(remember, the subscript 1 is never written)
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More Examples!
3
2
Aluminum sulphide
Al
S
Al S
Al2S3
3
2
2
1
Ca
Cl
Calcium chloride
Ca Cl
CaCl2
2
1
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Practice
Write the formula for the following compounds
a) Magnesium oxide
MgO
b) Calcium chloride
CaCl2
c) Aluminum bromide
AlBr3
d) Sodium sulphide
Na2S
e) Beryllium nitride
Be3N2
f) Sodium bromide
NaBr
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• Sometimes, metal ions have more than one
  combining capacity

e.g.
Cu
1 or 2
Fe
2 or 3
The name of the compound will include a roman
numeral that tells you which combining capacity
to use
Iron II oxide
e.g.
Iron III oxide
2
2
3
2
Fe
O
Fe
O
Fe O
Fe O
2
2
3
2
FeO
Fe2O3
(cannot be reduced)
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Assignment
Science 9 WorksheetFormula of ide Endings
Worksheet
In Class
Page 236 Instant Practice Q a-h
Homework
Quiz on Naming and Formulas Next Class!
SC9 1-11