Writing Chemical Formula Worksheet With Answer Key - Spes Unica Page 4
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28LI 3
2. USING VALENCY
The valency of an atom or ion is the number of electrons it shares, loses
or gains in a chemical reaction to become stable i.e. the number of bonds
it forms with other atoms
Group
1
2
3
4
5
6
7
0
number
Valency
1
2
3
4
3
2
1
0
We can use valency to work out chemical formulae for simple compounds
containing two elements i.e. compounds ending in –IDE
Remember HYDROXIDES and CYANIDES are exceptions to -IDE
rule
We can follow a set of simple rules to write chemical formula:
1. Write symbols of elements present in compound
2. Put valency above each symbol
3. Cross valency over (swap and drop)
4. Cancel down ratio if necessary
5. Write correct chemical formula
Example 1. Sodium chloride
1. Na
Cl
1
1
2. Na
Cl
3. Na
Cl
1
1
4. (Not needed)
5. NaCl
4
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