How do you write chemical formula?
When writing chemical formula for compounds it is important to know the combining powers or valencies of the elements. This allows you to determine the ratio the elements combine in. All compunds are made of of elements that are chemically combined in whole number ratios. eg. Water is made up of 2 parts hydrogen to 1 part water. Its chemical formula is therefore H2O
The elements in the periodic table are grouped together in columns with similar combining powers. These valencies also contain a charge. Eg. Mg is in group 2. It has a valency of 2+, O is in group 6 and has a valency of 2-
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Group |
1 |
2 |
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3 |
4 |
5 |
6 |
7 |
8 |
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Valency |
1+ |
2+ |
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3+ |
4 |
3- |
2- |
1- |
0 |
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Periodic table |
H |
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Li |
Be |
Transition block |
B |
C |
N |
O |
F |
Ne |
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First 20 elements |
Na |
Mg |
variable |
Al |
Si |
P |
S |
Cl |
Ar |
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K |
Ca |
valencies |
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A metal combines with a non-metal to produce an ionic compound. During the reaction ions or charge particles are formed. Refer to the valencies in the table above to help you determine the ionic charge. Eg. The sodium ion Na+ has a charge of +1, the calcium ion, Ca2+ is +2, the oxide ion, O2- is -2 and the chloride ion, Cl- is -1. Note: The 1 is ignored when determining the ionic charge in group I and group 7 elements.
When writing chemical formula determine the simplest ratio of positive and negative ions that are needed to produce a neutral compound. These numbers are written as subscripts. Eg. Al2O3
Look at the following worked examples on how to determine the chemical formula of compunds.
Chemical compound |
Make the simplest neutral compound from the ions |
Chemical formula (ignore ions) |
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Sodium chloride |
Na+ Cl- Check: + 1 + -1 = 0 |
NaCl |
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Magnesium chloride |
Mg2+ Cl- Check: 2+ + 2- = 0 |
MgCl2 |
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Aluminum chloride |
Al3+ Cl- Check: 3+ + 3- = 0 |
AlCl3 |
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Calcium oxide |
Ca2+ O2- Check: 2+ + 2- = 0 |
CaO |
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Sodium oxide |
Na+ O2- Check: 2+ + 2- = 0 |
Na2O |
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Lithium phosphide |
Li+ P3- Check: 3+ + 3- = 0 |
Li3P |
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Aluminum oxide |
Al3+ O2- Check: 6+ + 6- = 0 |
Al2O3 |