The use of chemical formula is a shorthand way of showing
| Compound | Molecular element | ||
| Water | H2O | Hydrogen | H2 |
| Carbon dioxide | CO2 | Oxygen | O2 |
| Glucose | C6H12O6 | Ozone | O3 |
| Sodium chloride | NaCl | Chlorine | Cl2 |
| Magnesium sulfate | MgSO4 | Sulfur | S8 |
The chemical symbols of the elements are shown in the Periodic table. For example the element hydrogen is given the chemical symbol H and the element oxygen the chemical symbol O.
In a chemical formula the chemical symbol of each element is shown with subscript numbers which tell us the numbers or ratio of atoms in the compound or molecular element.
For example the compound water has the chemical formula H2O. This tells us water is made up of two elements, hydrogen and oxygen. The subscript 2 in H2O tells us there are two parts hydrogen to one part oxygen. Note: The subscript 1 is never but taken for granted. That is why water has the formula H2O and not H2O1
Some elements also exist as molecules. Molecules are groups of atoms joined together by chemical bonds. Molecular elements like compounds also have a chemical formula. Examples of molecular elements include hydrogen, H2 and ozygen O2.
Hydrogen atoms are too unstable to exist by themseleves as free atoms. They pair up to with other hydrogen atoms to form hydrogen molecules which are much more stable. The chemical formula of the hydrogen molecule is therefore H2. This means two hydrogen atoms are joined together. The molecule O2 is also much more stable than single oxygen atoms.
The combining power or valency of an element determines the ratio that atoms will combine with one another. A knowledge of valencies can help us predict the chemical formula of a substance. Atoms achieve a stable outer shell of electrons by transferring or sharing electrons. They always combine with one another n simple whle number ratios.
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 |