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Writing chemical formulae

The formulae of covalent compounds

To work out the formulae of covalent compounds, we need to use the valency of the atoms in the compound. This is because the valency tells us how many bonds each atom can form. This in turn can help to work out how many atoms of each element are in the compound, and therefore what its formula is. The following are some examples where this information is used to write the chemical formula of a compound.

Write the chemical formula for water

  1. A molecule of water contains the elements hydrogen and oxygen .

  2. The valency of hydrogen is 1 and the valency of oxygen is 2. This means that oxygen can form two bonds with other elements and each of the hydrogen atoms can form one.

  3. Using the valencies of hydrogen and oxygen, we know that in a single water molecule, two hydrogen atoms will combine with one oxygen atom. The chemical formula for water is therefore:

    H 2 O .

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Write the chemical formula for magnesium oxide

  1. A molecule of magnesium oxide contains the elements magnesium and oxygen .

  2. The valency of magnesium is 2, while the valency of oxygen is also 2. In a molecule of magnesium oxide, one atom of magnesium will combine with one atomof oxygen.

  3. The chemical formula for magnesium oxide is therefore:


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Write the chemical formula for copper (II) chloride.

  1. A molecule of copper (II) chloride contains the elements copper and chlorine .

  2. The valency of copper is 2, while the valency of chlorine is 1. In a molecule of copper (II) chloride, two atoms of chlorine will combine with one atom of copper.

  3. The chemical formula for copper (II) chloride is therefore:

    CuCl 2

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The formulae of ionic compounds

The overall charge of an ionic compound will always be zero and so the negative and positive charge must be the same size. We can use this information to work out what the chemical formula of an ionic compound is if we know the charge on the individual ions. In the case of NaCl for example, the charge on the sodium is +1 and the charge on the chlorine is -1. The charges balance (+1-1=0) and therefore the ionic compound is neutral. In MgO, magnesium has a charge of +2 and oxygen has a charge of -2. Again, the charges balance and the compound is neutral. Positive ions are called cations and negative ions are called anions .

Some ions are made up of groups of atoms, and these are called compound ions . It is a good idea to learn the compound ions that are shown in [link]

Table showing common compound ions and their formulae
Name of compound ion formula
Carbonate CO 3 2 -
Sulfate SO 4 2 -
Hydroxide OH -
Ammonium NH 4 +
Nitrate NO 3 -
Hydrogen carbonate HCO 3 -
Phosphate PO 4 3 -
Chlorate ClO 3 -
Cyanide CN -
Chromate CrO 4 2 -
Permanganate MnO 4 -

In the case of ionic compounds, the valency of an ion is the same as its charge (Note: valency is always expressed as a positive number e.g. valency of the chloride ion is 1 and not -1). Since an ionic compound is always neutral , the positive charges in the compound must balance out the negative. The following are some examples:

Write the chemical formula for potassium iodide.

  1. Potassium iodide contains potassium and iodide ions.

  2. Potassium iodide contains the ions K + (valency = 1; charge = +1) and I - (valency = 1; charge = -1). In order to balance the charge in a single molecule, one atom of potassium will be needed for every one atom of iodine.

  3. The chemical formula for potassium iodide is therefore:


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Write the chemical formula for sodium sulfate.

  1. Sodium sulfate contains sodium ions and sulfate ions.

  2. Na + (valency = 1; charge = +1) and SO 4 2 - (valency = 2; charge = -2).

  3. Two sodium ions will be needed to balance the charge of the sulfate ion. The chemical formula for sodium sulfate is therefore:

    Na 2 SO 4

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Write the chemical formula for calcium hydroxide.

  1. Calcium hydroxide contains calcium ions and hydroxide ions.

  2. Calcium hydroxide contains the ions Ca 2 + (charge = +2) and OH - (charge = -1). In order to balance the charge in a single molecule, two hydroxide ions will be needed for every ion of calcium.

  3. The chemical formula for calcium hydroxide is therefore:

    Ca(OH) 2

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Chemical formulae

  1. Copy and complete the table below:
    Compound Cation Anion Formula
    Na + Cl -
    potassium bromide Br -
    NH 4 + Cl -
    potassium chromate
    potassium permanganate
    calcium phosphate
  2. Write the chemical formula for each of the following compounds:
    1. hydrogen cyanide
    2. carbon dioxide
    3. sodium carbonate
    4. ammonium hydroxide
    5. barium sulphate

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Source:  OpenStax, Siyavula textbooks: grade 11 physical science. OpenStax CNX. Jul 29, 2011 Download for free at http://cnx.org/content/col11241/1.2
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