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Giving names and formulae to substances

Module by: Free High School Science Texts Project. E-mail the author

Giving names and formulae to substances

It is easy to describe elements and mixtures. But how are compounds named? In the example of iron sulphide that was used earlier, which element is named first, and which 'ending' is given to the compound name (in this case, the ending is -ide)?

The following are some guidelines for naming compounds:

  1. The compound name will always include the names of the elements that are part of it.
    • A compound of iron (Fe) and sulphur (S) is iron
      sulphide (FeS)
    • A compound of potassium (K) and bromine (Br) is potassium
      bromide (KBr)
    • A compound of sodium (Na) and chlorine (Cl) is sodium
      chloride (NaCl)
  2. In a compound, the element that is on the left of the Periodic Table, is used first when naming the compound. In the example of NaCl, sodium is a group 1 element on the left hand side of the table, while chlorine is in group 7 on the right of the table. Sodium therefore comes first in the compound name. The same is true for FeS and KBr.
  3. The symbols of the elements can be used to represent compounds e.g. FeS, NaCl, KBr and H2O. These are called chemical formulae. In the first three examples, the ratio of the elements in each compound is 1:1. So, for FeS, there is one atom of iron for every atom of sulphur in the compound. In the last example (H2O) there are two atoms of hydrogen for every atom of oxygen in the compound.
  4. A compound may contain compound ions. An ion is an atom that has lost (positive ion) or gained (negative ion) electrons. Some of the more common compound ions and their formulae are given below.
    Table 1
    Name of compound ionFormula
    CarbonateCO32--
    SulphateSO42--
    HydroxideOH--
    AmmoniumNH4+
    NitrateNO3--
    Hydrogen carbonateHCO3--
    PhosphatePO43--
    ChlorateClO3--
    CyanideCN--
    ChromateCrO42--
    PermanganateMnO4--
  5. When there are only two elements in the compound, the compound is often given a suffix (ending) of -ide. You would have seen this in some of the examples we have used so far. For compound ions, when a non-metal is combined with oxygen to form a negative ion (anion) which then combines with a positive ion (cation) from hydrogen or a metal, then the suffix of the name will be ...ate or ...ite. NO3-- for example, is a negative ion, which may combine with a cation such as hydrogen (HNO3) or a metal like potassium (KNO3). The NO3-- anion has the name nitrate. SO32-- in a formula is sulphite, e.g. sodium sulphite (Na2SO3).
     SO42-- is sulphate and PO43-- is phosphate.
  6. Prefixes can be used to describe the ratio of the elements that are in the compound. You should know the following prefixes: 'mono' (one), 'di' (two) and 'tri' (three).
    • CO (carbon monoxide) - There is one atom of oxygen for every one atom of carbon
    • NO2 (nitrogen dioxide) - There are two atoms of oxygen for every one atom of nitrogen
    • SO3 (sulphur trioxide) - There are three atoms of oxygen for every one atom of sulphur

Tip:

When numbers are written as 'subscripts' in compounds (i.e. they are written below and to the right of the element symbol), this tells us how many atoms of that element there are in relation to other elements in the compound. For example in nitrogen dioxide (NO2) there are two oxygen atoms for every one atom of nitrogen. In sulphur trioxide (SO3), there are three oxygen atoms for every one atom of sulphur in the compound. Later, when we start looking at chemical equations, you will notice that sometimes there are numbers before the compound name. For example, 2H2O means that there are two molecules of water, and that in each molecule there are two hydrogen atoms for every one oxygen atom.

Naming compounds

  1. The formula for calcium carbonate is CaCO3.
    1. Is calcium carbonate a mixture or a compound? Give a reason for your answer.
    2. What is the ratio of Ca:C:O atoms in the formula?

    Click here for the solution

  2. Give the name of each of the following substances.
    1. KBr
    2. HCl
    3. KMnO4
    4. NO2
    5. NH4OH
    6. Na2SO4

    Click here for the solution

  3. Give the chemical formula for each of the following compounds.
    1. potassium nitrate
    2. sodium iodide
    3. barium sulphate
    4. nitrogen dioxide
    5. sodium monosulphate

    Click here for the solution

  4. Refer to the diagram below, showing sodium chloride and water, and then answer the questions that follow.
    Figure 1
    Figure 1 (CG10C2_004.png)
    1. What is the chemical formula for water?
    2. What is the chemical formula for sodium chloride?
    3. Label the water and sodium chloride in the diagram.
    4. Give a description of the picture. Focus on whether there are elements or compounds and if it is a mixture or not.

    Click here for the solution

  5. What is the formula of this molecule?
    Figure 2
    Figure 2 (CG10C1_005.png)
    1. C6H2O
    2. C2H6O
    3. 2C6HO
    4. 2CH6O

    Click here for the solution

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