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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.

Exercise 1: Formulae of covalent compounds

Write the chemical formula for water

Solution
  1. Step 1. Write down the elements that make up the compound. :

    A molecule of water contains the elements hydrogen and oxygen.

  2. Step 2. Determine the valency of each element :

    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. Step 3. Write the chemical formula :

    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:

    H2OH2O.

Exercise 2: Formulae of covalent compounds

Write the chemical formula for magnesium oxide

Solution
  1. Step 1. Write down the elements that make up the compound. :

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

  2. Step 2. Determine the valency of each element :

    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 atom of oxygen.

  3. Step 3. Write the chemical formula :

    The chemical formula for magnesium oxide is therefore:

    MgOMgO

Exercise 3: Formulae of covalent compounds

Write the chemical formula for copper (II) chloride.

Solution
  1. Step 1. Write down the elements that make up the compound. :

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

  2. Step 2. Determine the valency of each element :

    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. Step 3. Write the chemical formula :

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

    CuCl2CuCl2

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 NaClNaCl for example, the charge on the sodium is +1+1 and the charge on the chlorine is -1-1. The charges balance (+1-1=0+1-1=0) and therefore the ionic compound is neutral. In MgOMgO, magnesium has a charge of +2+2 and oxygen has a charge of -2-2. 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 Table 1

Table 1: Table showing common compound ions and their formulae
Name of compound ion formula Name of compound ion formula
Carbonate CO32-CO32- Nitrite NO2-NO2-
Sulphate SO42-SO42- Hydrogen sulphite HSO3-HSO3-
Hydroxide OH-OH- Hydrogen sulphate HSO4-HSO4-
Ammonium NH4+NH4+ Dihydrogen phosphate H2PO4-H2PO4-
Nitrate NO3-NO3- Hypochlorite ClO-ClO-
Hydrogen carbonate HCO3-HCO3- Acetate (ethanoate) CH3COO-CH3COO-
Phosphate PO43-PO43- Oxalate C2O42-C2O42-
Chlorate ClO3-ClO3- Oxide O2-O2-
Cyanide CN-CN- Peroxide O22-O22-
Chromate CrO42-CrO42- Sulphide S2-S2-
Permanganate MnO4-MnO4- Sulphite SO32-SO32-
Thiosulphate S2O32-S2O32- Manganate MnO42-MnO42-
Phosphide P3-P3- Hydrogen phosphate HPO43-HPO43-

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:

Exercise 4: Formulae of ionic compounds

Write the chemical formula for potassium iodide.

Solution
  1. Step 1. Write down the ions that make up the compound. :

    Potassium iodide contains potassium and iodide ions.

  2. Step 2. Determine the valency and charge of each ion. :

    Potassium iodide contains the ions K+K+ (valency=1valency=1; charge=+1charge=+1) and I-I- (valency=1valency=1; charge=-1charge=-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. Step 3. Write the chemical formula :

    The chemical formula for potassium iodide is therefore:

    KIKI

Exercise 5: Formulae of ionic compounds

Write the chemical formula for sodium sulphate.

Solution
  1. Step 1. Write down the ions that make up the compound. :

    Sodium sulphate contains sodium ions and sulphate ions.

  2. Step 2. Determine the valency and charge of each ion. :

    Na+Na+ (valency = 1; charge = +1) and SO42-SO42-(valency=2valency=2; charge=-2charge=-2).

  3. Step 3. Write the chemical formula. :

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

    Na2SO4Na2SO4

Exercise 6: Formulae of ionic compounds

Write the chemical formula for calcium hydroxide.

Solution
  1. Step 1. Write down the ions that make up the compound. :

    Calcium hydroxide contains calcium ions and hydroxide ions.

  2. Step 2. Determine the valency and charge of each ion. :

    Calcium hydroxide contains the ions Ca2+Ca2+ (charge=+2charge=+2) and OH-OH- (charge=-1charge=-1). In order to balance the charge in a single molecule, two hydroxide ions will be needed for every ion of calcium.

  3. Step 3. Write the chemical formula. :

    The chemical formula for calcium hydroxide is therefore:

    Ca(OH)2Ca(OH)2

Note:

Notice how in the last example we wrote OH-OH- inside brackets. We do this to indicate that OH-OH- is a complex ion and that there are two of these ions bonded to one calcium ion.

Chemical formulae

  1. Copy and complete the table below:
    Table 2
    CompoundCationAnionFormula
     Na+Na+Cl-Cl- 
    potassium bromide Br-Br-  
     NH4+NH4+Cl-Cl-  
    potassium chromate   
       PbIPbI
    potassium permanganate   
    calcium phosphate   
    Click here for the solution
  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
    Click here for the solution

Chemical compounds: names and masses

In Giving names and formulae to substances the names of chemical compounds was revised. The relative molecular mass for covalent molecules is simply the sum of the relative atomic masses of each of the individual atoms in that compound. For ionic compounds we use the formula of the compound to work out a relative formula mass. We ignore the fact that there are many molecules linked together to form a crystal lattice. For example NaClNaCl has a relative formula mass of 58g·mol-158g·mol-1 .

Figure 1

Summary

  • A chemical bond is the physical process that causes atoms and molecules to be attracted together and to be bound in new compounds.
  • Atoms are more reactive, and therefore more likely to bond, when their outer electron orbitals are not full. Atoms are less reactive when these outer orbitals contain the maximum number of electrons. This explains why the noble gases do not combine to form molecules.
  • When atoms bond, electrons are either shared or exchanged.
  • Covalent bonding occurs between the atoms of non-metals and involves a sharing of electrons so that the orbitals of the outermost energy levels in the atoms are filled.
  • The valency of an atom is the number of electrons in the outer shell of that atom and valence electrons are able to form bonds with other atoms.
  • A double or triple bond occurs if there are two or three electron pairs that are shared between the same two atoms.
  • A dative covalent bond is a bond between two atoms in which both the electrons that are shared in the bond come from the same atom.
  • Lewis and Couper notation are two ways of representing molecular structure. In Lewis notation, dots and crosses are used to represent the valence electrons around the central atom. In Couper notation, lines are used to represent the bonds between atoms.
  • An ionic bond occurs between atoms where the difference in electronegativity is greater than 1,7. An exchange of electrons takes place and the atoms are held together by the electrostatic force of attraction between oppositely-charged ions.
  • Ionic solids are arranged in a crystal lattice structure.
  • Ionic compounds have a number of specific properties, including their high melting and boiling points, brittle nature, the lattice structure of solids and the ability of ionic solutions to conduct electricity.
  • A metallic bond is the electrostatic attraction between the positively charged nuclei of metal atoms and the delocalised electrons in the metal.
  • Metals also have a number of properties, including their ability to conduct heat and electricity, their metallic lustre, the fact that they are both malleable and ductile, and their high melting point and density.
  • The valency of atoms, and the way they bond, can be used to determine the chemical formulae of compounds.

End of chapter exercises

  1. Explain the meaning of each of the following terms
    1. Valency
    2. Covalent bond
    Click here for the solution
  2. Which ONE of the following best describes the bond formed between an H+H+ ion and the NH3NH3 molecule?
    1. Covalent bond
    2. Dative covalent (coordinate covalent) bond
    3. Ionic Bond
    4. Hydrogen Bond
    Click here for the solution
  3. Which of the following reactions will not take place? Explain your answer.
    1. H+HH2H+HH2
    2. Ne + Ne Ne 2 Ne + Ne Ne 2
    3. Cl + Cl Cl 2 Cl + Cl Cl 2
    Click here for the solution
  4. Draw the Lewis structure for each of the following:
    1. calcium
    2. iodine (Hint: Which group is it in? It will be similar to others in that group)
    3. hydrogen bromide (HBrHBr)
    4. nitrogen dioxide (NO2NO2)
    Click here for the solution
  5. Given the following Lewis structure, where X and Y each represent a different element...
    Figure 2
    Figure 2 (CG11C1_027.png)
    1. What is the valency of XX?
    2. What is the valency of YY?
    3. Which elements could XX and YY represent?
    Click here for the solution
  6. A molecule of ethane has the formula C2H6C2H6. Which of the following diagrams (Couper notation) accurately represents this molecule?
    Figure 3
    Figure 3 (CG11C1_028.png)
    Click here for the solution
  7. Potassium dichromate is dissolved in water.
    1. Give the name and chemical formula for each of the ions in solution.
    2. What is the chemical formula for potassium dichromate?
    Click here for the solution

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