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Chemical Bonding

When you look at the matter, or physical substances, around you, you will realise that atoms seldom exist on their own. More often, the things around us are made up of different atoms that have been joined together. This is called chemical bonding. Chemical bonding is one of the most important processes in chemistry because it allows all sorts of different molecules and combinations of atoms to form, which then make up the objects in the complex world around us.

What happens when atoms bond?

A chemical bond is formed when atoms are held together by attractive forces. This attraction occurs when electrons are shared between atoms, or when electrons are exchanged between the atoms that are involved in the bond. The sharing or exchange of electrons takes place so that the outer energy levels of the atoms involved are filled and the atoms are more stable. If an electron is shared, it means that it will spend its time moving in the electron orbitals around both atoms. If an electron is exchanged it means that it is transferred from one atom to another, in other words one atom gains an electron while the other loses an electron.

Definition 1: Chemical bond
A chemical bond is the physical process that causes atoms and molecules to be attracted to each other, and held together in more stable chemical compounds.

The type of bond that is formed depends on the elements that are involved. In this chapter, we will be looking at three types of chemical bonding: covalent, ionic and metallic bonding.

You need to remember that it is the valence electrons that are involved in bonding and that atoms will try to fill their outer energy levels so that they are more stable (or are more like the noble gases which are very stable).

Covalent Bonding

The nature of the covalent bond

Covalent bonding occurs between the atoms of non-metals. The outermost orbitals of the atoms overlap so that unpaired electrons in each of the bonding atoms can be shared. By overlapping orbitals, the outer energy shells of all the bonding atoms are filled. The shared electrons move in the orbitals around both atoms. As they move, there is an attraction between these negatively charged electrons and the positively charged nuclei, and this force holds the atoms together in a covalent bond.

Definition 2: Covalent bond
Covalent bonding is a form of chemical bonding where pairs of electrons are shared between atoms.

Below are a few examples. Remember that it is only the valence electrons that are involved in bonding, and so when diagrams are drawn to show what is happening during bonding, it is only these electrons that are shown. Circles and crosses are used to represent electrons in different atoms.

Exercise 1: Covalent bonding

How do hydrogen and chlorine atoms bond covalently in a molecule of hydrogen chloride?

Solution
  1. Step 1. Determine the electron configuration of each of the bonding atoms. :

    A chlorine atom has 17 electrons, and an electron configuration of 1s22s22p63s23p51s22s22p63s23p5. A hydrogen atom has only 1 electron, and an electron configuration of 1s11s1.

  2. Step 2. Determine the number of valence electrons for each atom, and how many of the electrons are paired or unpaired. :

    Chlorine has 7 valence electrons. One of these electrons is unpaired. Hydrogen has 1 valence electron and it is unpaired.

  3. Step 3. Look to see how the electrons can be shared between the atoms so that the outermost energy levels of both atoms are full. :

    The hydrogen atom needs one more electron to complete its valence shell. The chlorine atom also needs one more electron to complete its shell. Therefore one pair of electrons must be shared between the two atoms. In other words, one electron from the chlorine atom will spend some of its time orbiting the hydrogen atom so that hydrogen's valence shell is full. The hydrogen electron will spend some of its time orbiting the chlorine atom so that chlorine's valence shell is also full. A molecule of hydrogen chloride is formed (Figure 1). Notice the shared electron pair in the overlapping orbitals.

    Figure 1: Covalent bonding in a molecule of hydrogen chloride
    Figure 1 (CG11C1_003.png)

Exercise 2: Covalent bonding involving multiple bonds

How do nitrogen and hydrogen atoms bond to form a molecule of ammonia (NH3NH3)?

Solution
  1. Step 1. Determine the electron configuration of each of the bonding atoms. :

    A nitrogen atom has 7 electrons, and an electron configuration of 1s22s22p31s22s22p3. A hydrogen atom has only 1 electron, and an electron configuration of 1s11s1.

  2. Step 2. Determine the number of valence electrons for each atom, and how many of the electrons are paired or unpaired. :

    Nitrogen has 5 valence electrons meaning that 3 electrons are unpaired. Hydrogen has 1 valence electron and it is unpaired.

  3. Step 3. Look to see how the electrons can be shared between the atoms so that the outer energy shells of all atoms are full. :

    Each hydrogen atom needs one more electron to complete its valence energy shell. The nitrogen atom needs three more electrons to complete its valence energy shell. Therefore three pairs of electrons must be shared between the four atoms involved. The nitrogen atom will share three of its electrons so that each of the hydrogen atoms now have a complete valence shell. Each of the hydrogen atoms will share its electron with the nitrogen atom to complete its valence shell (Figure 2).

    Figure 2: Covalent bonding in a molecule of ammonia
    Figure 2 (CG11C1_004.png)

The above examples all show single covalent bonds, where only one pair of electrons is shared between the same two atoms. If two pairs of electrons are shared between the same two atoms, this is called a double bond. A triple bond is formed if three pairs of electrons are shared.

Exercise 3: Covalent bonding involving a double bond

How do oxygen atoms bond covalently to form an oxygen molecule?

Solution
  1. Step 1. Determine the electron configuration of the bonding atoms. :

    Each oxygen atom has 8 electrons, and their electron configuration is 1s22s22p41s22s22p4.

  2. Step 2. Determine the number of valence electrons for each atom and how many of these electrons are paired and unpaired. :

    Each oxygen atom has 6 valence electrons, meaning that each atom has 2 unpaired electrons.

  3. Step 3. Look to see how the electrons can be shared between atoms so that the outer energy shells of all the atoms are full. :

    Each oxygen atom needs two more electrons to complete its valence energy shell. Therefore two pairs of electrons must be shared between the two oxygen atoms so that both valence shells are full. Notice that the two electron pairs are being shared between the same two atoms, and so we call this a double bond (Figure 3).

    Figure 3: A double covalent bond in an oxygen molecule
    Figure 3 (CG11C1_005.png)

You will have noticed in the above examples that the number of electrons that are involved in bonding varies between atoms. We say that the valency of the atoms is different.

Definition 3: Valency
The number of electrons in the outer shell of an atom which are able to be used to form bonds with other atoms.

In the first example, the valency of both hydrogen and chlorine is one, therefore there is a single covalent bond between these two atoms. In the second example, nitrogen has a valency of three and hydrogen has a valency of one. This means that three hydrogen atoms will need to bond with a single nitrogen atom. There are three single covalent bonds in a molecule of ammonia. In the third example, the valency of oxygen is two. This means that each oxygen atom will form two bonds with another atom. Since there is only one other atom in a molecule of O2O2, a double covalent bond is formed between these two atoms.

Tip:

There is a relationship between the valency of an element and its position on the Periodic Table. For the elements in groups 1 to 4, the valency is the same as the group number. For elements in groups 5 to 7, the valency is calculated by subtracting the group number from 8. For example, the valency of fluorine (group 7) is 8-7=18-7=1, while the valency of calcium (group 2) is 2. Some elements have more than one possible valency, so you always need to be careful when you are writing a chemical formula. Often, if there is more than one possibility in terms of valency, the valency will be written in a bracket after the element symbol e.g. carbon (IV) oxide, means that in this molecule carbon has a valency of 4.

Covalent bonding and valency

  1. Explain the difference between the valence electrons and the valency of an element.
    Click here for the solution.
  2. Complete the table below by filling in the number of valence electrons and the valency for each of the elements shown:
    Table 1
    ElementNo. of valence electronsNo. of electrons needed to fill outer shellValency
    FF   
    ArAr   
    CC   
    NN   
    OO   
    Click here for the solution.
  3. Draw simple diagrams to show how electrons are arranged in the following covalent molecules:
    1. Water (H2OH2O)
    2. Chlorine (Cl2Cl2)
    Click here for the solution.

Properties of covalent compounds

Covalent compounds have several properties that distinguish them from ionic compounds and metals. These properties are:

  1. Melting and boiling points: The melting and boiling points of covalent compounds is generally lower than that for ionic compounds.
  2. Flexibility: Covalent compounds are generally more flexible than ionic compounds. The molecules in covalent compounds are able to move around to some extent and can sometimes slide over each other (as is the case with graphite, this is why the lead in your pencil feels slightly slippery). In ionic compounds all the ions are tightly held in place.
  3. Solubility in water: Covalent compounds generally are not very soluble in water.
  4. Electrical conductivity: Covalent compounds generally do not conduct electricity when dissolved in water. This is because they do not dissociate as ionic compounds do.

Lewis notation and molecular structure

Although we have used diagrams to show the structure of molecules, there are other forms of notation that can be used, such as Lewis notation and Couper notation. Lewis notation uses dots and crosses to represent the valence electrons on different atoms. The chemical symbol of the element is used to represent the nucleus and the core electrons of the atom.

So, for example, a hydrogen atom would be represented like this:

Figure 4
Figure 4 (CG11C1_006.png)

A chlorine atom would look like this:

Figure 5
Figure 5 (CG11C1_007.png)

A molecule of hydrogen chloride would be shown like this:

Figure 6
Figure 6 (CG11C1_008.png)

The dot and cross in between the two atoms, represent the pair of electrons that are shared in the covalent bond.

Exercise 4: Lewis notation: Simple molecules

Represent the molecule H2OH2O using Lewis notation

Solution

  1. Step 1. For each atom, determine the number of valence electrons in the atom, and represent these using dots and crosses. :

    The electron configuration of hydrogen is 1s11s1 and the electron configuration for oxygen is 1s22s22p41s22s22p4. Each hydrogen atom has one valence electron, which is unpaired, and the oxygen atom has six valence electrons with two unpaired.

    Figure 7
    Figure 7 (CG11C1_009.png)
  2. Step 2. Arrange the electrons so that the outermost energy level of each atom is full. :

    The water molecule is represented below.

    Figure 8
    Figure 8 (CG11C1_010.png)

Exercise 5: Lewis notation: Molecules with multiple bonds

Represent the molecule HCNHCN (hydrogen cyanide) using Lewis notation

Solution

  1. Step 1. For each atom, determine the number of valence electrons that the atom has from its electron configuration. :

    The electron configuration of hydrogen is 1s11s1, the electron configuration of nitrogen is 1s22s22p31s22s22p3 and for carbon is 1s22s22p21s22s22p2. This means that hydrogen has one valence electron which is unpaired, carbon has four valence electrons, all of which are unpaired, and nitrogen has five valence electrons, three of which are unpaired.

    Figure 9
    Figure 9 (CG11C1_011.png)
  2. Step 2. Arrange the electrons in the HCNHCN molecule so that the outermost energy level in each atom is full. :

    The HCNHCN molecule is represented below. Notice the three electron pairs between the nitrogen and carbon atom. Because these three covalent bonds are between the same two atoms, this is a triple bond.

    Figure 10
    Figure 10 (CG11C1_012.png)

Exercise 6: Lewis notation: Atoms with variable valencies

Represent the molecule H2SH2S (hydrogen sulphide) using Lewis notation

Solution

  1. Step 1. Determine the number of valence electrons for each atom. :

    Hydrogen has an electron configuration of 1s11s1 and sulphur has an electron configuration of 1s22s22p63s23p41s22s22p63s23p4. Each hydrogen atom has one valence electron which is unpaired, and sulphur has six valence electrons. Although sulphur has a variable valency, we know that the sulphur will be able to form 2 bonds with the hydrogen atoms. In this case, the valency of sulphur must be two.

    Figure 11
    Figure 11 (CG11C1_013.png)
  2. Step 2. Arrange the atoms in the molecule so that the outermost energy level in each atom is full. :

    The H2SH2S molecule is represented below.

    Figure 12
    Figure 12 (CG11C1_014.png)

Another way of representing molecules is using Couper notation. In this case, only the electrons that are involved in the bond between the atoms are shown. A line is used for each covalent bond. Using Couper notation, a molecule of water and a molecule of HCNHCN would be represented as shown in figures Figure 13 and Figure 14 below.

Figure 13: A water molecule represented using Couper notation
Figure 13 (CG11C1_015.png)
Figure 14: A molecule of HCNHCN represented using Couper notation
Figure 14 (CG11C1_016.png)

Atomic bonding and Lewis notation

  1. Represent each of the following atoms using Lewis notation:
    1. beryllium
    2. calcium
    3. lithium
    Click here for the solution.
  2. Represent each of the following molecules using Lewis notation:
    1. bromine gas (Br2Br2)
    2. carbon dioxide (CO2CO2)
    Click here for the solution.
  3. Which of the two molecules listed above contains a double bond?
    Click here for the solution.
  4. Two chemical reactions are described below.
    • nitrogen and hydrogen react to form NH3NH3
    • carbon and hydrogen bond to form a molecule of CH4CH4
    For each reaction, give:
    1. the valency of each of the atoms involved in the reaction
    2. the Lewis structure of the product that is formed
    3. the chemical formula of the product
    4. the name of the product
    Click here for the solution.
  5. A chemical compound has the following Lewis notation:
    Figure 15
    Figure 15 (CG11C1_018.png)
    1. How many valence electrons does element YY have?
    2. What is the valency of element YY?
    3. What is the valency of element XX?
    4. How many covalent bonds are in the molecule?
    5. Suggest a name for the elements XX and YY.
    Click here for the solution.

Ionic Bonding

The nature of the ionic bond

You will remember that when atoms bond, electrons are either shared or they are transferred between the atoms that are bonding. In covalent bonding, electrons are shared between the atoms. There is another type of bonding, where electrons are transferred from one atom to another. This is called ionic bonding.

Ionic bonding takes place when the difference in electronegativity between the two atoms is more than 1,7. This usually happens when a metal atom bonds with a non-metal atom. When the difference in electronegativity is large, one atom will attract the shared electron pair much more strongly than the other, causing electrons to be transferred from one atom to the other.

Definition 4: Ionic bond
An ionic bond is a type of chemical bond based on the electrostatic forces between two oppositely-charged ions. When ionic bonds form, a metal donates one or more electrons, due to having a low electronegativity, to form a positive ion or cation. The non-metal atom has a high electronegativity, and therefore readily gains electrons to form a negative ion or anion. The two ions are then attracted to each other by electrostatic forces.

Example 1:

In the case of NaClNaCl, the difference in electronegativity is 2,1. Sodium has only one valence electron, while chlorine has seven. Because the electronegativity of chlorine is higher than the electronegativity of sodium, chlorine will attract the valence electron of the sodium atom very strongly. This electron from sodium is transferred to chlorine. Sodium loses an electron and forms an Na+Na+ ion. Chlorine gains an electron and forms an Cl-Cl- ion. The attractive force between the positive and negative ion holds the molecule together.

The balanced equation for the reaction is:

Na + Cl NaCl Na + Cl NaCl
(1)

This can be represented using Lewis notation:

Figure 16: Ionic bonding in sodium chloride
Figure 16 (CG11C1_019.png)

Example 2:

Another example of ionic bonding takes place between magnesium (MgMg) and oxygen (OO) to form magnesium oxide (MgOMgO). Magnesium has two valence electrons and an electronegativity of 1,2, while oxygen has six valence electrons and an electronegativity of 3,5. Since oxygen has a higher electronegativity, it attracts the two valence electrons from the magnesium atom and these electrons are transferred from the magnesium atom to the oxygen atom. Magnesium loses two electrons to form Mg2+Mg2+, and oxygen gains two electrons to form O2-O2-. The attractive force between the oppositely charged ions is what holds the molecule together.

The balanced equation for the reaction is:

2 Mg + O 2 2 MgO 2 Mg + O 2 2 MgO
(2)

Because oxygen is a diatomic molecule, two magnesium atoms will be needed to combine with one oxygen molecule (which has two oxygen atoms) to produce two molecules of magnesium oxide (MgOMgO).

Figure 17: Ionic bonding in magnesium oxide
Figure 17 (CG11C1_020.png)

Tip:

Notice that the number of electrons that is either lost or gained by an atom during ionic bonding, is the same as the valency of that element

Ionic compounds

  1. Explain the difference between a covalent and an ionic bond.
    Click here for the solution
  2. Magnesium and chlorine react to form magnesium chloride.
    1. What is the difference in electronegativity between these two elements?
    2. Give the chemical formula for:
      1. a magnesium ion
      2. a chloride ion
      3. the ionic compound that is produced during this reaction
    3. Write a balanced chemical equation for the reaction that takes place.
    Click here for the solution
  3. Draw Lewis diagrams to represent the following ionic compounds:
    1. sodium iodide (NaINaI)
    2. calcium bromide (CaBr2CaBr2)
    3. potassium chloride (KClKCl)
    Click here for the solution

The crystal lattice structure of ionic compounds

Ionic substances are actually a combination of lots of ions bonded together into a giant molecule. The arrangement of ions in a regular, geometric structure is called a crystal lattice. So in fact NaClNaCl does not contain one NaNa and one ClCl ion, but rather a lot of these two ions arranged in a crystal lattice where the ratio of NaNa to ClCl ions is 1:1. The structure of a crystal lattice is shown in Figure 18.

Figure 18: The crystal lattice arrangement in an ionic compound (e.g. NaClNaCl)
Figure 18 (CG11C1_021.png)

Properties of Ionic Compounds

Ionic compounds have a number of properties:

  • Ions are arranged in a lattice structure
  • Ionic solids are crystalline at room temperature
  • The ionic bond is a strong electrical attraction. This means that ionic compounds are often hard and have high melting and boiling points
  • Ionic compounds are brittle, and bonds are broken along planes when the compound is stressed
  • Solid crystals don't conduct electricity, but ionic solutions do

Metallic bonds

The nature of the metallic bond

The structure of a metallic bond is quite different from covalent and ionic bonds. In a metal bond, the valence electrons are delocalised, meaning that an atom's electrons do not stay around that one nucleus. In a metallic bond, the positive atomic nuclei (sometimes called the 'atomic kernels') are surrounded by a sea of delocalised electrons which are attracted to the nuclei (Figure 19).

Definition 5: Metallic bond
Metallic bonding is the electrostatic attraction between the positively charged atomic nuclei of metal atoms and the delocalised electrons in the metal.
Figure 19: Positive atomic nuclei (+) surrounded by delocalised electrons ()
Figure 19 (CG11C1_022.png)

The properties of metals

Metals have several unique properties as a result of this arrangement:

  • Thermal conductors Metals are good conductors of heat and are therefore used in cooking utensils such as pots and pans. Because the electrons are loosely bound and are able to move, they can transport heat energy from one part of the material to another.
  • Electrical conductors Metals are good conductors of electricity, and are therefore used in electrical conducting wires. The loosely bound electrons are able to move easily and to transfer charge from one part of the material to another.
  • Shiny metallic lustre Metals have a characteristic shiny appearance and are often used to make jewellery. The loosely bound electrons are able to absorb and reflect light at all frequencies, making metals look polished and shiny.
  • Malleable and ductile This means that they can be bent into shape without breaking (malleable) and can be stretched into thin wires (ductile) such as copper, which can then be used to conduct electricity. Because the bonds are not fixed in a particular direction, atoms can slide easily over one another, making metals easy to shape, mould or draw into threads.
  • Melting point Metals usually have a high melting point and can therefore be used to make cooking pots and other equipment that needs to become very hot, without being damaged. The high melting point is due to the high strength of metallic bonds.
  • Density Metals have a high density because their atoms are packed closely together.

Activity: Building models

Using coloured balls and sticks (or any other suitable materials) build models of each type of bonding. Think about how to represent each kind of bonding. For example, covalent bonding could be represented by simply connecting the balls with sticks to represent the molecules, while for ionic bonding you may wish to construct part of the crystal lattice. Do some research on types of crystal lattices (although the section on ionic bonding only showed the crystal lattice for sodium chloride, many other types of lattices exist) and try to build some of these. Share your findings with your class and compare notes to see what types of crystal lattices they found. How would you show metallic bonding?

You should spend some time doing this activity as it will really help you to understand how atoms combine to form molecules and what the differences are between the types of bonding.

Figure 20
Khan academy video on bonding - 1

Chemical bonding

  1. Give two examples of everyday objects that contain..
    1. covalent bonds
    2. ionic bonds
    3. metallic bonds
    Click here for the solution
  2. Complete the table which compares the different types of bonding:
    Table 2
     CovalentIonicMetallic
    Types of atoms involved   
    Nature of bond between atoms   
    Melting Point (high/low)   
    Conducts electricity? (yes/no)   
    Other properties   
    Click here for the solution
  3. Complete the table below by identifying the type of bond (covalent, ionic or metallic) in each of the compounds:
    Table 3
    Molecular formulaType of bond
    H2SO4H2SO4 
    FeSFeS 
    NaINaI 
    MgCl2MgCl2 
    ZnZn 
    Click here for the solution
  4. Which of these substances will conduct electricity most effectively? Give a reason for your answer.
    Click here for the solution
  5. Use your knowledge of the different types of bonding to explain the following statements:
    1. Swimming during an electric storm (i.e. where there is lightning) can be very dangerous.
    2. Most jewellery items are made from metals.
    3. Plastics are good insulators.
    Click here for the solution

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 7: 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 8: 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 9: 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 4

Table 4: Table showing common compound ions and their formulae
Name of compound ion formula Name of compound ion formula
Carbonate CO32-CO32- Nitrate 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 10: 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 11: 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 12: 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 5
    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
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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 21

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
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  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
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  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)
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  5. Given the following Lewis structure, where X and Y each represent a different element...
    Figure 22
    Figure 22 (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?
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  6. A molecule of ethane has the formula C2H6C2H6. Which of the following diagrams (Couper notation) accurately represents this molecule?
    Figure 23
    Figure 23 (CG11C1_028.png)
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  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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