5.2 Ionic bonding

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.

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 $\mathrm{NaCl}$ , 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 ${\mathrm{Na}}^{+}$ ion. Chlorine gains an electron and forms an ${\mathrm{Cl}}^{-}$ ion. The attractive force between the positive and negative ion holds the molecule together.

The balanced equation for the reaction is:

This can be represented using Lewis notation:

Example 2:

Another example of ionic bonding takes place between magnesium ( $\mathrm{Mg}$ ) and oxygen ( $\mathrm{O}$ ) to form magnesium oxide ( $\mathrm{MgO}$ ). 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 ${\mathrm{Mg}}^{2+}$ , and oxygen gains two electrons to form ${\mathrm{O}}^{2-}$ . The attractive force between the oppositely charged ions is what holds the molecule together.

The balanced equation for the reaction is:

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 ( $\mathrm{MgO}$ ).

Ionic compounds

1. Explain the difference between a covalent and an ionic bond.
   
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.
3. Draw Lewis diagrams to represent the following ionic compounds:
   1. sodium iodide ( $\mathrm{NaI}$ )
   2. calcium bromide ( $\mathrm{CaBr}{}_2$ )
   3. potassium chloride ( $\mathrm{KCl}$ )

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 $\mathrm{NaCl}$ does not contain one $\mathrm{Na}$ and one $\mathrm{Cl}$ ion, but rather a lot of these two ions arranged in a crystal lattice where the ratio of $\mathrm{Na}$ to $\mathrm{Cl}$ ions is 1:1. The structure of a crystal lattice is shown in [link] .

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