Ionic Bonding and Structure (OCR A-Level Chemistry A): Revision Notes

Ionic Bonding and Structure

What is ionic bonding?

Common cations include:

• Metal ions such as $\text{Na}^+$, $\text{Ca}^{2+}$, and $\text{Al}^{3+}$
• Ammonium ions ($\text{NH}_4^+$)

Common anions include:

• Non-metal ions such as $\text{Cl}^-$ and $\text{O}^{2-}$
• Polyatomic ions such as $\text{NO}_3^-$, $\text{SO}_4^{2-}$, and $\text{CO}_3^{2-}$

Formation of ionic compounds through electron transfer

The electron transfer model

The simplest ionic compounds form when metal atoms transfer their outer-shell electrons to non-metal atoms. This electron transfer process results in the formation of ions with stable electron configurations.

During ionic bonding:

• Outer-shell electrons from metal atoms are transferred to the outer shell of non-metal atoms
• Both positive and negative ions are created in the process
• The ions produced typically achieve electron configurations matching the nearest noble gas
• These stable configurations make the ions energetically favorable

Using dot-and-cross diagrams

Dot-and-cross diagrams provide a clear visual representation of ionic bonding. In these diagrams, electrons in the original atoms are shown as either dots or crosses, making it straightforward to track which electrons came from which atom and to determine the charge on each ion formed.

Example: potassium fluoride ($\text{KF}$)

Example: magnesium chloride ($\text{MgCl}_2$)

Giant ionic lattice structure

What is a giant ionic lattice?

While it is convenient to examine ionic bonding between just a few ions, in reality each ion attracts oppositely charged ions from all directions. This creates an extensive three-dimensional structure called a giant ionic lattice, containing billions upon billions of ions. The actual number of ions present is determined only by the size of the crystal.

Structure of sodium chloride

Sodium chloride provides an excellent example of a giant ionic lattice structure:

• Each $\text{Na}^+$ ion is surrounded by six $\text{Cl}^-$ ions
• Each $\text{Cl}^-$ ion is surrounded by six $\text{Na}^+$ ions
• Every ion is surrounded by oppositely charged ions, forming the extended giant ionic lattice

Properties of ionic compounds

The physical properties of ionic compounds can be understood by examining both the giant ionic lattice structure and the nature of ionic bonding within these structures.

High melting and boiling points

Nearly all ionic compounds exist as solids at room temperature. At room temperature, there is insufficient energy available to overcome the powerful electrostatic forces of attraction between the oppositely charged ions throughout the giant ionic lattice.

To melt or boil an ionic compound, high temperatures are required to provide the large quantity of energy needed to overcome these strong electrostatic attractions between the ions. Consequently, ionic compounds typically have very high melting and boiling points.

Factors affecting melting points

Ionic compound	Ions	Melting point / °C
$\text{NaF}$	$\text{Na}^+$ and $\text{F}^-$	993
$\text{CaF}_2$	$\text{Ca}^{2+}$ and $\text{F}^-$	1423
$\text{Na}_2\text{O}$	$\text{Na}^+$ and $\text{O}^{2-}$	1275
$\text{CaO}$	$\text{Ca}^{2+}$ and $\text{O}^{2-}$	2614

Comparing these values shows that melting points are higher for lattices containing ions with greater ionic charges. For instance, calcium oxide (with both doubly charged ions) has a much higher melting point than sodium fluoride (with singly charged ions).

Solubility in water

Many ionic compounds dissolve readily in polar solvents, particularly water. When dissolution occurs, polar water molecules break down the ionic lattice structure and surround each ion individually in solution. This process is called hydration.

However, in compounds composed of ions with large charges, the ionic attraction within the lattice may be too strong for water molecules to overcome and break down the lattice structure effectively. Such compounds will have very low solubility.

Comparing solubility values

Ionic compound	Ions	Solubility at 20°C / mol dm⁻³
$\text{NaCl}$	$\text{Na}^+$ and $\text{Cl}^-$	6.1
$\text{CaCl}_2$	$\text{Ca}^{2+}$ and $\text{Cl}^-$	0.62
$\text{Na}_2\text{CO}_3$	$\text{Na}^+$ and $\text{CO}_3^{2-}$	2.0
$\text{CaCO}_3$	$\text{Ca}^{2+}$ and $\text{CO}_3^{2-}$	$1.3 \times 10^{-4}$

Sodium chloride shows the highest solubility, while calcium carbonate (containing doubly charged ions) has extremely low solubility.

Important considerations about solubility

The solubility of an ionic compound in water therefore depends on the relative strengths of the attractions within the giant ionic lattice compared to the attractions between ions and water molecules. Generally, as ionic charge increases, the attractions within the giant ionic lattice become dominant, and solubility decreases.

Electrical conductivity

The ability of ionic compounds to conduct electricity depends on whether the ions are free to move and act as mobile charge carriers.

Solid state behavior

In the solid state, an ionic compound does not conduct electricity:

• The ions are held in fixed positions within the giant ionic lattice
• There are no mobile charge carriers available
• The compound acts as a non-conductor (insulator) of electricity

Liquid and aqueous states

When melted or dissolved in water, an ionic compound becomes able to conduct electricity:

• The solid ionic lattice breaks down completely
• The ions are now free to move throughout the liquid or solution
• These mobile ions act as charge carriers
• The compound becomes a conductor of electricity in these states

Summary of ionic compound properties

Real-world application: ionic compounds in teeth

Your body relies on ionic compounds for structural support. The main ionic compound in bones and teeth is calcium hydroxyapatite, which can be represented by the formula $\text{Ca}_5(\text{PO}_4)_3\text{OH}$. This compound serves as the primary constituent of tooth enamel.

Unfortunately, ions in tooth enamel can be removed under acidic conditions. Once the enamel breaks down, gaps form that allow tooth decay to develop underneath. Saliva helps to neutralize acidic food and can partially replace lost ions, but this may not always be sufficient protection.

Your water supply may also contain added fluoride depending on local regulations and policies in your area.

Remember!