Structure and Bonding - Ionic Lattices

1. Ionic Bonding in Context

An ionic compound is made of millions of oppositely charged ions arranged in a regular pattern. Think of it like a giant 3D chess board where positive ions (like Na+Na+) are surrounded by negative ions (like Cl−Cl−), and vice versa.

The ions are held together by strong electrostatic forces of attraction that pull equally in every direction throughout the crystal. This attraction in all directions is called ionic bonding and creates a rigid, giant lattice structure.

2. The Sodium Chloride Lattice

Sodium chloride (table salt) is the most common example you'll study at GCSE level.

• Each Na+Na+ ion is surrounded by six Cl−Cl− ions

• Each Cl−Cl− ion is surrounded by six Na+Na+ ions

• This pattern repeats millions of times in all directions

• Salt crystals look cubic because the internal arrangement is cubic

Visualising NaCl

Since real lattices are three-dimensional and contain millions of ions, chemists use different types of diagrams to help us understand them:

1. Dot-and-cross diagrams – show how electrons transfer when ions form

2. 2-D stick diagrams – show alternating ions in a flat view

3. Ball-and-stick models – show the 3D arrangement and relative sizes of ions

4. Space-filling diagrams – show how tightly the ions pack together

3. Strengths and Limitations of Lattice Diagrams

Understanding what each diagram can and cannot show helps you choose the right one:

Dot-and-cross

• Strength: clearly shows electron loss and gain during ion formation

• Limitation: gives no information about 3D structure or how big the lattice is

2-D stick

• Strength: quick way to show the alternating pattern of charges

• Limitation: makes it look like flat layers that could slide easily

Ball-and-stick

• Strength: shows bond angles and how many ions surround each ion; easy to count ions

• Limitation: the sticks aren't real bonds; makes distances look longer than they are

Space-filling

• Strength: most realistic view of how ions actually pack together

• Limitation: hides ions at the back; difficult to count ions or see the structure clearly

4. Recognising Ionic Compounds from Diagrams

You can identify an ionic compound from its diagram if it shows:

• A regular, repeating pattern of positive and negative ions

• No separate molecules – just a continuous network of ions

• Charges written as symbols like Mg2+Mg2+ rather than shared electrons

If these features are missing, the compound is probably covalent or metallic instead.

5. Working Out Empirical Formula from Lattice Pictures

The empirical formula tells you the simplest whole-number ratio of ions in the lattice. Even though a crystal contains millions of ions, the overall charge must be zero. The empirical formula reflects the ratio of ions needed to balance the charges.

Method:

1. Look at the diagram of the ionic lattice.

2. Count the total number of positive ions shown.

3. Count the total number of negative ions shown.

4. Express these two numbers as a ratio and simplify it to the lowest possible whole numbers.

For any diagram of sodium chloride, the number of Na+Na+ ions will equal the number of Cl−Cl− ions, giving a 1:1 ratio. The empirical formula is therefore NaCl.

Key terms

Ionic bond - The electrostatic attraction between oppositely charged ions

Lattice - A regular, repeating three-dimensional arrangement of ions

Electrostatic attraction - The force of attraction between oppositely charged particles

Empirical formula - The simplest whole-number ratio of ions in a compound

Coordination number - The number of oppositely charged ions immediately surrounding a given ion

Dot-and-cross diagram - A diagram that shows electrons as dots or crosses to show how ions form

Ball-and-stick model - A representation using spheres for ions connected by sticks to show the 3D structure

Worked example

Question: A ball-and-stick model of a small part of the magnesium chloride lattice is shown. It contains 4 magnesium (Mg2+Mg2+) ions and 8 chloride (Cl−Cl−) ions.

Deduce the empirical formula of magnesium chloride.

Solution:

Step 1: State the number of each type of ion shown in the model. Number of Mg2+Mg2+ ions = 4 Number of Cl−Cl− ions = 8

Step 2: Write the ratio of positive ions to negative ions. Ratio of Mg2+Mg2+ to Cl−Cl− = 4 : 8

Step 3: Simplify the ratio to its lowest whole numbers by dividing both sides by the smallest number (in this case, 4). 4 ÷ 4 = 1 8 ÷ 4 = 2 Simplest ratio = 1 : 2

Answer: The empirical formula is MgCl2MgCl2​.

This makes sense, as each Mg2+Mg2+ ion requires two Cl−Cl− ions to balance the charges.

Road Salt in Winter

Every winter, councils spread road salt (NaCl) on icy roads. If you look at the salt grains with a magnifying glass, they appear cubic - this reflects the cubic structure of the internal NaCl lattice.

When the salt dissolves in water on the road, the rigid lattice breaks apart into separate Na+Na+ and Cl−Cl− ions. These mobile ions can conduct electricity and, more importantly for winter driving, they lower the freezing point of water so ice melts at temperatures below 0°C.

Aim: To model the structure of an ionic lattice and understand why ionic compounds are hard but brittle

Apparatus:

• Two colours of polystyrene spheres (about 20 of each colour)

• Cocktail sticks

• Magnifying glass (optional)

Method:

1. Choose one colour to represent Na+Na+ ions and another for Cl−Cl− ions

2. Build a 3 × 3 × 3 cube by connecting the spheres with cocktail sticks

3. Make sure you alternate the colours in all three dimensions

4. Gently press on opposite faces of your model to test its rigidity

5. Try to shift one layer sideways to see what happens

Safety: Cocktail sticks have sharp points - handle with care to avoid injury

Observations: The model feels rigid when pressed but may break apart when layers are shifted

Conclusion: This demonstrates why real ionic compounds are hard (rigid structure) but brittle (break when like-charges are forced together)

Comparison table