Ionic Bonding and Ionic Compounds
In 1913, William and Lawrence Bragg used X-rays to map the NaCl crystal, revealing a perfect lattice of Na⁺ and Cl⁻ ions separated by just 0.28 nanometres.
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Q1 · Think about solid table salt versus salt dissolved in water, which one do you think would conduct electricity, and why might there be a difference?
Q2 · Why do you think opposite charges attract each other, and how might that idea explain why ionic compounds like salt are held together so strongly?
● Know
- How ionic bonds form through electron transfer
- The structure of an ionic lattice
- The properties of ionic compounds (high mp, brittle, conducts when dissolved/melted)
● Understand
- Why ionic compounds have high melting points
- Why solid NaCl does not conduct electricity but dissolved NaCl does
- Why ionic compounds are brittle
● Can do
- Draw and explain the formation of an ionic bond
- Predict the properties of an ionic compound from its structure
- Explain the conductivity misconception for solid ionic compounds
You already know that metals lose electrons to form positive cations and non-metals gain electrons to form negative anions. An ionic bond is what happens next: once these oppositely charged ions exist, they pull towards each other through a strong electrostatic attraction, the force between a positive charge and a negative charge. This attraction is the ionic bond. It is not a single link between just two ions; in a real ionic compound, every positive ion is pulled towards all the nearby negative ions, and every negative ion is pulled towards all the nearby positive ions.
In sodium chloride, sodium transfers its single outer electron to chlorine. Sodium becomes Na⁺ and chlorine becomes Cl⁻, and the strong attraction between Na⁺ and Cl⁻ is the ionic bond. Because each ion is attracted to many oppositely charged neighbours at once, ionic bonding holds enormous numbers of ions together very firmly. This is why ionic compounds are solid at room temperature and need a great deal of energy to pull apart, as you will see when you look at their lattice structure and properties next.
Magnesium oxide (MgO): magnesium loses 2 electrons to become Mg²⁺, and oxygen gains 2 electrons to become O²⁻. The attraction between the doubly charged Mg²⁺ and O²⁻ ions is even stronger than the attraction between the singly charged Na⁺ and Cl⁻ in salt, which is why MgO has a much higher melting point (2852 °C) than NaCl (801 °C).
The salt harvested at the Dampier and Port Hedland solar salt fields in Western Australia is held together entirely by ionic bonds between Na⁺ and Cl⁻ ions. Australia exports millions of tonnes of this salt each year for use in the chemical industry, where the strong ionic bonding must first be broken (by dissolving or melting) before the sodium and chlorine can be used.
Ionic compounds don't exist as individual pairs of ions, they form a giant three-dimensional ionic lattice. In sodium chloride, each Na⁺ ion is surrounded by 6 Cl⁻ ions, and each Cl⁻ is surrounded by 6 Na⁺ ions. This alternating arrangement maximises attractive forces between opposite charges while minimising repulsive forces between like charges. Millions of ions stack into this regular, repeating pattern, a crystal. The regularity is why salt crystals are always cubic in shape.
The lattice is very strong because every ion is held by multiple electrostatic forces simultaneously, it takes a great deal of energy to pull all those ions apart. This gives ionic compounds their characteristic high melting points (NaCl: 801 °C). However, the lattice is also brittle: if you hit a salt crystal with a hammer, the layers shift and like charges align, creating a line of repulsion that shatters the crystal along a cleavage plane. This brittleness is a key limitation of ionic materials in engineering.
Calcium fluoride (CaF₂, fluorite) forms beautiful cubic crystals. Each Ca²⁺ is surrounded by 8 F⁻ ions; each F⁻ by 4 Ca²⁺ ions. The crystal is so regular that it cleaves perfectly along planes when struck, producing flat triangular faces, a direct consequence of the regular lattice geometry.
Alumina (Al₂O₃) in its corundum crystal form has an ionic lattice strong enough to be used as grinding wheels and sandpaper at Australian manufacturing plants. Ruby and sapphire are corundum with trace impurity ions, precious gemstones and industrial abrasives both arise from the same ionic lattice structure.
The ionic lattice structure directly explains every bulk property of ionic compounds. They are hard but brittle because the lattice resists deformation but shatters along cleavage planes. They have high melting points because many strong electrostatic bonds must be broken simultaneously. They do not conduct electricity as solids because the ions are locked in fixed positions, charge cannot flow. However, when ionic compounds are dissolved in water or melted, the ions are free to move, making them good conductors of electricity, a property crucial to batteries, electrolysis, and nerve cells.
Many ionic compounds are soluble in water because water molecules are polar, their slightly positive hydrogen ends attract anions, and their slightly negative oxygen end attracts cations, pulling the lattice apart ion by ion. This is why NaCl dissolves readily in water but MgO does not, the higher charge density of Mg²⁺ and O²⁻ creates a stronger lattice that water molecules cannot easily disrupt. Each property follows logically from the structure, understanding the structure means you can predict all the properties.
Sodium chloride dissolved in water produces a conducting solution: 1 mol/L NaCl solution has a conductivity of about 10 S/m. Solid NaCl has near-zero conductivity. This switching behaviour, insulator when solid, conductor when dissolved, is exploited in electrolysis cells used by Orica (Newcastle, NSW) to produce chlorine for water treatment.
Orica's Kooragang Island facility near Newcastle uses the chlor-alkali process, electrolysis of dissolved NaCl (brine), to produce chlorine gas and sodium hydroxide. These two ionic compounds are feedstocks for PVC plastic, paper bleaching, and water purification across NSW and Queensland.
Ionic compounds are hard but because the lattice shatters along cleavage planes. They have high points because many strong electrostatic bonds must be broken at once. As solids they do not conduct electricity because the ions are locked in fixed . However, when dissolved or , the ions become free to move and can carry charge. Many ionic compounds dissolve in water because water molecules are .
At the start of this lesson, you heard that diamond and graphite are both pure carbon, yet diamond is the hardest natural substance on Earth while graphite is soft enough to write with. That enormous difference in properties comes entirely from how the electrons are shared, not from the atoms themselves.
Now that you've worked through the lesson, how has your understanding of covalent bonding changed? Can you now explain why the type of bond, not just the elements involved, is what determines a substance's properties?
Q1. Explain how an ionic bond forms between sodium and chlorine. Include the terms 'electron transfer', 'cation', and 'anion'.
Q2. A student dissolves NaCl in water and finds it conducts electricity. They then test solid NaCl and find it does NOT conduct. Explain both results using your knowledge of ionic structure.
Q3. Evaluate the suitability of an ionic compound (e.g., NaCl) as a structural building material. Refer to at least THREE properties in your response.
Model answers (click to reveal)
SAQ 1 (2 marks)
Model answer: A sodium atom has one electron in its outer shell, while a chlorine atom needs one more electron to fill its outer shell. During electron transfer, the sodium atom loses its outer electron to the chlorine atom. Sodium then has more protons than electrons, so it becomes a positively charged cation (Na⁺), and chlorine gains an electron to become a negatively charged anion (Cl⁻). The ionic bond is the strong electrostatic attraction between the oppositely charged Na⁺ cation and Cl⁻ anion, which holds them together in the compound.
SAQ 2 (3 marks)
Model answer: For a substance to conduct electricity, it must contain charged particles that are free to move and carry charge. In solid sodium chloride, the Na⁺ and Cl⁻ ions are locked tightly in fixed positions in the ionic lattice and cannot move, so even though charged particles are present, there are no mobile charge carriers and the solid does not conduct. When NaCl dissolves in water, the lattice breaks apart and the Na⁺ and Cl⁻ ions separate and become free to move through the solution. These mobile ions can now carry charge, so the salt solution conducts electricity.
SAQ 3 (3 marks)
Model answer: An ionic compound such as NaCl is not suitable as a structural building material. First, ionic compounds are brittle: when struck, layers of ions shift so that like charges line up and repel, causing the material to shatter rather than bend, so it could not safely bear structural loads. Second, NaCl is soluble in water, so rain and moisture would dissolve the structure over time, making it weather very poorly outdoors. A useful property is its high melting point (801 °C), which means it would not soften or deform in normal hot weather, but this single advantage does not outweigh its brittleness and solubility. Overall, NaCl is unsuitable as a building material because it is too brittle and dissolves in water.