Understanding Principles of chemistry: Ionic bonding to Electrolysis: Edexcel IGCSE Chemistry Explained

Bonding and electrolysis form a single logical sequence: how atoms join together, what properties those structures create, and how electrical energy can pull ionic compounds apart again.

The four topics covered here, ionic bonding, covalent bonding, metallic bonding and electrolysis, account for some of the most reliable mark-earning questions on the edexcel IGCSE Chemistry exam. They test whether you understand structure at the particle level, and whether you can connect that understanding to observable properties like melting point, conductivity and hardness. These edexcel igcse chemistry revision notes break down each topic systematically, with the worked examples and practice questions you need to lock the ideas in place.

Ionic bonding

Ionic bonds form when metals transfer electrons to non-metals. The metal atom loses one or more electrons and becomes a positively charged ion (cation). The non-metal gains those electrons and becomes a negatively charged ion (anion). The electrostatic attraction between opposite charges holds the ions together in a giant ionic lattice.

You need to know the charges of common ions:

You also need to memorise these specific ions: Ag⁺, Cu²⁺, Fe²⁺, Fe³⁺, Pb²⁺, Zn²⁺, H⁺, OH^-, NH₄⁺, CO₃^2-, NO₃^-, SO₄^2-.

Dot-and-cross diagrams show the transfer of electrons. Draw the outer shell electrons of each atom before the transfer, then show the resulting ions with their charges. Only outer electrons need to be shown.

Properties of ionic compounds follow directly from their structure:

• High melting and boiling points because it takes a lot of energy to overcome the strong electrostatic attractions between ions throughout the giant lattice.
• Do not conduct electricity when solid because the ions are held in fixed positions and cannot move.
• Conduct electricity when molten or dissolved in water because the ions become free to move and carry charge.

Covalent bonding

A covalent bond forms when two atoms share a pair of electrons. The electrostatic attraction between the shared electrons and the nuclei of both atoms holds the bond together. Covalent bonding typically occurs between non-metal atoms.

The igcse 4ch1 specification requires you to draw dot-and-cross diagrams for several molecules:

Simple molecular structures (like water, methane, carbon dioxide) have low melting and boiling points. The covalent bonds within each molecule are strong, but the intermolecular forces of attraction between molecules are weak. It is these weak intermolecular forces that break when a simple molecular substance melts or boils, which is why relatively little energy is needed. In general, larger molecules (higher relative molecular mass) have stronger intermolecular forces, so their melting and boiling points are higher.

Giant covalent structures are different. Diamond, graphite and C₆₀ fullerene are all forms of carbon with giant covalent structures:

• Diamond: Each carbon atom is bonded to four others in a rigid tetrahedral arrangement. Very hard. Very high melting point. Does not conduct electricity (no free electrons or ions).
• Graphite: Each carbon is bonded to three others in flat layers. The fourth electron from each carbon is delocalised between the layers. This makes graphite a good electrical conductor. The layers are held together by weak intermolecular forces, so they slide over each other, making graphite soft and slippery.
• C₆₀ fullerene: A hollow sphere made of 60 carbon atoms arranged in pentagons and hexagons. Each carbon is bonded to three others, similar to graphite.

Covalent compounds generally do not conduct electricity because they have no free ions or delocalised electrons (graphite being the notable exception).

Metallic bonding

In a metal, the atoms are arranged in a regular lattice. The outer electrons are delocalised, meaning they are free to move throughout the structure. Metallic bonding is the electrostatic attraction between these delocalised (free) electrons and the positive metal ions.

This model explains the typical properties of metals:

• Good electrical conductivity: The delocalised electrons can flow through the structure when a potential difference is applied.
• Malleability: The layers of ions can slide over each other without breaking the metallic bonds, because the delocalised electrons adjust their positions around the ions.
• High melting points (generally): The electrostatic attraction between the positive ions and the sea of delocalised electrons is strong.

Electrolysis

Electrolysis is the decomposition of an ionic compound using an electric current. The compound must be either molten or dissolved in water so that the ions are free to move. An electrolyte is the liquid or solution that conducts electricity during electrolysis.

The key terminology: the anode is the positive electrode, and the cathode is the negative electrode. Cations (positive ions) migrate to the cathode. Anions (negative ions) migrate to the anode. At the cathode, cations gain electrons (reduction). At the anode, anions lose electrons (oxidation). The mnemonic OILRIG helps: Oxidation Is Loss, Reduction Is Gain (of electrons).

You need to know the products for several specific electrolysis experiments:

Writing ionic half-equations:

At the cathode (reduction): Pb²⁺ + 2e^- → Pb

At the anode (oxidation): 2Br^- → Br₂ + 2e^-

For aqueous solutions, the products depend on what is in the solution. If the metal is more reactive than hydrogen, hydrogen gas is produced at the cathode instead of the metal. At the anode, if a halide ion (Cl^-, Br^-, I^-) is present, the halogen is produced; otherwise, oxygen is produced from hydroxide ions.

Common mistakes

• Drawing dot-and-cross diagrams without charges on the ions. The diagram must show the electron transfer and the resulting charge (e.g. [Na]⁺ and [Cl]^-). Missing the charge is a lost mark.
• Saying ionic compounds "contain molecules." They do not. They contain ions arranged in a giant lattice. The formula NaCl gives the ratio of ions (1:1), not a molecular unit.
• Confusing why graphite conducts. It is not because of the layers. It is because of the delocalised electrons between the layers. The layers explain why graphite is soft, not why it conducts.
• Writing "atoms" instead of "ions" when describing electrolysis. The species that move through the electrolyte are ions, not atoms. They become atoms (or molecules) only after gaining or losing electrons at the electrodes.
• Getting the electrodes mixed up. Cations go to the cathode. Anions go to the anode. "PANIC" helps: Positive Anode, Negative Is Cathode.

How these topics connect in the exam

The edexcel igcse chemistry principles of chemistry: ionic bonding to electrolysis topics form a coherent sequence that the exam often tests as a chain. A question might start with ionic bonding (draw the dot-and-cross diagram for sodium chloride), then move to properties (explain why it conducts when molten), and finish with electrolysis (write the half-equations for what happens at each electrode). Treating these topics as connected rather than isolated makes the igcse 4ch1 principles of chemistry: ionic bonding to electrolysis material much easier to revise and recall under timed conditions.

You should also expect edexcel igcse chemistry practice questions that blend bonding with properties. For instance: "Diamond and sodium chloride both have high melting points. Explain why their structures lead to this property, and describe one key difference between them." The answer requires you to discuss giant covalent structure (diamond) alongside giant ionic lattice (NaCl), using the correct bonding language for each.

Self-check questions

1. Draw a dot-and-cross diagram for the formation of magnesium oxide (MgO) from magnesium and oxygen atoms. Show the charges on the resulting ions.
2. Explain why diamond has a very high melting point but methane has a very low melting point, even though both substances contain only covalent bonds.
3. Explain why metals are good conductors of electricity.
4. Write ionic half-equations for the electrolysis of molten lead(II) bromide. Identify which reaction is oxidation and which is reduction.
5. During the electrolysis of copper(II) sulfate solution with inert electrodes, the blue colour fades. Explain why.

These principles of chemistry: ionic bonding to electrolysis edexcel igcse topics form a coherent unit. Ionic bonding explains how certain compounds are built; covalent bonding explains others; metallic bonding covers the metals. Electrolysis then uses electrical energy to reverse what ionic bonding created. If you can trace that logical thread from one topic to the next, the edexcel igcse chemistry explained here will stick far better than isolated facts ever could.

For additional edexcel igcse chemistry notes and practice questions on bonding and electrolysis, explore the Green Bridge CBT platform, where these topics are organised to match the specification exactly.