Structure–property links, metals and alloys

A material’s name alone is not an explanation of its properties. Identify what particles it contains, how they are arranged and what must move or separate in the change being considered.

Ionic solids: attraction and mobility answer different questions

In a giant ionic lattice, strong electrostatic attractions act between oppositely charged ions. A lot of energy is needed to overcome these attractions on melting, so ionic compounds generally have high melting points.

For electrical conduction, ask whether charged particles can travel through the sample. Solid sodium chloride contains ions, but they are fixed in their lattice positions and cannot carry charge through it. In molten sodium chloride the ions can move and carry current. When an ionic compound dissolves in water, its aqueous ions can also move. Dissolving is not melting: an insoluble solid placed in water does not automatically give a conducting solution of that compound.

Molecular substances: distinguish two levels of attraction

Strong covalent bonds hold atoms together within each molecule. Weaker intermolecular attractions act between the molecules. Melting or boiling separates molecules by overcoming intermolecular attractions; the covalent bonds remain intact. This is why a simple molecular substance can have a low boiling point despite strong bonds within its molecules.

Simple molecular substances usually do not conduct electricity because they have no mobile ions or electrons. Use the actual species in the stated sample: if a molecular substance reacts with water to produce ions, its solution need not have the conductivity of the pure substance.

Classify the particles before choosing a material

An element contains only one type of atom. A compound contains atoms of different elements chemically bonded in a fixed ratio. A mixture contains substances that are not chemically bonded to one another and can have varying proportions.

A molecule can contain only one element: O₂ is an element despite its bonded pairs. Identical H₂O molecules make a pure compound; a sample of both O₂ and H₂O molecules is a mixture. Mixture components can be separated by physical methods. An ionic compound is also a compound, but its formula describes a lattice ratio rather than a molecule.

Metals and alloys

Metals are generally solids with high melting and boiling points, are malleable (can be shaped by hammering), and conduct heat and electricity well. These are general properties, not a guarantee for every metal: mercury is liquid at room temperature. Choose a metal from the properties required for the job and any supplied measurements.

An alloy is a mixture of a metal with one or more other elements. Brass contains copper and zinc; stainless steel contains iron with other elements, including chromium. The proportions can vary, so an alloy is not represented by one fixed compound formula.

In the simple particle model of a pure metal, similar-sized atoms form regular layers that can slide past one another when a force is applied. Different-sized atoms in an alloy disrupt that regular arrangement and make sliding harder. This explains why an alloy can be harder than the pure metal. It does not establish every property of every alloy: use supplied data for conductivity, corrosion resistance and melting point. This layer model does not require the metallic-bonding or delocalised-electron model.

Choose using the required property

A saucepan base must transfer heat to its contents; its handle must limit heat transfer to the hand. Good thermal conduction supports a metal base, not a metal handle solely because “metals are strong”. For a shaped metal component, malleability may help manufacture; for a wear-resistant part, hardness may matter more. State the property and why it serves that purpose.

An illustrative test compares two candidate materials:

Choose one for a support that must resist bending, then cite the decisive observation. Would the same observation justify choosing the best heat conductor?

Build a complete structure–property explanation

The following comparison is formative writing practice. Use the worked solution to improve your explanation; self-marking is not an assessed result.

Deduce a structure from a combination of observations

Now compare three unknowns using the raw observations. Assume each is a pure substance and choose among an ionic, simple molecular or metallic structure.

For each sample write a conclusion, the most useful observation, and the particle explanation. Do not use high melting point alone to distinguish A from C.

Before topic practice, check that each explanation connects a particular observation to a particular particle model. For a dot-and-cross error, return to electron accounting; for a conductivity error, ask what carries charge and whether it can move.