Chemistry · Bonding and structure
Given an unknown substance's properties, how do I work out whether its bonding is ionic or covalent?
Three tests, and you usually only need two. Melting point, electrical conductivity, and solubility in water. A question giving you a table of properties is asking you to read exactly these.
The decision
High melting point, conducts when molten or dissolved but not when solid, usually dissolves in water → ionic.
Low melting point, does not conduct in any state, usually does not dissolve in water → simple covalent molecules.
Very high melting point, does not conduct (except graphite), does not dissolve → giant covalent.
That third category is why the first two tests are not always enough, and giant covalent structures are where most marks are lost.
Why each test works
Melting point measures how much energy is needed to separate the particles. In an ionic lattice every ion is held by strong electrostatic attraction to all its neighbours, so you must overcome a vast number of strong forces — hence high melting points.
In a simple covalent substance the covalent bonds are strong, but they are inside the molecules. Melting only has to overcome the weak forces between molecules. The bonds themselves do not break.
This distinction decides the question. Saying "covalent bonds are weak" is wrong and is penalised. They are strong; there just are not any holding one molecule to the next.
Conductivity requires charged particles free to move. Ionic solids have ions locked in a lattice, so they do not conduct. Melt or dissolve them and the ions become mobile — now they do. Covalent molecules have no charged particles at all, so they never conduct, in any state.
The exceptions examiners use
Graphite is giant covalent and conducts, because each carbon bonds to only three others, leaving one delocalised electron per atom free to move. It is also soft and slippery, because the layers have only weak forces between them. Diamond, with four bonds per carbon, does neither.
Metals conduct as solids, which neither ionic nor covalent substances do. Delocalised electrons move through a lattice of positive ions. If a table shows conduction in the solid state, the answer is metallic and not the two you were choosing between.
Writing the answer
A question giving you four substances and asking you to identify the bonding wants the property and the reason: "Substance B conducts when molten but not when solid, so it is ionic — the ions are fixed in the lattice as a solid and become free to move when molten."
Property, then "so", then the reason. The structure of that sentence is the mark scheme.
Working from a past paper? Mark your attempt against the published mark scheme before reading an answer like this one. Finding out where you lost the marks is worth more than seeing a correct solution, because the correct solution rarely tells you what you did instead.
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