This topic is about using evidence such as melting point, conductivity, and solubility to infer what particles a substance contains and how those particles are held together.

What this skill involves

In this part of the course, you are not just recalling properties. You are interpreting data to decide whether a substance has ionic, metallic, giant covalent, or simple molecular structure. The key is to connect each property to the particles present and whether those particles can move.

A strong answer identifies both:

• the type of structure

• the type of bonding

• and the evidence from the data

For example, conductivity data tells you whether there are mobile charged particles, while melting and boiling temperatures suggest whether the forces holding the substance together are relatively weak or very strong.

Giant lattices include giant ionic, giant metallic, and giant covalent structures.

Main structure types and their data patterns

Giant ionic lattice

A giant ionic lattice contains positive and negative ions held by strong electrostatic attraction.

A labeled model of the sodium chloride crystal lattice, showing alternating $\mathrm{Na^+}$ and $\mathrm{Cl^-}$ ions in a repeating 3D arrangement. This directly visualizes the “giant ionic lattice” idea: strong electrostatic attractions act throughout the entire structure rather than within discrete molecules. Source

Typical evidence:

• high melting temperature

• often brittle

• does not conduct electricity when solid

• does conduct when molten

• often does conduct in aqueous solution

• often soluble in water

The key clue is the change in conductivity. In a solid ionic lattice, ions are fixed in place, so they cannot carry charge. When molten or dissolved, the ions become mobile.

Simple molecular structure

A simple molecular substance contains discrete molecules. The covalent bonds inside each molecule are strong, but the forces between molecules are much weaker.

Typical evidence:

• low melting and boiling temperatures

• often gases, liquids, or soft solids at room temperature

• does not conduct in solid or liquid form

• often volatile

This pattern suggests there are no mobile ions and no delocalized electrons. A low boiling temperature is especially important because it shows that only weak attractions are being overcome.

Giant covalent structure

A giant covalent structure is a network of atoms joined by covalent bonds throughout the whole lattice.

Typical evidence:

• very high melting temperature

• usually very hard

• insoluble

• usually does not conduct electricity

This is because many strong covalent bonds must be broken to melt the substance. Most giant covalent substances do not conduct because they do not have mobile charged particles.

A very important exception is graphite and also graphene, which conduct because they have delocalized electrons.

A labeled structural diagram of graphite showing stacked hexagonal carbon layers (ABAB stacking) with the interlayer spacing marked. The layered arrangement helps explain graphite’s unusual property set: strong covalent bonding within sheets but much weaker forces between sheets, while delocalized electrons within the sheets enable electrical conductivity. Source

Metallic structure

A metallic structure consists of positive metal ions in a lattice with delocalized electrons.

A conceptual diagram illustrating metallic bonding, where positive metal ion cores form a lattice and the valence electrons are delocalized and mobile throughout the structure. This model links directly to data interpretation: metals conduct electricity in the solid state because these delocalized electrons can move and carry charge. Source

Typical evidence:

• usually high melting temperature

• conducts electricity when solid

• conducts when molten

• often malleable rather than brittle

• generally insoluble in water

The most distinctive clue is conductivity in the solid state. Metals conduct because delocalized electrons are free to move through the structure.

How to interpret data systematically

A good method is to work through the data in stages.

1. Look at melting and boiling temperatures

This often gives the first clue:

• low values suggest simple molecular

• high values suggest a giant structure

Do not treat this as an absolute rule. Some substances have intermediate values, so you must still use the rest of the data.

2. Look at electrical conductivity

This is often the most useful test.

• conducts when solid and molten: usually metallic

• does not conduct when solid, but conducts when molten: usually ionic

• does not conduct in any state: often simple molecular or giant covalent

• conducts despite being giant covalent: think about graphite or graphene

Always explain conductivity in terms of mobile ions or delocalized electrons.

3. Look at solubility and solution behavior

Solubility can support your answer, but it should not be used alone.

• if a substance dissolves in water and the solution conducts, this supports ionic structure

• if it is insoluble and has a very high melting temperature, it may be giant covalent or metallic

• if it has a low melting temperature and does not conduct, it is likely simple molecular

The most useful phrase is not just “it dissolves,” but “it forms a conducting solution because ions are present.”

4. Use mechanical properties if given

Properties such as hardness, brittleness, and malleability can separate similar options.

• brittle supports ionic

• malleable supports metallic

• very hard supports giant covalent

• soft supports simple molecular or layered structures such as graphite

Recognizing the most common patterns

Some combinations of data appear repeatedly in exam questions.

Pattern 1: High melting temperature, no conductivity as a solid, conductivity when molten

This strongly indicates a giant ionic lattice. The particles are ions, but they only carry charge when mobile.

Pattern 2: Low melting and boiling temperatures, no conductivity

This strongly indicates a simple molecular structure. The substance is made of molecules with weak intermolecular attractions between them.

Pattern 3: Very high melting temperature, no conductivity in any state

This usually indicates a giant covalent structure. The atoms are joined by many strong covalent bonds and there are no mobile charge carriers.

Pattern 4: Good conductivity as a solid

This usually indicates metallic bonding, unless other evidence points to graphite.

Common mistakes to avoid

• Using one property only to identify the structure

• Saying a substance conducts “because it has electrons” without stating whether they are mobile

• Forgetting that ionic solids do not conduct when solid

• Assuming that all substances that dissolve in water conduct electricity

• Forgetting the graphite exception

• Naming the structure correctly but not linking it to the data provided

In exam answers, the best approach is to make a clear identification and then justify it with precise property evidence and particle-level reasoning.