What you will learn

Explain differences in melting point and electrical conductivity only for the Period 3 oxides and chlorides listed in C1.3. The oxides are Na₂O, MgO, Al₂O₃, P₄O₁₀ and SO₂. The chlorides are NaCl, MgCl₂, Al₂Cl₆, SiCl₄ and PCl₅.

This is the Period 3 compound comparison in C1.4, not a replacement for the existing general B1.4 structure-and-properties page. SiO₂, other oxides and other phosphorus chlorides are outside this page’s comparison set. Reactions of chlorides with water and acid–base chemistry are later topics.

1. Use particles → attractions → property

A melting-point explanation identifies the particles and the attractions disrupted on changing from a solid to a liquid. A conductivity explanation identifies charged particles that can move. These are different questions: a compound may have strong electrostatic attractions but fail to conduct in its solid state.

In an ionic solid, ions occupy fixed lattice positions and cannot carry a sustained current by migration. In a suitable molten ionic compound, ions are mobile and can transport charge. Ordinary simple molecular substances contain neutral molecules and have no appreciable population of mobile ions or delocalised electrons, so pure molecular liquids generally do not conduct.

Do not use “contains electrons” as a conductivity explanation. Every one of these substances contains electrons, but most electrons are localised in bonds, lone pairs or filled shells rather than free to move through the material.

Melting-point values depend on pressure, structure and form. A substance that sublimes changes directly from solid to gas; a sublimation temperature is not an ordinary melting point. This distinction matters for aluminium chloride and phosphorus(V) chloride.

2. The three specified metal oxides

Na₂O, MgO and Al₂O₃ have extended structures with strong attractions between oppositely charged centres. At this level, ionic lattice models explain their high melting points and molten-state conductivity. Aluminium oxide also has significant covalent character; a simple charge-only model is an approximation, not a complete bonding description.

Na₂O contains Na⁺ and O²⁻ in a 2:1 ratio; MgO contains Mg²⁺ and O²⁻ in a 1:1 ratio; Al₂O₃ has the formula ratio 2Al³⁺:3O²⁻. In their solids, the charged centres cannot migrate freely. None has the delocalised electrons responsible for conduction in a metal.

In suitable molten samples, ions are mobile and the metal oxides can conduct and undergo electrolysis. The necessary temperatures are high. A question referring to molten Al₂O₃ means the oxide melt; industrial electrolysis often uses the oxide dissolved in another molten material, which is a different experimental situation.

MgO has particularly strong attractions between its small Mg²⁺ and O²⁻ ions and a very high melting point. Na₂O has lower cation charge and a lower melting point than MgO. Al₂O₃ also has a very high melting point, but a simplistic rule “Al³⁺ has the greatest charge, so Al₂O₃ must melt highest” fails: lattice structure and covalent character also matter. There is no smooth increasing melting-point line across the three.

3. P₄O₁₀ and SO₂: covalent oxides

Sulfur dioxide consists of small, bent SO₂ molecules. Its melting point is low and it is a gas at room temperature. Melting or freezing concerns the relatively weak forces between molecules, not wholesale breaking of the strong S–O bonds inside each molecule. Pure solid or liquid SO₂ does not have mobile ions or delocalised electrons and is a poor electrical conductor.

Phosphorus(V) oxide, P₄O₁₀, requires care because it has different structural forms. The common molecular form contains cage molecules and its transition temperatures are far below those of MgO and Al₂O₃. There are also extended/polymeric forms with different thermal behaviour, so identify the form if a question supplies it. Do not claim every P₄O₁₀ sample is an identical simple molecular solid.

For a question using the molecular P₄O₁₀ model, the larger cage has stronger total intermolecular dispersion attractions than small SO₂ molecules, giving a much higher solid-to-fluid transition temperature than SO₂. In that model, the primary comparison is between intermolecular attractions. These covalent oxides do not have the mobile charge carriers of an ionic melt or metal.

A solution formed when an oxide reacts with water is a different chemical system. If that solution conducts, it does not prove that pure molecular oxide was an ionic conductor.

4. NaCl and MgCl₂: high-melting ionic chlorides

Both sodium chloride and magnesium chloride have giant ionic lattices. Strong electrostatic attractions must be overcome to melt them, so their melting points are high compared with molecular SiCl₄. Solid NaCl and MgCl₂ are poor conductors because their ions are fixed; the pure molten salts conduct because their ions can move.

Do not force their melting points into a charge-only rule. Although Mg²⁺ has greater charge than Na⁺, NaCl’s melting point is higher than MgCl₂’s. Melting reflects the full crystal structure, the changing entropy and covalent character as well as electrostatic attractions. Use supplied data when a question asks for an exact ordering.

For both melts, ions carry the current. Do not write that the chloride electrons become a free electron sea like a metal. Electrode reactions are separate from the process of ions migrating through the liquid.

5. Aluminium chloride and SiCl₄

SiCl₄ is a simple molecular substance with tetrahedral molecules. It has a low melting point and is a liquid at room temperature. The attractions between molecules are much weaker than the ionic attractions in NaCl or MgCl₂. The pure liquid has no mobile ions or delocalised electrons, so it does not appreciably conduct electricity.

Aluminium chloride is represented by Al₂Cl₆ in C1.3.3. Its phase behaviour is more complex than an uncomplicated salt melt. Anhydrous solid aluminium chloride has an extended structure; the liquid and lower-temperature vapour contain molecular dimers, Al₂Cl₆. At ordinary pressure it sublimes, so do not treat its ordinary-pressure sublimation temperature as a melting point directly comparable with NaCl.

The molecular liquid, obtainable at an appropriate pressure, is a poor conductor because neutral dimers provide no ordinary mobile ion population. Its thermal behaviour and low conductivity distinguish it from the simple giant ionic model for NaCl and MgCl₂. Avoid explaining it as “Al³⁺ ions move freely as soon as it melts.”

At still higher vapour temperatures, more AlCl₃ monomers are present. The particular phase must be stated when describing the structure. Significant covalent character arises because the aluminium centre strongly polarises the chlorine electron cloud.

6. PCl₅: why the phase must be named

Solid phosphorus(V) chloride contains [PCl₄]⁺ and [PCl₆]⁻ ions. It is therefore not correct to call every phase of PCl₅ a simple molecular substance. The ions are fixed in the solid and cannot migrate, so the solid does not conduct electricity appreciably.

At ordinary pressure, PCl₅ sublimes rather than giving an ordinary freely available melt. Its vapour contains molecular PCl₅; dissociation becomes important as temperature increases. Its neutral molecular liquid, where it exists at suitable pressure, is a poor conductor; do not infer a highly conducting melt merely from the ionic structure of the original solid.

Compare it with molecular SiCl₄ using structure: the complex ionic solid has much stronger interparticle attractions than the intermolecular attractions in solid SiCl₄. This helps explain why PCl₅ is a solid at room temperature while SiCl₄ is a liquid. It does not justify calling the sublimation temperature a melting point.

7. Compare the specified compounds

The table is qualitative and does not assign one universal numerical melting point to a substance that changes structure, sublimes or has several polymorphs. Use the phase, pressure and data stated in an examination question.

8. Four worked examples

Common misconceptions

• “All high-melting compounds conduct as solids.” Lattice strength and mobile charge carriers are separate ideas.
• “All chlorides here are ionic.” SiCl₄ is molecular; aluminium chloride and PCl₅ require phase-dependent descriptions.
• “A molecular solid melts by breaking all its covalent bonds.” Ordinary molecular melting primarily overcomes intermolecular attractions.
• “Sublimation is another name for melting.” Solid-to-gas and solid-to-liquid are different changes.
• “SiO₂ belongs in this C1.4 comparison.” This page is restricted to the oxides and chlorides specified in C1.3.

Exam tips

• For conductivity, always state the physical state and identify the mobile charge carrier, or its absence.
• For melting point, name the structure and the attractions overcome before linking them to energy.
• Use supplied thermal data; do not force a perfectly smooth trend across compounds with different structures.
• Retain Al₂Cl₆ as the specified molecular representation while qualifying the solid’s structure.
• For PCl₅, distinguish fixed complex ions in the solid from neutral molecular fluid phases.

Check your understanding

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