What you will learn

Compare the reactions of sodium, magnesium, aluminium, silicon and phosphorus with chlorine, using the five products specified in C1.3.3: NaCl, MgCl₂, Al₂Cl₆, SiCl₄ and PCl₅. Describe visible observations, balance equations and connect bonding and structure to the metal-to-non-metal transition.

The product formula is fixed for this lesson. In particular, retain the dimer formula Al₂Cl₆ when representing molecular aluminium chloride, and use PCl₅ for phosphorus in excess chlorine. Other chlorides are not extra required products for this specification point.

1. Chlorine as an oxidising agent

Elemental chlorine is a pale green/yellow-green, diatomic gas, Cl₂. In the specified chlorides, its oxidation number is −1. Chlorine is therefore reduced in these reactions, while the Period 3 element is oxidised from zero to a positive oxidation number.

The formal reduction half-equation is Cl₂ + 2e⁻ → 2Cl⁻. It is useful for redox accounting, but does not mean that every product is a collection of simple metal and chloride ions. In covalent SiCl₄ and molecular Al₂Cl₆, oxidation numbers are formal bookkeeping; the bonded atoms share electron density.

Observations below describe controlled demonstrations, not experimental instructions. A bright flame, a white solid and the loss of chlorine colour are distinct observations. Moist-air fumes from a chloride are not automatically the colour of the pure compound.

2. Sodium and magnesium: ionic chlorides

Sodium: once a reaction is initiated, sodium reacts vigorously with chlorine with a bright yellow/orange-yellow flame. White sodium chloride forms. The equation is 2Na(s) + Cl₂(g) → 2NaCl(s). Each sodium atom changes from 0 to +1 and each chlorine atom from 0 to −1.

NaCl is a giant ionic lattice with a 1:1 ratio of Na⁺ and Cl⁻. Its formula gives the simplest ion ratio, not a separate NaCl molecule. Ionic bonds are electrostatic attractions throughout the lattice, rather than isolated sticks connecting one pair of atoms.

Magnesium: heated magnesium reacts vigorously with chlorine, producing a bright white light and white magnesium chloride. Mg(s) + Cl₂(g) → MgCl₂(s). One magnesium atom supplies two electrons in the formal transfer picture, balancing the two electrons accepted by a Cl₂ molecule.

MgCl₂ contains Mg²⁺ and Cl⁻ in a 1:2 ratio. Its larger subscript does not mean that it has two chlorine molecules per magnesium atom: it has two chlorine atoms, corresponding to two chloride ions, per magnesium ion.

3. Aluminium: keep the Al₂Cl₆ molecular representation

When initiated, aluminium reacts with chlorine to produce aluminium chloride. The reaction can be vigorous, with glowing/bright burning metal and a white deposit or fumes. The protective oxide film can delay the reaction of bulk aluminium; surface condition matters.

2Al(s) + 3Cl₂(g) → Al₂Cl₆
Product formula specified in C1.3.3; the product phase depends on temperature.

The empirical ratio is AlCl₃, but Al₂Cl₆ is the dimeric molecular representation required here. Two aluminium centres are connected by two bridging chlorine atoms; each aluminium also has two terminal chlorine atoms. There are six chlorine atoms altogether: four terminal and two bridging.

Aluminium’s small, highly charged centre strongly polarises chlorine’s electron cloud, giving substantial covalent character. Do not draw the molecular dimer as two isolated Al³⁺ ions beside six isolated Cl⁻ ions. Anhydrous solid aluminium chloride has an extended structure; its structure is phase-dependent and is not simply a collection of the same dimers in every state. The dimer is important in the liquid and lower-temperature vapour; at higher vapour temperatures it dissociates towards AlCl₃ monomers.

4. Silicon and phosphorus: specified covalent formulae

Silicon: heated silicon reacts with chlorine, forming silicon tetrachloride: Si(s) + 2Cl₂(g) → SiCl₄(l), with the product state shown after cooling to room temperature. The reaction can give a glow/burning solid. SiCl₄ itself is a colourless volatile liquid; it fumes in damp air because it reacts with moisture, not because pure SiCl₄ is a white solid.

SiCl₄ consists of discrete tetrahedral molecules, each with four Si–Cl covalent bonds and a Cl–Si–Cl angle of approximately 109.5°. Silicon has oxidation number +4, but this does not indicate free Si⁴⁺ ions in a chloride lattice.

Phosphorus: phosphorus reacts vigorously with chlorine. In excess chlorine, the specified phosphorus(V) chloride product is represented by P₄(s) + 10Cl₂(g) → 4PCl₅(s), with the product state after cooling. A bright flame and white solid/fumes can be observed. Product proportions depend on the chlorine supply, so specify excess chlorine when identifying PCl₅.

The P₄ reactant equation is a convenient molecular representation; red phosphorus is an extended allotrope and should not be described as a solid made of independent P₄ molecules.

PCl₅ is phase-dependent. A discrete gaseous PCl₅ molecule is trigonal bipyramidal. Solid PCl₅ is ionic, containing tetrahedral [PCl₄]⁺ and octahedral [PCl₆]⁻ ions. Two formula units give one of each ion: 2PCl₅ corresponds to [PCl₄]⁺[PCl₆]⁻. Neither this pair nor the gas should be labelled P⁵⁺ surrounded by five independent Cl⁻ ions.

5. Trends and observations at a glance

Across the set, the formal oxidation number of the Period 3 element rises +1, +2, +3, +4, +5. Covalent character becomes important as the electronegativity difference with chlorine narrows; aluminium’s polarising effect is significant. This is not a simple monotonic reaction-rate graph. Temperature, surface films, allotrope and chlorine supply affect observations.

6. Four worked examples

Common misconceptions

• “All chlorides of these elements are simple ionic compounds.” SiCl₄ is molecular; Al₂Cl₆ is a covalent dimer; PCl₅ has phase-dependent structure.
• “Al₂Cl₆ has six terminal Cl atoms plus two bridges.” It has four terminal and two bridging chlorine atoms in total.
• “PCl₅ is molecular in every state.” The solid contains [PCl₄]⁺ and [PCl₆]⁻ ions.
• “The white fumes from a moist-air reaction prove that SiCl₄ is a white solid.” Pure SiCl₄ is a colourless liquid at room temperature; moisture changes the chemical species.
• “A higher oxidation number proves faster reaction.” It records electron accounting, not rate.

Exam tips

• Start from Cl₂, not individual chlorine atoms, when balancing an element–chlorine equation.
• State excess chlorine for the specified PCl₅ product.
• Distinguish flame colour, product colour and moist-air fumes.
• Use Al₂Cl₆ when requested; its empirical ratio is AlCl₃.
• Explain bonding with reference to phase, especially for aluminium chloride and PCl₅.

Check your understanding

Select one option and Check Answer. Read the explanation, then use Try Again to reset.