Chlorides of the Period 3 Elements
This section explains the structures, physical properties, and behaviors, of the chlorides of the Period 3 elements (sodium to sulphur), especially when they interact with water.
Structures of Chlorides in Period 3
- Sodium Chloride (NaCl) and Magnesium Chloride (MgCl₂):
Both of these compounds are ionic and form giant ionic lattices at room temperature. The strong electrostatic forces between the oppositely charged ions in these structures account for their notable properties.
- Aluminium Chloride (AlCl₃) and Phosphorus (V) Chloride (PCl₅):
These chlorides exhibit dual characteristics, transitioning from ionic in their solid forms to covalent when melted or vaporized. This change complicates their behavior and properties.
- Other Chlorides (e.g., SiCl₄, PCl₃):
These exist as simple covalent molecules, with the bonds formed by shared pairs of electrons rather than ionic attractions.
Melting and Boiling Points
- Ionic Chlorides (NaCl and MgCl₂):
Due to their strong ionic bonds, these compounds have high melting and boiling points. A substantial amount of energy is required to break these bonds.
- Covalent Chlorides (AlCl₃, SiCl₄, PCl₃, PCl₅, and others):
These compounds generally have lower melting and boiling points compared to ionic chlorides. Their weaker intermolecular forces, such as Van der Waals forces, are easier to overcome. However, AlCl₃ and PCl₅ have complex behaviors due to their structural changes at different temperatures.
Electrical Conductivity
- Ionic Conductors (NaCl and MgCl₂):
These compounds conduct electricity when molten, as the ions are free to move and carry charge. In their solid state, however, the ions are fixed in place and cannot conduct electricity.
- Non-Conductors (AlCl₃, PCl₅, and others):
These compounds do not conduct electricity in either solid or molten states. In the case of AlCl₃ and PCl₅, this is because they convert to a covalent form when melted, lacking free-moving ions or mobile electrons necessary for conduction.
Detailed Breakdown of Chlorides
Sodium Chloride (NaCl)
- Structure: Giant ionic lattice.
- Properties: High melting and boiling points due to strong ionic bonds.
- Electrical Conductivity: Conducts when molten; solid state is non-conductive.
- Water Reaction: Dissolves in water to give a neutral solution.
Magnesium Chloride (MgCl₂)
- Structure: Complex ionic lattice.
- Properties: High melting and boiling points.
- Electrical Conductivity: Conducts when molten; non-conductive as a solid.
- Water Reaction: Dissolves to form a faintly acidic solution (pH ≈ 6), forming hexaaquamagnesium ions:
MgCl₂ + 6H₂O → [Mg(OH₂)₆]²⁺ + 2Cl⁻
Aluminium Chloride (AlCl₃)
- Structure: Ionic with covalent character at room temperature; converts to molecular Al₂Cl₆ at higher temperatures.
At higher temperatures, Al₂Cl₆ dimers dissociate, transforming Al₂Cl₆ units into individual AlCl₃ trigonal planar monomers, structurally similar to BF₃. 
AlCl₃ transforms into Al₂Cl₆ dimers upon melting. These dimers have a tetrahedral arrangement around the aluminum atom. - Properties: Does not conduct electricity in any state.
- Water Reaction: Reacts violently with water to produce hydrogen chloride gas:
Al₂Cl₆ + 6H₂O → 2Al(OH)₃ + 6HCl
Silicon Tetrachloride (SiCl₄)
- Structure: Simple covalent molecule.
Silicon tetrachloride (SiCl₄) is tetrahedral due to its central silicon atom bonding with four chlorine atoms. These bonds are formed using sp³ hybridization, resulting in a symmetrical, pyramid-like structure. - Properties: Colorless liquid at room temperature; non-conductive.
- Water Reaction: Reacts violently to produce silicon dioxide and hydrogen chloride gas:
SiCl₄ + 2H₂O → SiO₂ + 4HCl
Did you know that…?
Silicon tetrachloride, a colorless liquid at room temperature, which fumes in moist air, relies on weak van der Waals forces for its structure. Despite lacking electrical conductivity, it plays a critical role in producing ultra-pure silicon, the backbone of modern electronics.
Phosphorus Chlorides
- Phosphorus(III) Chloride (PCl₃): Covalent, fuming liquid at room temperature, reacts violently with water to produce phosphorous acid and hydrogen chloride gas:
PCl₃ + 3H₂O → H₃PO₃ + 3HCl
Phosphorus(V) Chloride (PCl₅): More structurally complex; reacts with water in two stages to produce phosphoric(V) acid and hydrogen chloride:
PCl₅ + H₂O → POCl₃ + 2HCl (With cold water)
POCl₃ + 3H₂O → H₃PO₄ + 3HCl (With boiling water)
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PCl₅ + 4H₂O → H₃PO₄ + 5HCl (Overall reaction in boiling water)
Did you know that…?
PCl₅ sublimes (turns directly into gas) at 163°C. As it gets even hotter, PCl₅ breaks down (dissociates) into two simpler molecules: phosphorus trichloride (PCl₃) and chlorine gas (Cl₂). PCl₅ actually contains positive (PCl4+) and negative ions (PCl6–) formed from interactions between two PCl₅ molecules! This unique ionic structure explains why it’s a solid at room temperature.
Disulphur Dichloride (S₂Cl₂)
- Structure: Simple covalent liquid, orange and smelly.
- Properties: Non-conductive; has van der Waals dispersion forces and dipole-dipole attractions.
- Water Reaction: Reacts slowly with water to produce a complex mixture including hydrochloric acid, sulfur, and various sulfur-containing acids and anions.
16S2Cl2 + 16H2O → 8SO2 + 32HCl + 3S.
Key Reactions with Water
- Aluminium Chloride:
Al₂Cl₆ + 6H₂O → 2Al(OH)₃ + 6HCl - Silicon Tetrachloride:
SiCl₄ + 2H₂O → SiO₂ + 4HCl - Phosphorus(III) Chloride:
PCl₃ + 3H₂O → H₃PO₃ + 3HCl - Phosphorus(V) Chloride:
PCl₅ + 4H₂O → H₃PO₄ + 5HCl - Disulphur Dichloride (S₂Cl₂)
16S2Cl2 + 16H2O → 8SO2 + 32HCl + 3S.
