Explain the variations of melting points of chlorides of period 3 elements

Physical properties of chlorides of period 3 elements

These are formed by reacting the elements with chlorine gas and the physical properties of their chlorides are shown in table 3.

 Table 3 The chlorides of period 3 elements

Explanation of melting points

NaCl and MgCl₂ have high melting points due to the strength of the ionic bonding.  AlCl₃ has a fairly high melting point because in the solid state, it consists of Al₂Cl₆ molecules and not simple AlCl₃. These molecules are produced through dative bonding between Al and Cl in the Al₂Cl₆ molecules.

SiCl₄ and S₂Cl₂ are liquids and consist of respective simple SiCl₄ and S₂Cl₂ molecules and in the solid state the molecules are held by weak van der Waals force which explains their very low melting points.  PCl₅ a pale yellow solid has a fairly high melting point because the solid undergoes partial ionization.

2PCl₅      ↔     PCl₄⁺ + PCl₆^–

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