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

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

ELEMENTSNaMgAlSiPSCl
CHLORIDESNaClMgCl2AlCl3SiCl4PCl5S2Cl2Cl2
BONDINGIonicIonicCovalentCovalentCovalentCovalentCovalent
STATESSolidSolidSolidLiquidSolidLiquidGas
MP\0C808714192-68160-76

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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