Explain the variations of melting points of elements of period 3

Table 2.15 The melting points of elements of the third period of the periodic table

From sodium to aluminium, the melting point increases due to increase in the strength of metallic bonds. The strength of metallic bonds increases as the number of electrons contributed to the formation of the metallic bond increases. Sodium contains one loosely bound electron in the valence shell, and this electron is readily contributed to the formation of the metallic bond.  Magnesium contains two electrons in the valence shell and both electrons are contributed to the formation of metallic bond.  The metallic bond in magnesium is thus stronger than that in sodium, which explains the large difference in the melting points.

However, the increase in melting point from Mg to Al is not as sharp as expected, probably because aluminium atoms use only two electrons of three valence electrons in the formation of the metallic bond.

Silicon has the highest melting point because it uses its four valence electrons to form an infinite three-dimensional assembly of atoms linked by a strong single covalent bond; thus, high temperatures are required to break these strong bonds.

The melting points of P, S, Cl are dependent on the sizes/masses of the molecules formed. The melting points of phosphorus and sulphur are relatively higher than expected because they exist as P₄ and S₈, rather than simple individual atoms; thus, the intermolecular forces are stronger. The boiling point of chlorine is very low because it exists as discrete diatomic molecules, Cl₂ held together by weak van der Waals forces.

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