Periodicity - Melting Point Trend.

In the understanding of periodicity, we will need to explain for the melting point trend across the period. Below is a picture of a graph which shows the melting point trend of the elements across the period.

Diagram to show melting point trend.

Across the period from Na to Ar: Na, Mg and Al exist as giant metallic structures. While Si exists as giant covalent molecule (macromolecular structure). While P, S, Cl and Ar exists as simple discrete molecules. These elements' standard states formula of P₄, S₈, Cl₂ and Ar respectively.

Hence, to explain the melting point trend across the period, it is recommended to define the structures the substance exists as first before proceeding further. The following diagram constructs the explanation needed.

Mindmap which constructs the explanation.

Lastly, Al contains 3 valence electrons. Although these 3 should contribute to the sea of electrons, it does not happen because it will be very difficult for Al to lose so many electrons to contribute to the sea of electrons. Therefore, the increase in melting point from Al to Mg is much smaller than the increase form Mg to Na.

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Article written by Kwok YL 2009.}

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