CAIE A-Level Chemistry 9.1.2 Period 3 Structure and Properties

Practise explaining Period 3 melting-point and conductivity trends through metallic, giant covalent and molecular structures.

Exam points

explain the Na-to-Al melting trend using increasing ion charge and stronger metallic attraction
contrast silicon’s giant covalent lattice with the weak intermolecular forces in P4, S8 and Cl2
link electrical conduction to mobile delocalised electrons and explain why molecular elements lack carriers

[Maximum number: 3]

The table gives some data for elements in the third period and some of their compounds.

Table for Question 9.1.2—Variation in melting point and electrical question 1 — CAIE A-Level Chemistry AS

Give three statements to explain your sketch.

1
2
3

Max. 3 from:

- increasing strength of metallic bond; Na<Mg<Al

OR stronger attraction between delocalised electrons and (positive) ion so melting point of Na<Mg<Al

- Si is giant covalent AND it has the highest melting point due to breaking / presence of strong (covalent) bonds OR Si requires the most energy because the covalent bonds in Si are stronger than metallic bonds (in Na / Mg / Al )

- P and S have weak(er) intermolecular forces / induced dipoles / van der Waals forces (than covalent / metallic bonds) so have low(er) melting points

- $\mathrm{S}\left({ }_{8}\right)$ has stronger/more intermolecular forces/van der Waals forces/induced dipoles than $\mathrm{P}_{(4)}$ so melting point of $\mathrm{S}\left({ }_{8}\right)$ is higher

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9.1.2—Variation in melting point and electrical question 1