Explain the increase in the melting point from sodium to aluminium.

Explain why sulfur, \({{\text{S}}_{\text{8}}}\), has a higher melting point than phosphorus, \({{\text{P}}_{\text{4}}}\).

Explain why silicon has the highest melting point and argon has the lowest melting point.

as (cat)ion becomes more positive / \({\text{N}}{{\text{a}}^ + }\), \({\text{M}}{{\text{g}}^{2 + }}\), \({\text{A}}{{\text{l}}^{3 + }}\) / size/radius decreases / charge density increases;

attraction for mobile/valence/delocalized/sea of electrons increases;

Do not accept “cloud of electrons”.

larger molecule / higher \({M_{\text{r}}}\) / \(M\) / greater number of electrons;

Do not accept “larger/higher/greater mass”.

greater van der Waals’/dispersion/London forces;

Accept intermolecular forces.

Si: giant/network/macromolecular/3-D covalent bonding;

No mark for strong bonding without reference to covalent and network.

No mark for molecular.

Ar: (simple) atomic / (only weak) van der Waals’/dispersion/London forces;

No mark for (simple) molecular.

Accept (only) weak intermolecular forces.

This proved to be the most difficult question in Section A. There was a general failure to recognize the relationship between different types of bonding with the melting points of period 3 elements. Many candidates did not appear to understand metallic bonding with much confusion between ionisation energy and melting points of metals. Many gave vague explanations or contradictory answers and there was some confusion of metallic bonding with covalent bonds or intermolecular forces. References to atoms instead of ions were more plentiful than expected with few candidates mentioning the increase in positive charge for the cations from Na to Al. There was also little mention of the increase in delocalized electrons from Na to Al or the decrease in ionic radii.

Trying to explain why \({{\text{S}}_{\text{8}}}\) has a higher melting point than \({{\text{P}}_{\text{4}}}\) caused many problems for candidates. There was a failure to appreciate that both \({{\text{S}}_{\text{8}}}\) and\({{\text{P}}_{\text{4}}}\) are simple molecular substances with van der Waals‟ forces between their molecules. Some answers lacked the precision necessary for the marks with many candidates stating that \({{\text{S}}_{\text{8}}}\) has a greater mass rather than molar or relative molecular mass.

Although the better candidates were able to state that Si has a giant structure some did not mention covalent bonding and a large number of candidates stated incorrectly that there were no intermolecular forces between argon atoms. In summary, intermolecular forces and structure and bonding were poorly understood.