Relating bonding and structure to the properties of substances: melting and boiling points, electrical conductivity and state, across ionic, simple molecular, giant covalent and metallic substances, and the limitations of bonding models.

What this dot point is asking

OCR wants you to relate the bonding and structure of a substance to its properties, especially melting and boiling points, electrical conductivity and state at room temperature. You should be able to look at a substance's structure (ionic, simple molecular, giant covalent or metallic) and predict its properties, or work backwards from properties to deduce the structure. You also need to know the limitations of the bonding models.

The four structure types and their properties

Why melting and boiling points differ

The melting and boiling point depends on the type and strength of the forces that must be overcome:

• Simple molecular substances only need their weak intermolecular forces broken, so they melt and boil at low temperatures. Larger molecules have stronger intermolecular forces and higher boiling points.
• Ionic, giant covalent and metallic substances all have strong bonds throughout a giant structure, so they need a lot of energy and have high melting points.

Within ionic compounds, more highly charged ions (such as $\text{Mg}^{2+}$ and $\text{O}^{2-}$) attract each other more strongly than singly charged ions (such as $\text{Na}^+$ and $\text{Cl}^-$), so they have higher melting points.

Why conductivity differs

Using properties to identify a structure

You can deduce the structure of an unknown substance from its properties:

• Low melting point and no conduction at all means simple molecular.
• High melting point, conducts when molten or dissolved but not as a solid means ionic.
• High melting point, conducts as a solid means metallic.
• Very high melting point and (usually) no conduction means giant covalent.

Limitations of the bonding models

OCR expects you to know the bonding models are simplified. Dot and cross diagrams show where electrons come from but not their true motion; ball and stick models show bonds and angles but exaggerate the space between particles and ignore the relative strength of forces; and the drawn structures do not show that the forces act in all directions. Real particles do not have hard edges, and the models can mislead about scale.