The properties of ionic compounds, simple molecular substances, giant covalent structures (diamond, graphite, graphene) and metals, related to their structure and bonding, and the properties of nanoparticles and polymers.

What this topic is asking

OCR wants you to explain the properties of ionic compounds, simple molecules, giant covalent structures and metals in terms of their structure and bonding, and describe the properties and uses of nanoparticles and polymers.

Ionic and simple molecular substances

Ionic compounds form a giant ionic lattice of oppositely charged ions held by strong electrostatic forces in all directions. A lot of energy is needed to overcome these forces, so they have high melting and boiling points. They conduct electricity only when molten or dissolved, because then the ions are free to move and carry charge; as solids the ions are fixed and cannot move, so they do not conduct. Simple molecular substances (such as water, carbon dioxide and methane) are made of small molecules held together by weak intermolecular forces. These weak forces are easily overcome, so simple molecular substances have low melting and boiling points (many are liquids or gases at room temperature) and do not conduct electricity because the molecules have no overall charge.

Giant covalent structures and metals

In a metal, the layers of ions can slide over each other when a force is applied, which is why metals are malleable and ductile, and the delocalised electrons are free to move and carry charge and heat, which is why metals are good conductors. An alloy is a mixture of a metal with other elements; the different-sized atoms distort the layers so they cannot slide as easily, which makes alloys harder than the pure metal (for example steel is harder than pure iron).

Nanoparticles and polymers

Nanoparticles are extremely small particles (roughly $1$ to $100$ nanometres across, where $1$ nm $= 1 \times 10^{-9}$ m). Because they are so small they have a very large surface area to volume ratio, which can make them more reactive or effective in small amounts, useful in catalysts, sun creams and medicines. Their tiny size also raises questions about safety that are still being researched. Polymers are very large molecules made of many small repeating units (monomers) joined in long chains; they generally have higher melting points than small molecules because the long chains have larger intermolecular forces, and their properties depend on the monomers used.