The three types of bonding (ionic, covalent and metallic), how to represent them, the states of matter and changes of state, and how the structures of ionic compounds, small molecules, giant covalent structures, polymers and metals explain their properties.

What this topic is asking

AQA wants you to describe ionic, covalent and metallic bonding and how to represent them, explain the states of matter and changes of state, and link the structures of ionic compounds, small molecules, giant covalent structures, polymers and metals to their properties.

The three types of bonding

When a metal reacts with a non-metal, electrons in the outer shell of the metal atom are transferred to the non-metal, so the metal becomes a positive ion and the non-metal a negative ion, and the oppositely charged ions attract. For example, sodium loses one electron to form $\text{Na}^+$ and chlorine gains it to form $\text{Cl}^-$. Ionic compounds form a regular giant ionic lattice of millions of ions. In covalent bonding the atoms share pairs of electrons so that both achieve a full outer shell; covalent substances can be small molecules (like water or methane), giant covalent structures (like diamond or silicon dioxide), or polymers. Dot-and-cross diagrams show which electrons come from which atom.

States of matter

The amount of energy needed to change state depends on the strength of the forces between the particles, not on the strength of the covalent bonds within molecules. The stronger the forces between particles, the more energy is needed and the higher the melting and boiling points. (The simple particle model has limits: it treats particles as solid spheres with no forces between them and ignores their actual size.)

Structure and properties

• Ionic compounds: high melting and boiling points because there are many strong electrostatic attractions throughout the lattice that take a lot of energy to overcome; they conduct electricity only when molten or dissolved, because the ions must be free to move to carry charge.
• Small molecules: low melting and boiling points because the intermolecular forces between molecules are weak and easily overcome (note the strong covalent bonds inside the molecule are not broken when it melts or boils); they do not conduct electricity because the molecules have no overall charge.
• Giant covalent structures: very high melting points because many strong covalent bonds must be broken. Diamond is very hard, with each carbon bonded to four others; graphite is soft and conducts because each carbon bonds to three others in layers with delocalised electrons and weak forces between layers; graphene is a single layer of graphite, strong and conducting.
• Polymers: very large molecules with strong covalent bonds along the chain and relatively strong intermolecular forces, so they are solid at room temperature.
• Metals: good conductors of heat and electricity because the delocalised electrons are free to move and carry charge and energy; they are malleable and ductile because the layers of positive ions can slide over each other.

Nanoparticles (1 to 100 nanometres across) have a very large surface area to volume ratio, so they are used in catalysts, sun creams and electronics, though their long-term effects on health are still being researched.