At GCSE, three types of strong chemical bonding are studied. Which one a substance has explains most of its physical properties, including melting point, hardness and whether it conducts.
There are three types of strong chemical bonding studied at GCSE. Ionic bonding is the electrostatic attraction between oppositely charged ions; the GCSE examples usually form when a metal reacts with a non-metal, and electrons are transferred. Covalent bonding happens between non-metals: electrons are shared. Metallic bonding happens in metals: outer electrons are delocalised and shared across the whole structure.
A metal atom loses its outer electrons to become a positive ion; a non-metal gains them to become a negative ion. In the simple ionic compounds studied here both end up with a full outer shell, and the oppositely charged ions attract strongly.
The result is a giant ionic lattice, a regular three-dimensional arrangement of ions held by strong electrostatic forces acting in all directions. The formula gives the ratio of ions, not a molecule: NaCl means one sodium ion for each chloride ion.
Because the electrostatic forces are strong and act throughout the lattice, ionic compounds have high melting and boiling points.
They do not conduct when solid, because the ions are locked in place, but they do conduct when molten or dissolved, because the ions are then free to move and carry charge. They are also brittle: shifting the layers brings like charges together, which repel and split the crystal.
A covalent bond is a shared pair of electrons, and it is strong.
But in a small molecule the forces between the molecules are weak, and it is those that must be overcome to melt or boil the substance, not the covalent bonds themselves. So small molecular substances have low melting and boiling points and are often gases or liquids at room temperature.
They do not conduct electricity, because they have no mobile ions and no delocalised electrons. As molecules get larger the intermolecular forces generally get stronger, so boiling points usually rise; melting points are less regular, because how the molecules pack also matters.
In a giant covalent structure every atom is covalently bonded to its neighbours throughout, so melting it means breaking strong covalent bonds and the melting point is very high.
Diamond: each carbon is bonded to four others in a rigid three-dimensional lattice. Very hard, very high melting point, and does not conduct because it has no free electrons.
Graphite: each carbon is bonded to three others in layers of hexagons, with only weak forces between the layers rather than covalent bonds, so the layers slide, which makes it soft and a good lubricant. The fourth electron of each atom is delocalised, so graphite conducts electricity, unusually for a non-metal.
Graphene is a single layer of graphite, extremely strong for its mass and a good conductor. Fullerenes are hollow carbon molecules. The first was C60, buckminsterfullerene, and carbon nanotubes are cylindrical fullerenes with high tensile strength.
In a metal, positive metal ions sit in a giant regular lattice and the outer electrons are delocalised, free to move through the whole structure. The attraction between the positive ions and this sea of electrons is the metallic bond.
Metals conduct electricity and heat because the delocalised electrons can carry charge and energy. They generally have high melting points because the bonding is strong. They are malleable because layers of similarly sized positive ions can slide over one another while still being attracted to the delocalised electrons.
In an alloy, differently sized positive metal ions disrupt the regular layers, making it harder for the layers to slide over one another, so alloys are harder than pure metals.
Whether a substance is solid, liquid or gas depends mainly on the strength of the attractions between its particles, together with the temperature and pressure. The stronger those forces, the higher the melting and boiling points.
State symbols in equations are (s) solid, (l) liquid, (g) gas and (aq) aqueous, meaning dissolved in water. (l) represents a pure liquid, whereas (aq) means dissolved in water.
A polymer is a very large molecule made of many small repeating units joined by covalent bonds.
The bonds within each chain are strong, and the long chains also experience substantial intermolecular forces. As a result, many common polymers are solids at room temperature, unlike the small molecules they were made from.
Nanoparticles are between 1 and 100 nanometres across, far smaller than fine particles, and smaller again than coarse particles such as dust.
Their key feature is a very high surface area to volume ratio. Reducing the side of a cube by a factor of ten increases that ratio by a factor of ten, so a small quantity exposes a large surface. Nanoparticles made from suitable materials can therefore be effective catalysts.
They are used in sun creams, deodorants and medicines, and in some electronics. Because their properties differ from the bulk material, and because some can enter cells, their long-term effects on health and the environment are not yet fully understood.
