- The macromolecular compounds have giant, covalent molecules with extremely large molecular lattices.
- They have very high melting and boiling points.
- They don’t conduct electricity — not even when molten (except for graphite).
- They’re usually insoluble in water.
- Examples of such macromolecules are diamond, silica and graphite.
Diamond and Silica(Sand)
|(3 dimensional structure macromolecular compound – Diamond)|
- A diamond crystal or a grain of sand is just one giant molecule. Such molecules, because they are so rigid and strong, have very high melting points.
- Each carbon atom forms four covalent bonds in a very rigid giant covalent structure, which makes diamond the hardest natural substance. This makes diamonds ideal as cutting tools.
- All those strong covalent bonds give diamond a very high melting point.
- It doesn’t conduct electricity because it has no free electrons.
- Diamond is an allotrope of carbon. Allotropes are different forms of the same element in the same physical state
|(3 dimensional layer structure: graphite)|
- Carbon also occurs in the form of graphite. The carbon atoms form joined hexagonal rings forming layers 1 atom thick.
- Graphite is black and opaque.
- Each carbon atom only forms three covalent bonds, creating sheets of carbon atoms which are free to slide over each other. This makes graphite slippery, so it’s useful as a lubricant.
- The layers are held together so loosely that they can be rubbed off onto paper to leave a black mark — that’s how pencils work.
- Graphite has a high melting point — the covalent bonds need lots of energy before they break.
- Only three out of each carbon’s four outer electrons are used in bonds, so there are lots of spare electrons. This means graphite conducts electricity — it’s used for electrodes.