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Not all covalent substances are small molecules. Some carbon and silicon structures are giant — billions of atoms joined by covalent bonds into one enormous network — and these have utterly different properties from the gases and liquids of the last lesson: diamond is the hardest natural material, while graphite (also pure carbon) is soft enough to write with. This lesson, part of Topic C2 of OCR Gateway Science A, covers giant covalent structures, metallic bonding and alloys, and finishes with nanoparticles. Throughout, the single skill being tested is the same one as in the bonding lessons: explaining properties from structure and bonding.
By the end of this lesson you should be able to describe the structures of diamond, graphite, graphene and fullerenes, explain their properties from structure, explain metallic bonding and the properties of metals and alloys, and describe nanoparticles and their uses.
A giant covalent structure (or macromolecule) is a huge network of atoms all joined by strong covalent bonds. Because there are so many strong covalent bonds to break, these substances have very high melting points — quite unlike simple molecular substances. The most important examples are forms of carbon.
In diamond, each carbon atom forms four covalent bonds to four other carbon atoms, building a rigid, three-dimensional lattice that extends throughout the crystal. This structure gives diamond its properties:
In graphite, each carbon atom forms only three covalent bonds, arranging the atoms into flat layers of hexagons. The layers are held to one another only by weak intermolecular forces, and the fourth outer electron of each carbon is delocalised (free to move). This very different structure gives very different properties:
Diamond and graphite are both pure carbon — they are allotropes of carbon (different structural forms of the same element). Their wildly different properties come entirely from how the atoms are arranged and bonded, which is a perfect illustration of structure → property.
Exam Tip: The key contrast: in diamond each carbon makes 4 bonds (rigid, hard, no free electrons → no conduction); in graphite each carbon makes 3 bonds in layers that slide (soft lubricant) with 1 delocalised electron per atom (conducts). Both are allotropes of carbon.
Silicon dioxide (SiO2, found in sand and quartz) is another giant covalent structure, with each silicon bonded to oxygen in a hard, high-melting lattice, similar in form to diamond.
In a metal, the atoms are packed in a regular giant lattice, and the outer electrons become delocalised — they leave the individual atoms and are free to move throughout the whole structure. This leaves the atoms as positive metal ions sitting in a "sea" of delocalised electrons. The strong electrostatic attraction between the positive ions and the sea of electrons is the metallic bond.
This structure explains the properties of metals:
Exam Tip: Metals conduct because of delocalised electrons that are free to move — not because the ions move. State "metallic bond = electrostatic attraction between positive metal ions and a sea of delocalised electrons" to score the bonding mark.
A pure metal is often too soft for many uses, because its layers of identical atoms can slide over each other easily. An alloy is a mixture of a metal with one or more other elements (often another metal). Alloys are harder than the pure metal because the different-sized atoms distort the regular layers, so the layers cannot slide over each other as easily. Examples include steel (iron with carbon) and brass (copper with zinc).
Exam Tip: An alloy is a mixture, not a compound. It is harder than the pure metal because the different-sized atoms disrupt the layers, stopping them sliding. This is a frequent 2-mark question.
Nanoparticles are particles between about 1 and 100 nm (nanometres) across — only a few hundred atoms. At this tiny scale, a material can have different properties from the bulk (normal-sized) material, largely because of an enormous surface-area-to-volume (SA:V) ratio: as a particle gets smaller, a much larger fraction of its atoms sit on the surface.
This large surface area makes nanoparticles especially useful as catalysts (more surface for reactions), and in sunscreens (nanoparticle titanium dioxide or zinc oxide block UV without leaving a white film), medicines (delivering drugs efficiently) and electronics. Because their properties differ from the bulk, smaller quantities can achieve the same effect.
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