Diamond and silicon dioxide: giant covalent structures

Why can lightning turn beach sand into glass, when a bonfire on the same beach leaves the sand as sand? Beach sand is mostly quartz, which is silicon dioxide. A strike can fuse the grains into a hollow, glassy tube called a fulgurite. A bonfire, hot enough to cook on all evening, does not even soften them. Saying lightning melts anything skips the real puzzle: why sand needs that much. Diamond is the cleaner case of the same puzzle, so it goes before the sand.

A giant covalent structure is a huge network of atoms. Every atom is joined to its neighbours by covalent bonds, the pattern repeats in every direction, and there are no separate molecules anywhere. Diamond is pure carbon. A carbon atom has four outer electrons, and in diamond it uses all four to form four covalent bonds, one to each of four other carbon atoms. Count the bonds on one atom: four. Pick another atom anywhere in the network and count again: four. Four bonds from every carbon atom, the same all the way through. Diamond being only carbon surprised people once. In seventeen ninety-seven, a chemist named Smithson Tennant burned a diamond, and it gave the same amount of carbon dioxide as the same weight of charcoal. Explaining diamond from its structure is expected at Foundation and at Higher, so the exact wording matters at both. Two students each wrote one sentence about diamond's structure. The first wrote: diamond contains four carbon atoms. The second wrote: each carbon atom forms four bonds. Which student's sentence about diamond is right, the first or the second? The second. A diamond holds a vast number of carbon atoms, so the first sentence attaches the right number to the wrong thing. One examiner's report saw that slip often: "A common response was that diamond contains four carbon atoms rather than each carbon atom having four bonds, which would have been sufficient to be awarded the second marking point." The number was right and the thing it counted was wrong. The fix is to keep the number welded to its object: four bonds from every carbon atom.

To melt diamond, the carbon atoms have to be pulled away from each other. What holds one carbon atom to the next is a covalent bond, so here, melting means breaking covalent bonds. That is the difference from small molecules and polymers, where melting only overcomes the forces between molecules. In a giant covalent structure there are no molecules, so the covalent bonds themselves have to break. There are an enormous number of these strong bonds, and a very large number of them must be broken. That needs an enormous amount of energy, so the melting point is very high. This is an answer many students already give well. One examiner's report says: "Most candidates correctly identified diamond as having the highest melting point and there were some excellent answers describing it as a giant covalent structure requiring large amounts of energy to break the many strong covalent bonds." Good answers can still be undone by one word. Intermolecular means between molecules, and diamond has no molecules, so there is nothing for an intermolecular force to act between. A student writes: diamond has a high melting point because it has strong covalent bonds and strong intermolecular forces. Half of that sentence is excellent. Which half of the student's sentence about diamond is wrong, and why? The intermolecular forces half. Diamond has no molecules, so it has no intermolecular forces, and naming them contradicts the covalent bonds that came first. A second examiner's report, on a question about diamond, describes the cost: "Many did mention covalent bonds but then contradicted themselves by referring to intermolecular forces; these are not present, so the type of bonding mark could not be awarded." The students knew the chemistry and cancelled it. The fix is a rule for any giant covalent structure: covalent bonds, many of them, broken, and the word intermolecular never appears.

The same four bonds explain why diamond is hard. Each atom is held in place by four strong bonds pointing in different directions. That makes a rigid network in three dimensions. There are no layers and nothing that can slide, so moving any atom means breaking strong bonds. Diamond is very hard. Conduction is next, and the number four answers it as well. Using the number four, why does diamond not conduct electricity? All four outer electrons on each carbon atom are used in bonds. None is left over to move through the structure, and there are no ions, so nothing charged is free to carry a current. Saying diamond does not conduct because it is not a metal cannot be the reason. The video on graphite shows a substance made of nothing but carbon that does conduct. Notice that every answer so far started with the structure and then gave the property. One examiner's report shows why that order matters: "Many students gave properties of diamond, which were not asked for and gained no credit." Saying diamond is hard does not explain anything. The fix is a fixed order: structure first, property second.

Silicon dioxide is the second giant covalent structure. Each silicon atom is bonded to four oxygen atoms, each oxygen atom to two silicon atoms, and the network runs right through with no separate molecules. Its formula, S i O two, gives the ratio of atoms: one silicon for every two oxygen. It is not the formula of a molecule, because there are no molecules to have one. Silicon dioxide melts at about one thousand seven hundred and ten degrees C. Pause and explain silicon dioxide's melting point using giant, covalent, many and energy. Silicon dioxide is a giant covalent structure. To melt it, many strong covalent bonds must be broken, which needs a large amount of energy, so its melting point is very high. That is the beach answered. A bonfire cannot supply enough energy to break that many strong covalent bonds. A lightning strike, briefly hotter than the surface of the Sun, can fuse the sand into glass. Now three substances, with melting points of minus one hundred and fourteen, about one hundred and thirty, and one thousand seven hundred and ten degrees C. One is small molecules, one a polymer, one giant covalent. Which structure, small molecules, polymer or giant covalent, goes with each melting point, and why does the middle one sit where it does? Small molecules at minus one hundred and fourteen, the polymer at about one hundred and thirty, giant covalent at one thousand seven hundred and ten. The polymer's long chains give relatively strong forces between chains, but no covalent bonds break, so it stays far below the giant structure.

Time to explain it back, with the structure first in every answer. Why is diamond so hard? Start with its structure. It is a giant covalent structure with four strong bonds from every carbon atom, in a rigid network. Nothing can slide without breaking strong bonds. Now an unseen one: the element silicon forms a giant covalent structure. High or low melting point, and why? High. Many strong covalent bonds must be broken to melt it, which needs a large amount of energy. A different angle: why does diamond not conduct? Every outer electron is used in a bond, so there are no free electrons, and no charged particles can move. One more: why does beach sand need lightning, not a bonfire, to turn to glass? Sand is silicon dioxide, a giant covalent structure. Fusing it means breaking many strong covalent bonds, and only something as energetic as lightning supplies enough.

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