Giant covalent structures: diamond, graphite and graphene

Science · Matter & Materials · ages 15-16

Name ______________________ Date ____________

Why is diamond so hard?
- every atom is bonded to its neighbours in every direction
- its atoms are the heaviest of any solid element
- it is squeezed so tightly that nothing can get in
- its layers grip each other far more strongly than graphite's
How many of each carbon atom's four outer electrons are free to move in graphite?
Answer: ______________
In graphite, the layers of carbon atoms can slide past each other because they are held together only by weak forces between layers, not by covalent bonds.
Circle one: True False
How many covalent bonds does each carbon atom make in diamond?
Answer: ______________
Graphene can be made by peeling thinner and thinner layers off a piece of graphite with sticky tape, until only one layer of atoms is left. Based on this, how is graphene related to a single layer of graphite?
- Graphene is exactly one single layer of graphite's carbon sheet, just isolated on its own.
- Graphene is a completely different element from graphite.
- Graphene has extra atoms added compared to a graphite layer.
- Graphene is a 3D structure like diamond.
Why does no giant covalent substance dissolve in water?
- water cannot reach a solid surface
- the whole crystal is one connected piece
- they are all far too heavy for water to lift them
- water only dissolves things that conduct electricity
Graphite conducts electricity because its sheets slide over each other.
Circle one: True False
Diamond grit is bonded to the rim of a cutting disc. Which property makes that work?
- it melts into the cut and seals it
- it is transparent, so the cut can be seen
- it is light enough to spin quickly
- it is harder than whatever is being cut
Breaking diamond apart means pulling whole molecules away from each other.
Circle one: True False
Nadia rubs a block of graphite between her fingers, gets a grey smear with almost no effort, and says its covalent bonds must be weak. Where is the slip?
- graphite has no covalent bonds at all, only weak forces
- pencil lines come from the paper, not from the graphite
- graphite really is soft because its bonds are unusually weak
- the strong bonds are inside a sheet, not between sheets

Answer key

For grown-ups. Fold this page away before handing over the rest.

Giant covalent structures: diamond, graphite and graphene W1-mt_YTEj9srwPG-s1

every atom is bonded to its neighbours in every direction · There is no weak point anywhere in the structure. Scratching a diamond means breaking covalent bonds, and covalent bonds are strong.
1 · Three go into bonds within the sheet, which leaves one. That single delocalised electron per atom is what carries the current.
True · Inside each layer, carbon atoms are joined by strong covalent bonds. But the layers themselves are held together only by weak forces, so they can slip past one another easily. That's why graphite is soft and used in pencils.
4 · Carbon has four outer electrons and is four short of a full shell, so it makes four bonds. In diamond all four go to other carbon atoms.
Graphene is exactly one single layer of graphite's carbon sheet, just isolated on its own. · Graphite is just many honeycomb carbon layers stacked on top of each other. When you peel away layers until only one is left, that single sheet is graphene. It has the same honeycomb pattern of carbon atoms, just on its own instead of stacked with others. Because it's so thin, strong, and conducts well, graphene is used in flexible touchscreens.
the whole crystal is one connected piece · Dissolving means separating particles and surrounding them. A giant covalent structure has no particles to separate, only one connected network held by covalent bonds.
False · Sliding and conducting are two separate results of the same structure. The current is carried by the spare electron on each carbon, not by any movement of the sheets.
it is harder than whatever is being cut · Cutting works by scratching, and only a harder material can scratch a softer one. Diamond's four-bond network makes it the hardest natural substance there is.
False · There are no molecules in diamond. The bonding runs through the entire crystal, so taking it apart has to break covalent bonds themselves, which is why it takes thousands of degrees.
the strong bonds are inside a sheet, not between sheets · What gives way is the attraction between one sheet and the next, and that was never a covalent bond. Inside a sheet the bonding is as strong as diamond's, which is why graphite also stays solid at thousands of degrees.