Giant Covalent Structures: Diamond and Graphite

A network of bonds, not separate molecules

A covalent bond forms when atoms share a pair of electrons. In a simple molecule, only a small, fixed number of atoms are joined together. In a giant covalent structure, bonding continues through an extended network containing very many atoms. There are no separate small molecules.

Diamond and graphite are both made entirely of carbon. They are allotropes: different structural forms of the same element in the same physical state. Their carbon atoms are arranged and bonded differently, so the materials have very different properties. Hardness belongs to the whole structure—not to an individual carbon atom.

Diamond: a rigid network in three dimensions

In diamond, each carbon atom forms four strong covalent bonds to four other carbon atoms. The four bonds point in different directions towards the corners of a tetrahedron, rather than lying in a flat cross. This arrangement repeats throughout the crystal, producing a continuous three-dimensional network.

Diamond is very hard because its atoms are held firmly in place by strong covalent bonds extending in all directions. There are no layers that can easily slide over one another. Its resistance to scratching makes it useful on the cutting edges of tools.

Diamond also has a very high melting point. Breaking down the giant structure requires many strong covalent bonds to be broken, which needs a large amount of energy.

It does not conduct electricity: its outer electrons are involved in covalent bonds, so there are no mobile charged particles to carry a current through the structure.

Graphite: strong sheets with weak forces between them

In graphite, each carbon atom forms three strong covalent bonds to three other carbon atoms. These bonds form flat sheets of joined hexagonal rings. Many sheets are stacked to make the solid.

The distinction between bonding within a sheet and forces between sheets explains graphite’s properties. Strong covalent bonds hold each sheet together, but only weak forces hold neighbouring sheets together. There are no covalent bonds joining one sheet to the next.

Graphite has a very high melting point because breaking down its structure requires strong covalent bonds within the sheets to be broken. Simply sliding the sheets past each other does not break down those networks.

However, graphite is soft and slippery because the weak forces between sheets are easily overcome, allowing the sheets to slide over one another. This makes graphite useful as a lubricant, reducing friction between moving surfaces. Layers can also rub off onto paper, explaining its use in pencils.

Carbon has four outer electrons. In graphite, three are used to form the three covalent bonds around each atom. The fourth becomes delocalised: it is not confined to one atom or one bond and can move along the sheet. These mobile electrons carry charge, so graphite conducts electricity. This makes it useful for electrodes and electrical contacts.

Reading a three-dimensional structure on a flat page

A ball-and-stick model represents atoms as balls and covalent bonds as sticks. A drawing of that model must show depth on a flat page. Perspective can make some bonds appear shorter or make atoms overlap; this does not mean that the actual bonds are shorter or that overlapping atoms are bonded.

For diamond, start by identifying a carbon atom and tracing its four bonds. Imagine its neighbours surrounding it in three dimensions, not sitting together on one flat sheet. One way to draw this local arrangement uses two ordinary lines for bonds in the plane of the page, a solid wedge for a bond pointing towards you and a dashed wedge for a bond pointing away. To reconstruct the shape, place those last two neighbours in front of and behind the page. Extend the same four-neighbour arrangement to represent more of the lattice.

For graphite, a view from above shows the joined hexagonal rings within a sheet. A view from the side reveals the stack of sheets and the gaps between them. To turn the flat hexagonal drawing into a three-dimensional model, keep each sheet flat and stack several sheets above one another; do not add covalent bonds across the gaps.

When drawing from a model, preserve which atoms are connected and their spatial arrangement, even though the viewing angle changes. A small drawing represents only a fragment: the network continues beyond its edges. Ball-and-stick models make connections easy to see, but atoms are not literally hard balls joined by rods, and the drawn gaps are not an accurate picture of their relative sizes.

Diamond and graphite

Both are carbon allotropes with giant covalent structures and very high melting points: many strong covalent bonds require much energy to break.