Discovering chemistry
Discovering chemistry

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Discovering chemistry

2.2 Allotropes

Different solid forms of the same element are called allotropes (or polymorphs).

Carbon, sulfur and phosphorous are examples of elements that form a number of allotropes.

Let’s focus on carbon.

Diamond is an allotrope of carbon. The arrangement of its atoms is shown in Figure 15; for each atom are four surrounding carbon atoms at the corners of a regular tetrahedron, and the C—C distance is 154 pm.

Described image
Figure 15 The structure of diamond. Note here distances are reported in picometeres (pm), where 1pm ≡ 10-12m

Figure 16 shows the structure of graphite, the form of carbon used in pencil leads. In this allotrope, there are regular hexagons of carbon atoms arranged in parallel sheets. Within the sheets, the C—C distance is only 142 pm, but the shortest distances between the sheets is 340 pm.

Described image
Figure 16 The structure of graphite

In 1985 Robert Curl, Harry Kroto and Richard Smalley discovered a further form of carbon. This is known as buckminsterfullerene and its structure is shown in Figure 17.

There are sixty carbons in its structure.

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Figure 17 The structure of buckminsterfullerene C60.

Describe the arrangement of the carbon atoms in C60.


The atoms are arranged in pentagons and hexagons. In fact there are twelve pentagons and twenty hexagons, and the structure resembles a football – hence the frequently used term buckyball.

There is in fact a bigger family of fullerenes comprising cage-like molecules (such as C60) and tube fullerenes called nanotubes.

In addition to carbon, other elements that exist as allotropes include, phosphorus, sulfur and tin. At this point take a look at the following video which shows you the allotropes of sulfur and how they can be interconverted.

Download this video clip.Video player: Video: Sulfur and its allotropes.
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Transcript: Video: Sulfur and its allotropes.

Yellow powdered sulfur consists of rings of 8 sulfur atoms arranged in the shape of a crown. In the oil bath, we’re melting sulfur. This amber liquid contains s8 rings. Let’s cool the liquid. Here we can see needle-shaped crystals. This is the monoclinic allotrope. Now we heat sulfur to a higher temperature with a direct flame. Here we produce a dark brown liquid which contains short sulfur chains. Quickly cooling in water, we produce plastic sulfur – a stretchy substance. In case you’re wondering whether these rubbery properties can be put to good use, let’s look at the sample after one month. It’s turned very hard and brittle – not very promising.
End transcript: Video: Sulfur and its allotropes.
Video: Sulfur and its allotropes.
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