Discovering chemistry
Discovering chemistry

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

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.

Described image
Figure 17 The structure of buckminsterfullerene C60.

Describe the arrangement of the carbon atoms in C60.

Answer

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.
Skip transcript: Video: Sulfur and its allotropes.

Transcript: Video: Sulfur and its allotropes.

INSTRUCTOR:
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.
Interactive feature not available in single page view (see it in standard view).
DC_1

Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371