Belgium

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Atomium, Brussels, Belgium

50° 53′ 41″ N, 4° 20′ 28″ E

An Iron Crystal

The Atomium building in Brussels was built for the International Exhibition in 1958. It represents the crystal structure of iron (actually just one of the allotropes of iron; see sidebar) and is constructed from steel with an aluminum skin. Like the Eiffel Tower before it, the Atomium was intended to be a temporary structure, but survived because of its popularity.

The Atomium consists of 9 spheres representing iron atoms, connected by 20 tubes representing the bonds between the iron atoms, forming a cube with iron atoms at the vertexes and a single iron atom at the center. The cube structure sits balanced on one vertex for aesthetic reasons, and is supported by extra pillars connected to spheres near the ground. The entire structure is over 100 meters tall.

Between 2004 and 2006, the Atomium was extensively renovated. The corroded aluminum was removed and replaced with stainless steel (which is made from a different allotrope of iron than the allotrope represented by the Atomium itself).

There’s a lot going on inside the Atomium. In the sphere at the base, there’s an exhibition covering the 1950s and the International Exhibition of 1958. One of the other spheres contains an exhibition space that houses temporary exhibits. The topmost sphere offers a restaurant and a panoramic view over Brussels. There’s even a sphere used exclusively by school trips—the children get to sleep inside.

If you’re wondering why there’s no photograph of the Atomium here to show you just how cool it is, it’s because you can’t photograph it and publish the picture. Through Belgian copyright law, the Atomium insists that it owns the copyright of photographs even if they’re taken by a third party, and that a large fee must be paid for any reproduction. Happily, we have the Web. To see this incredible structure, do a Google Image search for “Atomium”.

Although the interior of the Atomium is interesting, there’s no real science to be found there, so you can easily avoid the entrance fee and view the structure from the outside. After all, copyright law doesn’t apply to images on the retina.

Practical Information

Information about the Atomium and details on how to visit it are available at http://www.atomium.be/ (click “EN” for English).

Iron Allotropes

The iron crystal represented by the Atomium is actually just one of three possible structures (or allotropes) of iron: alpha, beta, and delta. Allotropy (forming different structures from a single element) is not limited to iron; other elements (such as carbon and oxygen) also have allotropes.

When molten iron starts to cool down, it first forms the delta allotrope, which has an iron atom at each vertex of a cube and a single iron atom in the middle of the cube. Each atom connects to four other atoms (their adjacent neighbors along the sides of the cube and the central atom). This structure is called a body-centered cubic, and is the structure represented by the Atomium (see Figure 3-1).

Figure 3-1. Body-centered cubic

The Delta Allotrope of Iron

As iron cools further (below 1394°C), it then forms the gamma allotrope, which has a different structure. Gamma iron is a face-centered cubic (see Figure 3-2); it has a cubic structure with atoms at the vertexes, but also a single atom in the center of each of the cube’s faces. Gamma iron is used in the production of stainless steel, where it is alloyed with chromium. (The chromium forms an invisible oxide layer on the outside of the stainless steel, which protects it from corrosion.)

Figure 3-2. Face-centered cubic

When below 912°C, iron forms its final allotrope, the alpha form. This has the same structure as the delta allotrope, but with the important property that once it falls below 770°C it becomes ferromagnetic (770°C is the so-called Curie point, above which a ferromagnetic material loses its magnetism). The alpha allotrope is used in cast iron and steel making.

Carbon Allotropes

Probably the most interesting allotropes are those made from carbon. Carbon has many different allotropes including diamonds, graphite, and carbon nanotubes.

Diamonds form a complex structure that consists of tetrahedrons of four carbon atoms joined together to fit inside a cube. This structure is known as a diamond cubic (see Figure 3-3).

Figure 3-3. Diamond structure

If you don’t have diamonds at home, you’ve probably got another carbon allotrope: graphite, which is found in pencil leads. Graphite is almost the polar opposite of diamonds—it’s soft, black, and opaque. The carbon in graphite consists of hexagonal units that link together into flat sheets (Figure 3-4). The sheets lie on top of each other and in the presence of air they can slip around, making graphite a useful lubricant.

Figure 3-4. Graphite structure

Yet another allotrope of carbon forms tubes (known as nanotubes). A nanotube consists of a sheet of carbon atoms in the hexagonal structure seen in graphite, but that has been rolled up to form a tube (Figure 3-5).

Figure 3-5. Carbon nanotube

Nanotubes are extremely strong and stiff, are very good conductors of heat, and can form either semiconductors or good conductors. But they are a relatively recent discovery, and have only lately become available commercially. They are likely to be as important to the 21st century as iron was to the 19th.