Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 99

At centres where there are two areas of electron density, the bonded atoms arrange themselves at 180° to each other, generating a linear structure. One of the simplest molecules that adopts this shape is beryllium chloride, BeCl₂. Beryllium has electron configuration 1s²2s² with just two electrons in its outer shell. Even by sharing electrons with two chlorine atoms, the beryllium atom cannot gain a full outer shell of electrons but shares each of its valence electrons with the unpaired electron of a chlorine atom. In this way, the chlorine atom gains an octet of electrons in its outer shell, and the beryllium atom has just four, as shown in Figure 2.12. Each chlorine atom forms a single bond to the central beryllium atom, and as there are just two centres of electron density, these arrange themselves at 180° to give a linear structure.

Figure 2.12 Dot‐and‐cross diagram for beryllium chloride, BeCl₂ showing two centres of electron density and linear shape.

The Be centre is said to be electron deficient because it has only four electrons in its outer shell when it has bonded to the chlorine atoms. In the solid state the Be centres accept lone pairs of electrons from chlorine atoms in neighbouring molecules to form long polymer chains. Single molecules of BeCl₂ are only present in the gas phase.

Carbon dioxide is another linear molecule. We saw in Section 2.1.4 that each central carbon atom has a double bond joining it to an oxygen atom. These two areas of electron density (the two double bonds) spread themselves as far apart as possible in a linear O=C=O arrangement, as shown in Figure 2.13.

Figure 2.13 A linear centre in carbon dioxide, CO₂.