Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 100

Centres that have three areas of electron density adopt a trigonal planar structure, with angles of approximately 120° between each of the centres of electron density. A simple molecule with this structure is BCl₃.

Boron has three electrons in its outer shell, and each chlorine atom has seven. Thus boron shares each of its valence electrons with one unpaired electron from chlorine. This gives the boron atom six outer electrons, and the chlorine atoms each have eight outer electrons. The six electrons around boron form three single bonds to the chlorine atoms, thus creating three areas of electron density. These three bonds spread so that they are 120° apart and form a trigonal planar (or triangular planar) shape, as shown in Figure 2.14.

Figure 2.14 (a) Bonding in boron trichloride, BCl₃ with a trigonal planar centre; (b) showing the bonding angles in boron trichloride, BCl₃.

The boron centre is electron deficient because it has only six electrons in its outer shell once it has bonded to the chlorine atoms.

Another molecule that has a trigonal planar arrangement around its central atoms is ethene, C₂H₄. Each carbon atom has four electrons in its outer shell; to attain a complete octet, carbon shares two of these electrons with the single electrons of two hydrogen atoms and shares the remaining two electrons with two electrons from the other bonded carbon atom. Thus, each carbon atom in C₂H₄ has two single C—H bonds and one double C=C bond, giving it three distinct areas of electron density. Again, the atoms around each carbon atom spread so that they are approximately 120° apart in a trigonal planar arrangement, as shown in Figure 2.15.

Figure 2.15 (a) Bonding in C₂H₄, ethene with trigonal planar centre; (b) showing the bonding angles in ethene.