The basic shapes

Key points: In the VSEPR model, regions of enhanced electron density take up positions as far apart as possible, and the shape of the molecule is identified by referring to the locations of the atoms in the resulting structure. The primary assumption of the VSEPR model is that regions of enhanced electron density, by which we mean bonding pairs, lone pairs, or the concentrations of electrons associated with multiple bonds, take up positions as far apart as possible so that the repulsions between them are minimized. For instance, four such regions of electron density will lie at the corners of a regular tetrahedron, five will lie at the corners of a trigonal bipyramid, and so on (Table 2.2). Although the arrangement of regions of electron density, both bonding regions and regions associated with lone pairs, governs the shape of the molecule, the name of the shape is determined by the arrangement of atoms, not the arrangement of the regions of electron density (Table 2.3). For instance, the NH3 molecule has four electron pairs that are disposed tetrahedrally, but as one of them is a lone pair the molecule itself is classified as trigonal pyramidal. One apex of the pyramid is occupied by the lone pair. Similarly, H2 O has a tetrahedral arrangement of its electron pairs but, as two of the pairs are lone pairs, the molecule is classified as angular (or ‘bent’). To apply the VSEPR model systematically, we first write down the Lewis structure for the molecule or ion and identify the central atom. Next, we count the number of atoms and lone pairs carried by that atom because each atom (whether it is singly or multiply bonded to the central atom) and each lone pair counts as one region of high electron den sity. To achieve lowest energy, these regions take up positions as far apart as possible, so we identify the basic shape they adopt by referring to Table 2.2. Finally, we note which locations correspond to atoms and identify the shape of the molecule from Table 2.3. Thus, a PCl5 molecule, with five single bonds and therefore five regions of electron density around the central atom, is predicted (and found) to be trigonal bipyramidal (7).

The VSEPR model is highly successful, but sometimes runs into difficulty when there is more than one basic shape of similar energy. For example, with five electron-dense regions around the central atom, a square-pyramidal arrangement is only slightly higher in energy than a trigonal-bipyramidal arrangement, and there are several examples of the former (12). Similarly, the basic shapes for seven electron-dense regions are less readily predicted than others, partly because so many different conformations correspond to similar energies. However, in the p block, seven-coordination is dominated by pentagonal-bipyramidal structures. For example, IF7 is pentagonal bipyramidal and XeF5 , with five bonds and two

lone pairs, is pentagonal planar. Lone pairs are stereochemically less influential when they belong to heavy p-block elements. The SeF₆² and TeCl₆² ions, for instance, are octahedral despite the presence of a lone pair on the Se and Te atoms. Lone pairs that do not influence the molecular geometry are said to be stereochemically inert and are usually in the non-directional s orbitals.

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مواضيع ذات صلة

Polyatomic molecules

Homonuclear diatomic molecules

The hydrogen molecule

Resonance

Polarizability

Electronegativity

Electron affinity

Atomic and ionic radii

The format of the periodic table

The periodic table

Penetration and shielding

The angular variation of atomic orbitals