Molecular Geometry
Lewis structures provide a simple method of estimating molecular shapes. The geometry about any atom covalently bonded to two or more other atoms is found by counting the number of electron groups around the atom. Each unshared pair counts as one groups and each bond, weather single or multiple counts as one group. The number of electron groups around and atom is therefore equal to the sum of the number of electron pairs on the atom and the number of other atoms bonded to it. The geometry is linear if the number of electron groups is two, trigonal if its 3, and tetrahedral if its 4.
The rule is based on teh electron pair repulsion model, which postulates that because electron pairs repel each other, they will try to stay as far apart as possible. In trigonal and tetrahedral geometries, the shape will be exactly trigonal (120° bond angles), or exactly tetrahedral (109.5° bond angles) if the electron groups are all equivalent, as for example in BH3, or CH3+ (trigonal), or in CH4, or NH4+ (tetrahedral).
If the groups are not all equivalent, the angles will deviate from the ideal values. Thus in NH, (four electron groups, three in N-H bonds, one an unshared pair), the unshared pair, being attracted only by the nitrogen nucleus, will be closer to the nitrogen on the average than will the bonding pairs, which are also attracted by a hydrogen nucleus. Therefore the repulsion between the unshared pair and a bonding pair is greater than between two bonding pairs, and the bonding pairs will be pushed closer to each other. The H-N-H angle should therefore be less than 109.5°. It is found experimentally to be 107°. Similarly, in H2O (four electron groups, two unshared pairs, and two 0-H bonds), the angle is 104.5°.
Ambiguity may arise when more than one structure contributes. Then unshared pairs in one structure may become multiple bonds in another, so that the number of electron groups around a given atom is not the same in both structures. An example is methyl azide (19). The central nitrogen is clearly linear (two electron groups), but the nitrogen bonded to CH3, has three electron groups in
19a and four in 19b. In such a situation, the number of electron groups is determined from the structure with the larger number of honds. Thus the nitrogen in question in 19 is trigonal, not tetrahedral.
on September 10, 2009 at 10:22 pm
Permalink
had recently herpes virus found in my blood. what I haveto do??? I’m in panic…