Molecular shape - Non-ABn Molecules
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Molecular shape - Non-ABn Molecules

1. Define the following: (a) ABn Molecules (b) Non-ABn Molecules

2. Given the following Lewis Structure of acetic acid (fig 1). Predict the molecular geometry of acetic acid from its Lewis structure and the bond angles associated with its geometry
Lewis Structure Acetic Acid

3. Write the general representative strings for Non-ABn Molecules.

1(a) ABn molecules are molecules (or ions) that have a single central atom bonded to two or more atoms of the same type. For example, CH4 (methane) has a central atom, carbon, bonded to four hydrogen atoms.

1(b) Non-ABn molecules are usually molecules larger than ABn molecules. Unlike ABn molecules, they do not have a single central atom. Instead they have two or more central atoms that are also called interior atoms. An electron-domain is determined using each of the interior atoms as a central atom. These individual electron-domain geometries are then integrated to form the electron-domain geometry of the non-ABn molecule.

2. The Lewis structure of acetic acid as shown, has three interior atoms: the leftmost C atom, the central C atom and the rightmost O atom. Each of these atoms is considered as a central atom and the electron-domain geometry is determined.
Leftmost C atom as a central atom: there are 4 electron-domains and all are bonding domains. Therefore the predicted electron-domain geometry and molecular geometry are both tetrahedral. Consequently the predicted bond angle is 109.5o.
Central C atom as a central atom: there are 3 electron-domains and all are bonding domains. Therefore the predicted electron-domain geometry and molecular geometry are both trigonal planar. The predicted bond angle will deviate slightly from the usual 120o because of the space demand of the double bond.
Rightmost O atom as a central atom: there are 4 electron-domains. 2 bonding domains and 2 nonbonding domains. Therefore the predicted electron-domain geometry is tetrahedral and its molecular geometry is bent because of the 2 nonbonding domains which also cause a slight deviation of the bond angle from 120o.
The integrated molecular geometry is as follows:
Molecular Geometry Acetic Acid

3. Non-ABn molecules are compounds. S7P1A17 is the general representative string for the structure of a compound because the location of the constituents of a compound relative to one another establishes its structure. (S7P3A31)chemical bonds is the key string implicit in the representative string.

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