Final answer:
Electron pair geometry is based on the regions of electron density, including both bonding and lone pairs, while molecular structure is determined by the arrangement of these regions and lone pairs. Examples include octahedral geometry leading to a square pyramidal structure, or a trigonal planar geometry with one lone pair creating a bent molecular structure. Lone pairs affect the geometry as they require more space and create repulsion that must be minimized.
Step-by-step explanation:
To predict the electron pair geometry and the molecular structure of molecules, you must first consider the number of regions of electron density around the central atom.
The electron pair geometry is determined by these regions which can include bonding pairs and lone pairs of electrons. Then, based on the number of lone pairs of electrons, you can determine the molecular structure which defines the shape of the molecule.
Let's look at some examples:
- The electron-pair geometry of a molecule with six regions of electron density is octahedral, but if one of these regions is a lone pair, the molecule has a square pyramidal structure.
- For a molecule with three regions of electron density, the electron-pair geometry is trigonal planar. If there is one lone pair in this arrangement, the structure is bent or angular, often at an angle of about 120 degrees.
- An electron-pair geometry that is tetrahedral arises from four regions of electron density. If one of the regions is a lone pair, we may have a trigonal pyramidal molecular structure. If two regions are lone pairs, the structure is bent, often with bond angles close to 109 degrees.
- Finally, molecules with two regions of electron density have a linear electron-pair geometry and a linear molecular structure.
Remember, the arrangement of lone pairs aims to minimize repulsions, which is why they occupy specific positions that minimize the repulsion within a given electron-pair geometry.