Final answer:
The electron-pair geometry and molecular structure of molecules are determined by the regions of electron density around the central atom. Examples include trigonal planar, tetrahedral, linear, and octahedral shapes. Lone pair repulsions can also influence the molecular structure.
Step-by-step explanation:
The electron-pair geometry and the molecular structure of molecules or ions are determined by the regions of electron density around the central atom. The electron-pair geometry defines the arrangement of bonding pairs and lone pairs of electrons in a molecule. The number of these regions impacts the shape that minimizes the repulsion between them, leading to the various molecular geometries.
Examples include:
Trigonal Planar: This electron-pair geometry occurs with three regions of electron density, resulting in a flat, triangular shape. When all electron pairs are bonding pairs, the molecular structure is also trigonal planar.
Tetrahedral: With four regions of electron density, the electron-pair geometry is tetrahedral. If there are lone pairs, the molecular structure may change to bent or trigonal pyramidal.
Linear: With two regions of electron density, both the electron-pair geometry and molecular structure are linear.
Octahedral: Six regions of electron density create an octahedral electron-pair geometry. The molecular structure could be square pyramidal if there is one lone pair or square planar if there are two lone pairs.
When determining these geometries, one must also consider lone pair repulsions, as they take more space than bonding pairs, affecting the molecular structure.