Why do some molecules have distorted geometries?

Some molecules have distorted geometries due to differences in electronegativity and the presence of lone pairs of electrons.

In more detail, the shape of a molecule is determined by the arrangement of the electron pairs around the central atom. This arrangement is described by the Valence Shell Electron Pair Repulsion (VSEPR) theory. According to this theory, electron pairs will arrange themselves to be as far apart as possible to minimise repulsion. This results in the geometric shape of the molecule.

However, not all electron pairs are shared between atoms. Some atoms have lone pairs of electrons, which are not involved in bonding. These lone pairs occupy more space than bonding pairs, causing greater repulsion and distorting the geometry of the molecule. For example, water (H2O) has two lone pairs of electrons on the oxygen atom, resulting in a bent or V-shaped geometry instead of the expected linear shape.

Furthermore, differences in electronegativity between atoms can also cause distortion. Electronegativity is a measure of an atom's ability to attract shared electrons. When atoms with different electronegativities form a bond, the shared electrons are not evenly distributed. The more electronegative atom will pull the electrons closer to itself, creating an uneven electron distribution and a polar bond. This can cause a distortion in the molecule's geometry. For instance, in the molecule BF3, the boron atom is less electronegative than the fluorine atoms, causing the molecule to have a trigonal planar shape instead of the expected tetrahedral shape.

In conclusion, the geometry of a molecule is not always what we would expect based on the number of atoms and bonds alone. The presence of lone pairs and differences in electronegativity can cause distortions in the molecule's shape. Understanding these factors is crucial for predicting the geometry and properties of molecules.