What causes deviation from ideal VSEPR shapes in certain molecules?

The deviation from ideal VSEPR shapes in certain molecules is caused by the presence of lone pairs of electrons.

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a simple model used in chemistry to predict the geometry of individual molecules based on the number of electron pairs surrounding their central atoms. However, in certain molecules, the observed shapes deviate from the ideal shapes predicted by the VSEPR theory. This deviation is primarily due to the presence of lone pairs of electrons on the central atom.

Lone pairs of electrons are pairs of valence electrons that are not shared with another atom and are not involved in a chemical bond. They occupy more space around the central atom than bonding pairs, causing a greater repulsion between them and the bonding pairs. This repulsion distorts the shape of the molecule, causing it to deviate from the ideal VSEPR shape.

For example, consider the water molecule (H2O). According to the VSEPR theory, with two bonding pairs and two lone pairs of electrons, the water molecule should have a tetrahedral shape. However, due to the greater repulsion of the lone pairs, the actual shape of a water molecule is bent or V-shaped.

Another factor that can cause deviation from the ideal VSEPR shapes is the difference in electronegativity between the atoms in a molecule. Electronegativity is a measure of how strongly atoms attract bonding electrons to themselves. When there is a significant difference in electronegativity, the bonding electrons are drawn more towards the more electronegative atom. This uneven distribution of electrons can distort the shape of the molecule, causing it to deviate from the ideal VSEPR shape.

In conclusion, while the VSEPR theory provides a useful model for predicting the shapes of molecules, it is not always accurate. The presence of lone pairs of electrons and differences in electronegativity can cause molecules to deviate from the ideal VSEPR shapes.