What is the underlying principle on which the VSEPR model is based?

The concept of electron pair repulsion is fundamental in understanding why molecules take on certain shapes. Electrons possess negative charges, and because like charges repel each other, electron pairs in a molecule naturally repel one another. This repulsion is not limited to the electrons engaged in bonds, known as bonding pairs, but also includes the lone pairs of electrons that are not involved in bonding. Both types of electron pairs push away from each other, striving for the position of least repulsion which, in turn, dictates the molecular shape.
Consider the water molecule, H₂O, which has two hydrogen atoms and two lone pairs of electrons on the oxygen atom. These lone pairs exert a repulsive force, making the bonds between the oxygen and hydrogen atoms angle away from the lone pairs. This results in the bent shape of the water molecule.

Predicting the shape of molecules is a crucial aspect of chemistry and the VSEPR model serves as a valuable resource for this purpose. Molecular shape prediction involves determining how electron pairs—both bonding and lone pairs—arrange themselves around a central atom. The ideal arrangement is the one that minimizes repulsion and therefore maximizes distance between these electron pairs.
For instance, methane (CH₄) has four bonding pairs of electrons around the central carbon atom. According to VSEPR theory, these pairs will position themselves as far apart as possible, leading to a tetrahedral shape for methane. By utilizing the VSEPR model, students can predict the three-dimensional structure of a molecule which is critical for understanding its reactivity, polarity, and interactions with other molecules.

The term molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. Factors influencing molecular geometry include the number of bonding pairs, the presence of lone pairs, and the size of the atoms involved. Unlike molecular shape, which concerns the position of atoms, molecular geometry accounts for the spatial position of all electron pairs, including those not involved in chemical bonds.
Take carbon dioxide (CO₂), for example; the molecule displays a linear geometry because the three atoms—itself and the two oxygen atoms—align in a straight line. This is due to the double bonds between the carbon and oxygen atoms and the absence of lone pair repulsions. Understanding molecular geometry is vital for recognizing how molecules will interact with one another, and how they will behave in various chemical reactions and physical processes.