What principle explains the shape of a molecule based on electron pairs?

The shape of a molecule is primarily explained by the electron pair repulsion theory, which is based on the idea that electron pairs around a central atom will arrange themselves as far apart as possible to minimize repulsion between them. This theory is essential for predicting the geometry of molecules.

According to this principle, the spatial arrangement of the electron pairs surrounding the nucleus determines the molecular shape. Electron pairs can be either bonding pairs (which are involved in bonds between atoms) or lone pairs (which are not involved in bonding). Lone pairs repel more strongly than bonding pairs, affecting the overall shape of the molecule. For instance, in a molecule with four electron pairs, such as methane (CH4), the shape is tetrahedral due to the equal repulsion between all four bonding pairs. However, in ammonia (NH3), which has one lone pair and three bonding pairs, the shape is trigonal pyramidal because the lone pair exerts greater repulsive force, altering the traditional tetrahedral angle.

This theory allows chemists to predict not just the likely shapes of various molecules but also to understand how molecular geometry can influence reactivity and physical properties of substances. It is a foundational concept in molecular modeling and chemical reactivity.