How does VSEPR theory predict the shape of molecules?

VSEPR theory predicts the shape of molecules by considering the repulsion between electron pairs in the molecule's outer shell.

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a simple yet powerful approach used to predict the shape of atoms that form covalent bonds. According to this theory, the electron pairs in the outermost shell, or valence shell, of an atom repel each other. They try to get as far away from each other as possible, leading to a specific molecular shape.

The VSEPR theory is based on the principle that electron pairs in the valence shell of an atom repel each other because they carry the same negative charge. This repulsion is what determines the shape of the molecule. The electron pairs arrange themselves in a way that minimises this repulsion, leading to the most stable configuration.

The theory considers both bonding electron pairs (those involved in forming covalent bonds) and lone pairs (those not involved in bonding). Lone pairs repel more strongly than bonding pairs, so they influence the shape more. For example, water (H2O) has two bonding pairs and two lone pairs of electrons around the oxygen atom. The repulsion between these pairs gives water its bent shape.

The VSEPR theory uses a notation system to describe the shapes of molecules. The central atom is represented by the letter A, a bonding pair by the letter X, and a lone pair by the letter E. For instance, methane (CH4) is represented as AX4, meaning it has one central atom and four bonding pairs, giving it a tetrahedral shape.

In summary, the VSEPR theory predicts the shape of molecules by considering the repulsion between electron pairs in the valence shell of an atom. The electron pairs arrange themselves to minimise this repulsion, resulting in a specific molecular shape. The theory takes into account both bonding and lone pairs of electrons, with lone pairs having a greater influence on the shape due to their stronger repulsion.