Why do bond angles change?

Bond angles can change in molecules and compounds for several reasons, primarily due to the influence of various factors that affect the distribution of electrons around the central atom. The most common factors that can cause bond angles to change include:

Steric Repulsion: Electrons in the electron clouds of atoms and lone pairs of electrons tend to repel each other. When two atoms or lone pairs of electrons are too close to each other, they experience strong repulsive forces, which can lead to a deviation from the ideal bond angle. This is known as steric repulsion, and it causes bond angles to be larger or smaller than their ideal values.

Hybridization: The concept of hybridization explains how atomic orbitals mix to form hybrid orbitals that influence bond angles. Different hybridization states result in different bond angles. For example, in a molecule with tetrahedral geometry like methane (CH4), carbon undergoes sp3 hybridization, resulting in bond angles of approximately 109.5 degrees. In contrast, in a molecule like ethene (C2H4), carbon undergoes sp2 hybridization, leading to bond angles of approximately 120 degrees.

Electron Pair Geometry: The arrangement of electron pairs around a central atom, as predicted by VSEPR (Valence Shell Electron Pair Repulsion) theory, affects bond angles. VSEPR theory predicts different geometries (e.g., linear, trigonal planar, tetrahedral) based on the number of bonding pairs and lone pairs of electrons around the central atom, which in turn influences the bond angles.

Presence of Lone Pairs: Lone pairs of electrons on the central atom can exert greater repulsive forces than bonding pairs, leading to changes in bond angles. For example, in water (H2O), the presence of two lone pairs on the oxygen atom causes the bond angles to be approximately 104.5 degrees, deviating from the ideal tetrahedral angle.

Molecular Distortions: Some molecules undergo distortions due to various factors, such as resonance effects, charge distribution, or external influences. These distortions can lead to deviations from the expected bond angles.

Bonding with Different Atoms: Bond angles can also be affected by the identity of the atoms involved in the chemical bond. Different atoms have different sizes and electronegativities, which can influence the bond angles in a molecule. For instance, the bond angles in a molecule like ammonia (NH3) are different from those in methane (CH4) due to the presence of a more electronegative nitrogen atom.

In summary, bond angles change primarily due to the interplay of electron repulsion, hybridization, lone pairs, molecular geometry, and the specific atoms involved in the bonding. Understanding these factors is essential for predicting and explaining the geometry of molecules and their bond angles.