Why is water a bent molecule while carbon dioxide is linear?

Water is a bent molecule due to its two lone pairs of electrons, while carbon dioxide is linear because it has no lone pairs.

Water (H2O) and carbon dioxide (CO2) are both simple molecules, but they have different shapes due to the arrangement of their electrons. In water, the central oxygen atom is bonded to two hydrogen atoms and also has two lone pairs of electrons. These lone pairs occupy space around the oxygen atom and repel the bonds holding the hydrogen atoms, causing the molecule to adopt a bent or V-shaped structure. This is a result of the molecule trying to minimise repulsion and achieve the most stable configuration. The bond angle in a water molecule is approximately 104.5 degrees.

On the other hand, carbon dioxide has a central carbon atom bonded to two oxygen atoms. Unlike water, carbon dioxide does not have any lone pairs of electrons on the central atom. Each oxygen atom forms a double bond with the carbon atom. The repulsion between these bonds is equal, causing the molecule to be linear in shape. The bond angle in a carbon dioxide molecule is 180 degrees.

The shapes of these molecules can be predicted using Valence Shell Electron Pair Repulsion (VSEPR) theory. According to this theory, the shape of a molecule is determined by the repulsion between the electron pairs in the valence shell of the central atom. The electron pairs arrange themselves in a way that minimises this repulsion, resulting in the observed shape of the molecule. In the case of water, the two lone pairs of electrons on the oxygen atom cause greater repulsion than the bonding pairs, leading to a bent shape. In contrast, the absence of lone pairs on the carbon atom in carbon dioxide results in a linear shape.

To further understand the influence of electron arrangements in complex molecules, you can refer to the discussion on transition metals. Additionally, the concepts of polar and non-polar covalent bonds play a significant role in the properties and arrangements of these molecular structures. For a deeper exploration of how molecular structures are influenced by bonding, check out our notes on covalent bonding and molecular structures.

IB Chemistry Tutor Summary: Water is bent and carbon dioxide is linear because of the different electron arrangements around their central atoms. Water's oxygen has two lone electron pairs pushing its hydrogen atoms into a V-shape, making it bent. Carbon dioxide's carbon doesn't have lone pairs, so its oxygen atoms line up straight. VSEPR theory explains these shapes by how electron pairs repel each other to minimise repulsion.