Question

What is the observed bond angle in a water molecule?

Short answer

The bond angle in a water molecule is approximately 104.5°.

Step-by-step solution

Understand the Question

The question asks us to determine the bond angle in a water (H2O) molecule. This involves identifying the geometric arrangement of the molecule's atoms and understanding how the bonds interact within that structure.

Determine Molecular Shape

Water, H2O, is composed of two hydrogen atoms bonded to one oxygen atom. According to VSEPR (Valence Shell Electron Pair Repulsion) theory, the molecule forms a bent or V-shaped structure. This is due to the two lone pairs of electrons on the oxygen atom that repel the bonded hydrogen atoms, creating the bent geometry.

Apply VSEPR Theory

According to VSEPR theory, lone pairs occupy more space and repel more than bonded pairs. In H2O, the lone pairs lead to a decrease in the bond angle. For a perfect tetrahedral angle, the angle is 109.5°, but the repulsion of the lone pairs causes the H—O—H bond angle to be less.

Identify the Bond Angle

Due to the presence of the lone pairs, the bond angle is reduced from the tetrahedral angle. For a water molecule, this bond angle is approximately 104.5°. Therefore, the H—O—H bond angle exhibits this reduced arrangement.

Key concepts

Molecular Geometry

Understanding molecular geometry is crucial when discussing the structure of molecules like water. Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. This spatial configuration determines many physical and chemical properties, including reactivity, polarity, and biological activity. In the case of water (H\(_2\)O), the molecular geometry can be described as bent or V-shaped. This shape is due to the repulsion forces between electron pairs, which will generally arrange themselves as far apart as possible to minimize repulsion. This arrangement affects how water molecules interact with each other and other substances, making water an excellent solvent.

Bond Angles

Bond angles are an essential aspect of molecular shape, representing the angle formed between two bonds that originate from a shared atom. These angles result from the spatial arrangement of atoms in a molecule and are influenced by the presence of lone pairs of electrons. In molecules with tetrahedral electron pair arrangements like water, the ideal bond angle is 109.5°. However, due to the presence of lone pairs, these angles usually decrease as lone pairs tend to occupy more space and repel bonded pairs more strongly. Thus, in a water molecule, the actual bond angle is about 104.5° instead, showcasing the influence of lone pairs.

Lone Pairs

Lone pairs are non-bonding pairs of electrons that reside in the outermost valence shell of an atom. Often overlooked by beginners, they play a significant role in determining the molecular shape and bond angles. In a molecule like H\(_2\)O, the oxygen atom has two lone pairs. These lone pairs repel the hydrogen atoms more strongly than the hydrogen atoms repel each other due to their larger electron cloud density. Consequently, the bond angle between hydrogen atoms is reduced as lone pairs require more space and push bonded atoms closer together. This is a perfect example of how lone pairs can alter a molecule's idealized geometry, leading to phenomena like bent shapes.

Bent Molecular Shape

The bent molecular shape is a widely recognized configuration that stems from specific compounds and electron pair interactions. In water, the bent shape is a direct consequence of the two lone pairs on the oxygen atom. These lone pairs act as if they are large, invisible balloons that push the hydrogen atoms on either side closer together, resulting in a bent structure. Unlike a linear shape where atoms are arranged in a straight line, a bent shape has angles typically less than the perfect tetrahedral angle of 109.5°, often around 104.5° for water. This bent structure is vital for allowing water to have its unique properties, such as a high boiling point, surface tension, and its role as a universal solvent.