Question

The shape of water molecule which should be tetrahedral has a bent or distorted tetrahedral shape with a bond angle \(104.5^{\circ}\). What could be the reason for this? (a) lp-lp repulsion is more than lp-bp repulsion. (b) Ip-bp repulsion is more than Ip-ip repulsion. (c) lp-lp repulsion is equal to lp-bp repulsion. (d) Presence of lone pair does not affect the bond angle.

Short answer

The bent shape of the water molecule is due to the lp-lp repulsion being stronger than lp-bp repulsion, causing the bond angle to reduce to 104.5 degrees.

Step-by-step solution

Understand the effect of electron pair repulsion

VSEPR theory (Valence Shell Electron Pair Repulsion theory) states that electron pairs around a central atom tend to orient themselves as far apart as possible. This is because electron pairs are negatively charged and repel each other.

Identify lone pairs and bonding pairs in water molecule

In a water molecule (H2O), the oxygen atom has two lone pairs (lp) and two bonding pairs (bp) with hydrogen atoms. A tetrahedral shape is expected when there are four bonding pairs, but the presence of lone pairs distorts this geometry.

Recognize the types of repulsion

Electron pair repulsion can be classified into lp-lp, lp-bp, and bp-bp repulsion. Lone pair-lone pair (lp-lp) repulsion is generally stronger than lone pair-bonding pair (lp-bp) repulsion, which in turn is stronger than bonding pair-bonding pair (bp-bp) repulsion.

Determine the reason for the bent shape of water

Due to the strong repulsion between the two lone pairs present on oxygen, the bond angle is reduced from the expected 109.5 degrees in a perfect tetrahedral arrangement to 104.5 degrees, resulting in a bent shape of the water molecule. Therefore, option (a) lp-lp repulsion is more than lp-bp repulsion is the correct reason for the observed geometry of the water molecule.

Key concepts

Lone Pair Repulsion

Understanding the concept of lone pair repulsion is crucial for explaining the shape and geometry of molecules, such as water. In accordance with the VSEPR theory, the lone pair repulsion plays a significant role in determining the molecular structure. Lone pairs are valence electrons that are not shared with other atoms and are located around the central atom. They occupy more space than bonding pairs because they are localized closer to the nucleus and have a stronger repulsive force, primarily due to their higher electron density.

Lone pair-lone pair (lp-lp) repulsion is the strongest type of electron pair repulsion. This is because the electrons in lone pairs only feel the attraction from one nucleus, thus they spread out more in space, leading to a greater repulsive force when compared to bonding pairs. In a water molecule, the two lone pairs on the oxygen atom push away from each other as much as possible, which significantly affects the molecular geometry. The repulsion between these lone pairs forces the hydrogen-oxygen-hydrogen bond angle to become smaller than what is found in a perfect tetrahedral structure.

Molecular Geometry

The molecular geometry of a compound is a three-dimensional arrangement of atoms within a molecule. It is one of the foundational concepts in chemistry because it determines the molecule's reactivity, polarity, color, phase of matter, and biological activity.

The geometry can be predicted using VSEPR theory, which provides a simple way of rationalizing the shape of molecules by considering lone pairs and bonding pairs of electrons. For example, a molecule with four bonding pairs, like methane (CH₄), would have a perfect tetrahedral shape. However, in the case of water (H₂O), the presence of two lone pairs distorts this ideal geometry, leading to a 'bent' shape. The two lone pairs on the central oxygen atom repel each other and push the bonding pairs closer together. This modification is essential to predict the physical and chemical properties since the molecular geometry influences the molecule's dipole moment and its interactions with other molecules.

Bond Angle

The bond angle is the angle formed between three atoms across at least two bonds. In VSEPR theory, the bond angles are affected by the repulsion forces between electron pairs in the valence shell of the central atom. In a perfectly tetrahedral molecule, the bond angles are 109.5 degrees. However, due to lone pair repulsions, actual bond angles in molecules can be different.

In water, the idealized tetrahedral angle is reduced to 104.5 degrees because the lone pairs occupy more space than bonding pairs, and as they push down the hydrogen atoms, the bond angle tightens. This is an essential concept for students to understand as bond angles influence molecular polarity and the interactions that molecules have with each other, affecting everything from the boiling point of a substance to its biochemical interactions.