Question

Which of the statements given below are true for the structure of water molecule? (i) Oxygen undergoes sp \({ }^{3}\) hybridisation. (ii) Due to presence of two lone pairs of electrons on oxygen the \(\mathrm{H}-\mathrm{O}-\mathrm{H}\) bond angle is \(118.4^{\circ}\) (iii) Due to angular geometry the net dipole moment of water is not zero, \(\mu=1.84 \mathrm{D}\). (a) (i) and (ii) (b) (ii) and (iii) (c) (i) and (iii) (d) only (ii)

Short answer

Statements (i) and (iii) are true. Oxygen undergoes sp3 hybridization and due to the angular geometry of water resulting from the two lone pairs on oxygen, the net dipole moment of water is not zero.

Step-by-step solution

Hybridization of Oxygen in Water

Examine the electronic configuration of oxygen and the hybridization necessary for forming two sigma bonds with hydrogen atoms and maintaining two lone pairs. Oxygen has six valence electrons and uses four of these (two for bonding with hydrogen atoms and two as lone pairs) to form a tetrahedral electron geometry. This corresponds to sp3 hybridization.

Determining the H-O-H Bond Angle

Inspect the influence of lone pairs on the bond angle. Lone pairs repel more strongly than bonding pairs, resulting in a reduction of the bond angle from the tetrahedral 109.5 degrees. The actual bond angle in water is approximately 104.5 degrees, not 118.4 degrees as given.

Assessing the Net Dipole Moment

Analyze the molecular geometry and the electronegativity difference between hydrogen and oxygen. Water has a bent shape due to the two lone pairs on oxygen. Oxygen is more electronegative than hydrogen, creating a dipole along each of the H-O bonds. These dipoles do not cancel out due to the molecular geometry, resulting in a net dipole moment.

Key concepts

Hybridization in Water Molecule

Understanding the hybridization in a water molecule is crucial for grasping its chemical behavior and properties. In a water molecule, the oxygen atom has six valence electrons. It forms two sigma bonds with hydrogen atoms and also has two lone pairs of electrons.

Hybridization is a model that explains how atomic orbitals combine to form new hybrid orbitals, which can then accommodate the pairs of electrons in bonds and lone pairs. For oxygen in the water molecule, the s-orbital and all three p-orbitals hybridize to form four equivalent sp3 hybrid orbitals. Each of these sp3 hybrid orbitals contains one electron from oxygen and can form a sigma bond with hydrogen or accommodate a lone pair. This results in a tetrahedral electron pair geometry, though the molecular shape is bent due to the lone pairs exerting more repulsion.

H-O-H Bond Angle

The H-O-H bond angle in a water molecule is another fundamental aspect to understand. According to VSEPR (Valence Shell Electron Pair Repulsion) theory, electron pairs, both bonding and lone pairs, repel each other and try to stay as far apart as possible.

If an atom has four pairs of electrons in sp3 hybridization, like oxygen does in a water molecule, the pairs would ideally situate themselves at the vertices of a tetrahedron with bond angles of 109.5 degrees. However, the two lone pairs on the oxygen atom in water repel each other and the bonding hydrogen atoms more strongly than the hydrogen atoms repel each other. This repulsion results in a decrease in the bond angle from the ideal tetrahedral angle to approximately 104.5 degrees. Thus, the statement given in the exercise, suggesting a bond angle of 118.4 degrees, is incorrect.

Dipole Moment of Water

Finally, the dipole moment of water is a measurable quantity that arises due to the asymmetric distribution of charges in the molecule. The electronegativity of oxygen is higher than that of hydrogen, meaning oxygen pulls the shared electrons in the H-O bonds closer to itself. This creates two polar bonds within the molecule.

Since water has a bent shape due to its two lone pairs, the polarity of the H-O bonds does not cancel out. Instead, they add up to produce a net dipole moment across the molecule. This makes water a polar molecule with a dipole moment of 1.84 Debye (D). The net dipole moment is an important factor contributing to the unique properties of water, such as its high boiling point and its ability to dissolve many ionic and polar substances efficiently. The correct statement, therefore, is that the net dipole moment of water is not zero.