Question

What is the geometric structure of the water molecule? How many pairs of valence electrons are there on the oxygen atom in the water molecule? What is the approximate \(\mathrm{H}-\mathrm{O}-\mathrm{H}\) bond angle in water?

Short answer

The geometric structure of a water molecule is bent or V-shaped, with one oxygen atom connected to two hydrogen atoms by covalent bonds. The oxygen atom has four pairs of valence electrons, two bonding pairs shared with hydrogen atoms, and two nonbonding pairs (lone pairs). The electron pairs repel each other, causing the H-O-H bond angle to be less than the ideal tetrahedral angle of 109.5 degrees. Experimentally, the H-O-H bond angle in a water molecule is approximately \(104.5^{\circ}\).

Step-by-step solution

Geometric structure of a water molecule

The water molecule H2O is composed of one oxygen atom and two hydrogen atoms connected by covalent bonds. The oxygen atom has six valence electrons: two lone pairs and two bonding pairs that are shared with the hydrogen atoms to form the H-O bonds. The electrons around the oxygen atom repel each other and push the hydrogen atoms to form an angle with the oxygen atom. This molecular geometry is known as Bent or V-shaped.

Number of pairs of valence electrons on the oxygen atom

The oxygen atom has a total of six valence electrons, from which two are shared with the hydrogen atoms to form the covalent bonds. The remaining four valence electrons are unshared and are considered as two pairs of nonbonding electrons or lone pairs. Hence, there are four pairs of valence electrons on the oxygen atom, two bonding pairs, and two nonbonding pairs.

H-O-H bond angle in the water molecule

The electron pairs (both bonding and nonbonding) surrounding the oxygen atom in a water molecule repel each other and arrange themselves to be as far apart as possible. Since nonbonding electrons create stronger repulsion compared to bonding electrons, the two lone pairs on the oxygen atom push the bonding pairs closer together, resulting in the H-O-H bond angle being less than the ideal tetrahedral angle of 109.5 degrees. Experimentally, the H-O-H bond angle in a water molecule is measured to be approximately \(104.5^{\circ}\).

Key concepts

Valence Electrons

Valence electrons are the electrons located in the outermost shell of an atom. They play a critical role in chemical bonding because they are the electrons that atoms use to form bonds.
In the case of a water molecule, also represented as \( \text{H}_{2}\text{O} \), the oxygen atom has six valence electrons.
These electrons are shared with the hydrogen atoms to form bonds. Being in the outermost shell makes them highly interactive with other atoms, allowing chemical reactions to occur.
It's important to remember:

• Oxygen contributes six valence electrons.
• Each hydrogen contributes one valence electron, totaling eight electrons that influence the molecule's properties.
• This configuration influences how oxygen participates in forming the structure of the water molecule.

Covalent Bonds

Covalent bonds are a type of chemical bond where atoms share pairs of electrons. These bonds form between nonmetals and involve the mutual sharing of valence electrons between atoms.
In a water molecule, the oxygen atom forms covalent bonds with two hydrogen atoms.
Here's how it works:

• Oxygen shares one of its valence electrons with each hydrogen.
• This sharing creates two covalent bonds: two hydrogen-oxygen bonds.
• The sharing allows the atoms involved to attain a more stable electron configuration.

Each hydrogen atom shares its single valence electron with the oxygen, forming the covalent bonds that hold the water molecule together in a stable arrangement.

Bond Angle

The bond angle is the angle formed between three atoms across at least two bonds. It is a critical aspect of molecular geometry, determining the shape of the molecule.
In the water molecule, the \( \mathrm{H}-\mathrm{O}-\mathrm{H} \) bond angle is approximately \(104.5^{\circ}\).
This decreased angle compared to the ideal tetrahedral angle of \(109.5^{\circ}\) results from the repulsion between lone pairs being stronger than between bonding pairs.
Remember:

• Lone pairs take up more space, causing the bond angle to reduce.
• The electron cloud of lone pairs creates greater repulsive forces.
• These angles affect how the molecule interacts physically and chemically.

The bond angle is a determinant factor in the uniqueness of the water molecule's properties, such as its bent shape.

Lone Pairs

Lone pairs refer to pairs of valence electrons that are not involved in bonding but still influence the shape and reactivity of a molecule. They are also known as nonbonding electron pairs.
In a water molecule, the oxygen atom has two lone pairs: four electrons that do not participate in the bonding with hydrogen.
Key aspects of lone pairs include:

• Lone pairs exert a significant repulsive force.
• This repulsion is greater than that of bonding pairs, causing the molecular shape to adjust.
• In water, these lone pairs force the H-O-H bonds closer together, contributing to its bent shape.

Lone pairs are crucial in the spatial arrangement of water molecules, affecting everything from bond angles to intermolecular forces.