Question

Use the localized electron model to describe the bonding in \(\mathrm{H}_{2} \mathrm{O}\).

Short answer

In the water molecule, H₂O, the oxygen atom serves as the central atom and forms two single covalent bonds with hydrogen atoms, sharing a pair of electrons in each bond. The molecule exhibits a bent or angular molecular geometry due to the influence of the two lone pairs of electrons on the oxygen atom, which also results in a tetrahedral electron-pair geometry according to the localized electron model.

Step-by-step solution

1. Determine the valence electrons

Each atom in the molecule has a certain number of valence electrons that can be involved in bonding. For hydrogen atoms, there is one valence electron; for oxygen, there are six valence electrons.

2. Calculate the total number of valence electrons

Combine the valence electrons from all atoms in the molecule. For a water molecule, this is \( 2 \times (\text{1 valence electron for hydrogen}) + \text{6 valence electrons for oxygen} = 2 + 6 = 8 \text{ valence electrons} \)

3. Place the central atom

Identify the central atom, which is the atom with the highest bonding capacity or the lowest electronegativity. In the case of the water molecule, the central atom is oxygen since it has more available bonding sites than hydrogen.

4. Distribute the electrons around the atoms

Arrange the remaining atoms and valence electrons around the central atom to form bonds and satisfy the octet rule. Each bond requires 2 electrons. In the case of water, connect the oxygen atom with the two hydrogen atoms using covalent bonds (single bonds), which account for \(2 \times 2 = 4\) valence electrons. The remaining \(8 - 4 = 4\) valence electrons should be placed on the oxygen atom as two lone pairs, satisfying the octet rule for oxygen.

5. Determine the electron-pair geometry

Based on the distribution of electrons around the central atom (oxygen) and the orientation of the bonded atoms (hydrogen), determine the electron-pair geometry of the molecule. In the case of water, the electron-pair geometry is tetrahedral since there are two bonding electron pairs and two lone pairs.

6. Determine the molecular geometry

Ignoring the lone pairs, consider the positions of the atoms only. In the case of water, the molecular geometry is bent or angular since the hydrogen atoms are connected to the central oxygen atom in an angle.

7. Describe the bonding using the localized electron model

In the water molecule, the bonding can be described using the localized electron model as follows: the oxygen atom forms two single covalent bonds with each of the hydrogen atoms, sharing two bonding electron pairs between them. This forms a bent molecular geometry, with two lone pairs of electrons remaining on the oxygen atom, causing the molecule to have a tetrahedral electron-pair geometry.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a crucial role in chemical bonding. These electrons are found in the highest occupied energy level and are the ones involved in forming bonds between atoms. In the localized electron model, understanding the number of valence electrons helps determine how atoms will interact and bond with each other.
For example, in a water molecule (\( \mathrm{H}_2\mathrm{O} \)), the hydrogen atom contributes one valence electron, while the oxygen atom contributes six. Together, they share these electrons to form a stable molecule by filling their outer electron shells. Knowing the count of valence electrons helps predict the molecule's structure and properties.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of a molecule's atoms in space. It dictates how the molecule looks and can significantly affect its chemical and physical properties, such as polarity and reactivity.
For a water molecule, the oxygen atom is at the molecule's center, surrounded by two hydrogen atoms and two lone pairs of electrons. This arrangement gives water a bent or angular shape, as the lone pairs push the hydrogen atoms closer together. This specific molecular geometry explains why water is a polar molecule, essential for its role in many chemical reactions and its unique solvent capabilities.

Covalent Bonds

Covalent bonds are formed when two atoms share one or more pairs of valence electrons. In the localized electron model, covalent bonding is a key concept as it describes how the atoms within a molecule are held together.
In the case of water, the oxygen atom forms covalent bonds with two hydrogen atoms. Each bond consists of a shared pair of electrons, one from the hydrogen and one from the oxygen, contributing to a stable electron configuration. These single covalent bonds help achieve a full outer shell for each atom involved, satisfying their valence electron requirements and enabling the formation of a stable compound. This sharing of electrons leads to the strong bonding seen in water molecules, giving them unique properties such as high boiling and melting points.

Octet Rule

The octet rule is a classic rule in chemistry where atoms tend to form bonds until they are surrounded by eight valence electrons, leading to a more stable electron configuration, similar to that of noble gases. This rule helps predict how atoms will bond and is foundational in understanding molecular formation.
In water, the octet rule applies to the oxygen atom. While each hydrogen atom shares its one electron through covalent bonding, the oxygen atom aims to fill its outer shell with eight electrons. Through sharing its six valence electrons with those from hydrogen, oxygen completes an octet with two pairs of non-bonding electrons left on itself—commonly known as lone pairs. These lone pairs are critical in determining water's overall shape and resulting properties.