Question

Why is the molecular structure of \(\mathrm{H}_{2} \mathrm{O}\) nonlinear, whereas that of \(\mathrm{BeF}_{2}\) is linear, even though both molecules consist of three atoms?

Short answer

The molecular structure of H2O is nonlinear (bent) because the oxygen atom has two non-bonding electron pairs, which repel the bonding electron pairs and result in a bent arrangement of the O-H bonds. In contrast, BeF2 is linear because the beryllium atom has no non-bonding electron pairs, allowing the Be-F bonds to arrange themselves in a straight line to minimize repulsion between the two bonding electron pairs.

Step-by-step solution

Determine the valence electron pairs around the central atom

First, let's identify the central atom in both H2O and BeF2. In H2O, oxygen (O) is the central atom, and in BeF2, beryllium (Be) is the central atom. Next, we will determine the number of valence electron pairs around the central atom in each molecule. Oxygen has 6 valence electrons, two of which are shared with each hydrogen atom to form covalent bonds. Beryllium has 2 valence electrons, one of which is shared with each fluorine atom to form covalent bonds.

Determine the number of bonding and non-bonding electron pairs around the central atom

In H2O, there are two bonding electron pairs (one for each O-H bond) and two non-bonding electron pairs on the oxygen atom. In BeF2, there are two bonding electron pairs (one for each Be-F bond) and no non-bonding electron pairs on the beryllium atom.

Describe the molecular structure based on VSEPR theory

According to the Valence Shell Electron Pair Repulsion (VSEPR) theory, the molecular shape is determined by minimizing the repulsion between electron pairs around the central atom. In H2O, the repulsions between the two bonding electron pairs and two non-bonding electron pairs on the oxygen atom cause them to be arranged in a tetrahedral fashion. However, the molecular shape only takes into account the atomic positions, so the H2O molecule has a bent or angular geometry. In BeF2, there are only two bonding electron pairs around the beryllium atom, which causes the repulsion between them to be minimized when they are positioned opposite each other, leading to a linear geometry.

Explain the difference in molecular structure

The molecular structure of H2O is nonlinear (bent) because of the presence of two non-bonding electron pairs on the oxygen atom, which repel the bonding electron pairs and result in a bent arrangement of the O-H bonds. In contrast, the molecular structure of BeF2 is linear due to the absence of non-bonding electron pairs on the beryllium atom, allowing the Be-F bonds to arrange themselves in a straight line to minimize repulsion between the two bonding electron pairs.

Key concepts

Molecular Geometry

Understanding molecular geometry is crucial for predicting the shape of molecules like water (\(\text{H}_2\text{O}\)) and beryllium fluoride (\(\text{BeF}_2\)).Molecular geometry refers to the 3D arrangement of atoms within a molecule. It is governed by the Valence Shell Electron Pair Repulsion (VSEPR) theory.
According to this theory, electron pairs around a central atom arrange themselves to minimize repulsion. This leads to the overall shape of the molecule:

• Bent Geometry: In \(\text{H}_2\text{O}\), the molecule has a bent shape due to two bonding pairs and two lone pairs on oxygen.
• Linear Geometry: In \(\text{BeF}_2\), the molecule is linear because it only has two bonding pairs on beryllium, with no lone pairs.

These different geometries affect how molecules behave in chemical reactions and how they interact with other substances.

Valence Electrons

Valence electrons play a pivotal role in chemical bonding and molecular structure. They are the outermost electrons and are crucial for determining how atoms connect.
Here's a quick look:

• Oxygen in \(\text{H}_2\text{O}\): Has 6 valence electrons; forms two covalent bonds with hydrogen, leaving two electrons as lone pairs.
• Beryllium in \(\text{BeF}_2\): Has 2 valence electrons; forms two covalent bonds with fluorine, using up all its valence electrons.

Understanding valence electrons helps us predict bonding patterns and molecular shapes, making it easier to understand chemical reactions and interactions.

Bonding Electron Pairs

Bonding electron pairs are those involved in forming covalent bonds between atoms. They are shared by two atoms to hold a molecule together.
For example, in the molecules:

• \(\text{H}_2\text{O}\): The oxygen shares its electrons with two hydrogen atoms, making two bonding pairs (O–H).
• \(\text{BeF}_2\): The beryllium shares its electrons with two fluorine atoms, making two bonding pairs (Be–F).

Bonding pairs influence the molecule's shape and its chemical properties, impacting how a molecule functions and interacts in various environments.

Non-bonding Electron Pairs

Non-bonding electron pairs, or lone pairs, are electrons that are not involved in bonding but significantly influence molecular geometry.
They occupy space around the central atom, affecting the shape and angles:

• \(\text{H}_2\text{O}\): Oxygen has two lone pairs that repel the bonding pairs, resulting in the bent shape.
• \(\text{BeF}_2\): Beryllium has no lone pairs, allowing the bonding pairs to align linearly.

These lone pairs are crucial for understanding deviations from idealized shapes predicted by VSEPR theory, leading to the diverse structures we observe in different substances.