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

In short, the molecular structure of H2O is nonlinear due to the presence of 2 bond pairs and 2 lone pairs surrounding the central oxygen atom, which arrange in a tetrahedral configuration, resulting in a bent shape. In contrast, BeF2 has a linear structure because there are only 2 bond pairs surrounding the central beryllium atom and no lone pairs, leading to the least repulsive arrangement being a straight line with a bond angle of 180°. This difference is explained by the VSEPR theory, which states that electron pairs in the valence shell arrange themselves to minimize repulsion.

Step-by-step solution

Understand the VSEPR theory

The valence shell electron pair repulsion (VSEPR) theory states that, in a molecule, electron pairs in the valence shell of a central atom arrange themselves in a way that minimizes their mutual electrostatic repulsion. Thus, the VSEPR theory gives us valuable insight into the shapes of molecules. The repulsions between electron pairs surrounding an atom depend on the number of lone pairs and bond pairs of electrons in the valence shell.

Determine the electron pair configuration of H2O

To apply the VSEPR theory, we first need to figure out the electron pair configuration for H2O. Oxygen has 6 valence electrons, and each hydrogen atom contributes 1 electron. In an H2O molecule, each hydrogen atom forms a single bond with the oxygen atom. This means there are two bond pairs surrounding oxygen. Since there are 6 valence electrons for oxygen and 2 of them participate in bonding, the remaining 4 form 2 lone pairs.

Apply VSEPR theory to H2O

According to VSEPR theory, with 4 electron pairs (2 bond pairs and 2 lone pairs) surrounding the oxygen atom, these pairs will arrange themselves in a way that minimizes repulsion. A tetrahedral geometry provides the arrangement with the least repulsion between electron pairs. However, due to the presence of the 2 lone pairs, the H2O molecule forms a bent (nonlinear) shape. So, the molecular structure of H2O is nonlinear, with the bond angle at around 104.5°.

Determine the electron pair configuration of BeF2

Now let's apply the VSEPR theory for BeF2. Beryllium has 2 valence electrons, and each fluorine atom has 7 valence electrons. Beryllium forms single bonds with both fluorine atoms, resulting in 2 bond pairs and no lone pairs surrounding the beryllium atom. The 2 bond pairs occupy the valence shell of beryllium.

Apply VSEPR theory to BeF2

Since there are only 2 bond pairs around the beryllium atom and no lone pairs, the least repulsive arrangement is a linear geometry with bond pairs at opposite ends of the beryllium atom. Thus, the molecular structure of BeF2 is linear with a bond angle of 180°. In conclusion, the molecular structure of H2O is nonlinear and that of BeF2 is linear due to the differences in the number of bond pairs and lone pairs surrounding the central atom, according to the VSEPR theory.