Question

Predict the electron pair geometry, the molecular shape, and the bond angle for a hydrogen sulfide molecule, \(\mathrm{H}_{2} \mathrm{~S}\), using VSEPR theory.

Short answer

H₂S has a bent shape with a bond angle of approximately 104.5°, based on the tetrahedral electron pair geometry.

Step-by-step solution

Determine the Valence Electrons

Sulfur (S) has 6 valence electrons, and each hydrogen (H) has 1 valence electron. In total, the hydrogen sulfide (H₂S) molecule has 8 valence electrons.

Draw the Lewis Structure

The Lewis structure for H₂S places the sulfur atom in the center with two single bonds to each hydrogen atom, leaving two lone pairs of electrons on the sulfur.

Identify the Electron Pair Geometry

According to VSEPR theory, the central atom (sulfur with 2 bonding pairs and 2 lone pairs) is surrounded by four electron groups in total. This corresponds to a tetrahedral electron pair geometry.

Determine the Molecular Shape

With two bond pairs and two lone pairs, the actual molecular shape is 'bent' (or angular), similar to the shape of water (H₂O) since lone pairs take up more space and repel the bonding pairs.

Predict the Bond Angle

The tetrahedral electron pair geometry ideally has bond angles of 109.5°. However, the presence of two lone pairs results in greater repulsion, reducing the H-S-H bond angle to approximately 104.5°.

Key concepts

Electron Pair Geometry

In the context of molecules, electron pair geometry considers all electron groups around the central atom, regardless if they are bonding or non-bonding (lone pairs). This perspective is pivotal when employing VSEPR (Valence Shell Electron Pair Repulsion) Theory to predict molecular shapes.
According to VSEPR theory, because electrons repel each other, electron groups tend to position themselves as far apart as possible around the central atom, minimizing repulsion.
This results in specific geometric arrangements. For hydrogen sulfide (H₂S), the central atom is sulfur, which is surrounded by a total of four electron groups, comprising two bond pairs with hydrogen atoms and two lone pairs. This configuration leads to a tetrahedral electron pair geometry. Tetrahedral geometry is a common result when a central atom has four groups of electrons crowding its valence shell. Yet, this geometric name doesn't necessarily describe the molecule’s actual shape, which will be refined based on the nature of these groups.

Molecular Shape

While electron pair geometry considers all electron pairs, molecular shape only accounts for the spatial arrangement of the atoms themselves. It ignores non-bonding electrons for the moment, yet these lone pairs are crucial in determining the final shape, as their presence can effectively "push" the bond pairs closer together. In the case of H₂S, the electron pair geometry is tetrahedral as derived in the previous section. However, out of the four electron groups, two are lone pairs which exert more repulsion force than bonding pairs. This causes the hydrogen atoms to be pushed closer together than they would be in a perfect tetrahedral geometry.
The actual molecular shape of H₂S is then described as 'bent' or 'angular'. This type of shape is notably similar to the shape of the water molecule (H₂O), where two lone pairs also distort the ideal angles. The presence of these lone pair electrons is a key factor influencing the molecular shape.

Lewis Structure

A Lewis structure is a simplified model that helps to visualize the arrangement of valence electrons around atoms in a molecule. It is particularly useful for determining both the electron pair geometry and molecular shape of a molecule. In the case of hydrogen sulfide (H₂S), start with determining the total count of valence electrons. Sulfur contributes 6 valence electrons and each hydrogen contributes 1, leading to a total of 8 electrons.
The next step is to draft the Lewis structure, which is quite straightforward for H₂S. Place the sulfur atom in the center, forming single bonds with two hydrogen atoms. The rest of the electrons manifest as two lone pairs on the sulfur atom, represented by dots on the Lewis diagram. The Lewis structure provides a visual representation that showcases both bonding and lone pairs, forming the basis for predicting molecular geometry via VSEPR theory. By examining the Lewis structure, one can deduce the necessary information to predict both electron pair geometry and molecular shape.