Question

In the molecule \(\mathrm{H}_{2} \mathrm{~S}\), the four electron pairs around the sulfur atom are arranged in a tetrahedral geometry. However, the shape of the molecule is called bent. Why does the shape of the molecule have a different name from the name of the electron pair geometry?

Short answer

The molecule \(\text{H}_2S\) is 'bent' because of the lone pairs of electrons that repel the bonding pairs, altering the shape from tetrahedral to bent.

Step-by-step solution

Identify the Electron Pair Geometry

The molecule \(\text{H}_2S\) has four pairs of electrons around the sulfur atom. This arrangement typically adopts a tetrahedral shape because it minimizes the repulsion between electron pairs.

Determine Lone Pairs and Bonding Pairs

Sulfur has six valence electrons. Two of them form bonds with hydrogen atoms, and the remaining four electrons form two lone pairs. Thus, there are two bonding pairs and two lone pairs on sulfur.

Understand the Effect of Lone Pairs

Lone pairs occupy more space compared to bonding pairs because lone pairs are localized on the sulfur atom, while bonding pairs are shared between sulfur and hydrogen. This leads to greater repulsion between lone pairs than between bonding pairs.

Describe the Molecular Shape

Due to the repulsion caused by the lone pairs, the bonding pairs are pushed closer together. This results in the \(\text{H}_2S\) molecule adopting a 'bent' shape, rather than a tetrahedral shape.

Conclude the Difference in Naming

Although the electron pairs around sulfur are arranged in a tetrahedral geometry, the shape of the molecule is referred to as 'bent' due to the position of the hydrogen atoms as influenced by the lone pairs.

Key concepts

Electron Pair Geometry

The electron pair geometry around a central atom considers all electron pairs. Both bonding pairs and lone pairs are included in this arrangement. In the case of the molecule \(\text{H}_2S\), sulfur has four pairs of electrons around it.
Because electron pairs repel each other, they position themselves as far apart as possible. This minimizes repulsion and leads to a specific arrangement. For four pairs of electrons, this arrangement is tetrahedral.
In a tetrahedral arrangement, the four pairs of electrons are situated at the corners of a tetrahedron, creating a 109.5-degree angle between them.
This electron pair geometry is what ultimately determines the initial configuration of the molecule's electron pairs.

Lone Pairs

In the \(\text{H}_2S\) molecule, sulfur has two lone pairs of electrons in addition to the two pairs of bonding electrons that connect to hydrogen atoms.
Lone pairs are electron pairs that are not involved in bonding; they are localized on the central atom. This localization causes lone pairs to occupy more space compared to bonding pairs.
Because lone pairs take up more room, they exert greater repulsive force on nearby electron pairs. As a result, lone pairs can significantly alter the shape of a molecule by pushing bonding pairs closer together.
For example, in \(\text{H}_2S\), the lone pairs on sulfur push the bonding pairs closer, creating a bent shape for the molecule.

Molecular Shape

The molecular shape describes the arrangement of the atoms in a space, not the electron pairs. It only considers the positions of the atoms.
In the molecule \(\text{H}_2S\), although the electron pairs are arranged tetrahedrally, the actual shape of the molecule is different. This is due to the influence of lone pairs.
The two lone pairs on sulfur push the hydrogen atoms closer together, resulting in a 'bent' molecular shape. The bond angle between the hydrogen atoms is less than the tetrahedral angle of 109.5 degrees, and is typically around 104.5 degrees in bent molecules like \(\text{H}_2S\).
This discrepancy between electron pair geometry and molecular shape is a key concept in VSEPR Theory.

Bonding Pairs

Bonding pairs are pairs of electrons that are shared between two atoms, creating a bond. Each bonding pair represents a bond which holds the atoms together.
In \(\text{H}_2S\), there are two bonding pairs of electrons that connect sulfur to two hydrogen atoms. These bonding pairs form the chemical bonds in the molecule.
Unlike lone pairs, bonding pairs are shared between atoms. This shared nature means they occupy slightly less space and exert less repulsion on adjacent electron pairs.
The number and position of bonding pairs determine how the atoms are arranged, which helps in identifying the molecular shape. When considering \(\text{H}_2S\), the two bonding pairs are pushed closer together due to the repulsion from the lone pairs, resulting in the bent shape.