Problem 19 - Chemistry: An Atoms-Focused Approach

Question

Why does the seesaw structure have lower energy than a trigonal pyramidal structure when \(\mathrm{SN}=5 ?\)

Step-by-Step Solution

Answer

Answer: The seesaw structure has lower energy when the Steric Number (SN) is 5. This is due to its larger bond angles and reduced electron repulsion, as the lone pair is located in the equatorial plane, making it more stable compared to the trigonal pyramidal structure.

Step 1: Understanding Steric Number (SN) and Molecular Geometries

The Steric Number (SN) is the number of electron groups around the central atom in a molecule. When SN is 5, there are 5 electron groups, which can be a combination of bonding pairs and lone pairs. The seesaw structure has 4 bonding pairs and 1 lone pair, while the trigonal pyramidal structure has 3 bonding pairs and 2 lone pairs.

Step 2: Analyzing Bond Angles

The ideal bond angle for the seesaw structure is 90° and 120°, while the ideal bond angle for the trigonal pyramidal structure is 107°. The larger bond angles in the seesaw structure mean that the bonding pairs of electrons are more spread out, reducing electron-electron repulsion.

Step 3: Evaluating Electron Repulsion

In the seesaw structure, the lone pair of electrons is in the equatorial plane, which reduces its repulsive force on the other bonding pairs. In the trigonal pyramidal structure, the two lone pairs are in the axial positions, which cause a stronger repulsive force on the bonding pairs. The higher electron-electron repulsion in the trigonal pyramidal structure increases its energy compared to the seesaw structure.

Step 4: Comparing the Energy of the Structures

The seesaw structure has larger bond angles and reduced electron repulsion due to the lone pair's location in the equatorial plane. Conversely, the trigonal pyramidal structure has smaller bond angles and increased electron repulsion due to the axial position of its lone pairs. As a result, the seesaw structure has lower energy than the trigonal pyramidal structure, making it more stable when SN=5.

Key Concepts

Steric Number

The Steric Number (SN) is a fundamental concept in determining the molecular geometry of a molecule. It helps you understand how atoms are arranged around a central atom. In essence, the steric number is the count of both bonding pairs of electrons and lone pairs around a central atom.
For example, when you have SN=5, it means there are five groups of electrons that are interacting with each other around the central atom. These groups can include:

• Bonding pairs - which are shared electrons forming bonds with neighboring atoms.
• Lone pairs - which are unshared electrons residing on the central atom.

This collection of pairs around the central atom influences the geometric shape of the molecule. Different combinations of bonding and lone pairs will result in different molecular structures.

Seesaw Structure

The Seesaw Structure is a unique molecular geometry that occurs when the steric number is 5. It specifically forms when there are 4 bonding pairs and 1 lone pair.
The seesaw shape arises because of the specific placement of the lone pair in what is known as the equatorial plane. By positioning the lone pair here, the molecule achieves optimal energy configuration by reducing electron repulsion. Here's how it works:

• The equatorial plane allows the lone pair to push less strongly against the bonding pairs.
• This results in broader bond angles, with ideal angles being around 90° and 120°.
• Such an arrangement minimizes energy and stabilizes the molecule.

Overall, the seesaw structure is more stable due to these broader bond angles and lessened repulsive forces among the electrons.

Trigonal Pyramidal Structure

The Trigonal Pyramidal Structure is another possibility when SN=5, but it comes with certain distinctive features due to its arrangement. This shape forms when there are 3 bonding pairs and 2 lone pairs.
In this geometry, the lone pairs tend to occupy the axial positions, leading to:

• Smaller bond angles, approximately 107°, due to the increased electron repulsion from lone pairs.
• Stronger repulsive forces between lone pairs and bonding pairs because they are closer together.

Consequently, the trigonal pyramidal arrangement results in a higher energy configuration. Therefore, compared to the seesaw structure, it is less stable due to its increased electron-electron repulsion.