Question

The structure of \(\mathrm{TeF}_{5}-\) is Draw a complete Lewis structure for \(\mathrm{TeF}_{5}-\), and explain the distortion from the ideal square pyramidal structure. (See Exercise 116.)

Short answer

The Lewis structure for the \(\mathrm{TeF}_{5}^-\) ion consists of a central Te atom surrounded by five F atoms and a lone pair on the Te atom. The distortion from the ideal square pyramidal structure is caused by the presence of the lone pair on the Te atom, which creates repulsion and pushes the bond pairs closer together due to the larger volume and stronger repulsion of the lone pair compared to the bond pairs.

Step-by-step solution

Finding the total number of valence electrons

First, let's find the total number of valence electrons for \(\mathrm{TeF}_{5}^-\). Tellurium (Te) belongs to group 16 and has 6 valence electrons. Fluorine (F) belongs to group 17 and has 7 valence electrons. There are five F atoms and a negative charge which adds one additional electron. Therefore, the total number of valence electrons is: \(6 + (5 \times 7) + 1 = 6 + 35 + 1 = 42 \)

Drawing the Lewis Structure

We now proceed to draw the Lewis structure for \(\mathrm{TeF}_{5}^-\). 1. Place the least electronegative atom (Te) in the center and arrange the F atoms around it. 2. Connect each F atom to the central Te atom using a single bond (this uses 2 valence electrons for each bond, thus using 10 valence electrons). 3. Complete the octet of all the F atoms by placing lone pairs (each F atom needs 6 more valence electrons, which uses the remaining 32 valence electrons). 4. The central Te atom will have a lone pair. The resulting Lewis structure is shown below: F | F - Te - F | F | F with a lone pair on the Te atom. Notice that based on the Lewis structure, the electron-domain geometry around the central Te atom is octahedral, with six electron domains (5 bond pairs and 1 lone pair).

Explain the distortion in geometry

In the ideal square pyramidal structure, the central atom is bonded to five peripheral atoms, and its electron-domain geometry is octahedral without any lone pairs. However, in \(\mathrm{TeF}_{5}^-\), the central atom (Te) has a lone pair in addition to the five surrounding F atoms. Due to the presence of the lone pair, the electron domains will repel each other. The lone pair holds a relatively larger volume and has stronger repulsion compared to the bond pairs. This results in the distortion of geometry from the ideal square pyramidal structure, as the lone pair pushes the bond pairs closer together.

Conclusion

In conclusion, the Lewis structure for the \(\mathrm{TeF}_{5}^-\) ion shows the central Te atom surrounded by five F atoms and a lone pair. The distortion from the ideal square pyramidal structure is due to the presence of the lone pair on the Te atom, which creates repulsion and pushes the bond pairs closer together.

Key concepts

Valence Electrons

Understanding valence electrons is fundamental to drawing Lewis structures. Valence electrons are the outermost electrons of an atom and are responsible for an atom's chemical properties. They play a crucial role in bond formation. In the case of \(\mathrm{TeF}_{5}^{-}\), we calculate the number of valence electrons by considering each atom's contribution. Tellurium (Te), located in group 16 of the periodic table, contributes 6 valence electrons. Fluorine (F), in group 17, has 7 valence electrons, and since there are 5 F atoms, they contribute 35 electrons in total. Furthermore, the negative charge on the molecule adds one additional electron, giving a total of 42 valence electrons.

Valence electrons:

• Determine the bonding capacity of atoms.
• Are used to form chemical bonds (shared in covalent bonds, transferred in ionic bonds).
• Help predict molecular geometry and reactivity.

Electron-Domain Geometry

Electron-domain geometry considers all electron domains around the central atom, including bonds and lone pairs, to predict the 3D shape of molecules. For \(\mathrm{TeF}_{5}^{-}\), the central tellurium atom is surrounded by five fluorine atoms and one lone pair of electrons. This results in six electron domains, which corresponds to an octahedral electron-domain geometry.

Key points about electron-domain geometry:

• All electron pairs, bonded or not, are considered, which affects the molecule's shape.
• Electron-domain geometry is often different from molecular geometry, which only considers the placement of atoms.
• The VSEPR (Valence Shell Electron Pair Repulsion) theory helps predict geometry by minimizing repulsion between electron pairs.

Lone Pair Repulsion

In molecular geometry, lone pair repulsion is crucial to understanding why molecules might deviate from ideal shapes. Lone pairs are unbonded electrons located on the central atom in a molecule, and they occupy more space than bonded pairs. In \(\mathrm{TeF}_{5}^{-}\), the presence of a lone pair on the tellurium atom results in electron-electron repulsion that is stronger than that between bonded pairs, causing the geometry to be distorted.

This increased repulsion can be explained by:

• Lone pairs are not shared between atoms and thus have a higher electron density.
• The repulsion causes bonded pairs to be pushed closer, altering ideal angles.
• Lone pair repulsion is often the reason structures differ from basic geometric expectations.

Octahedral Geometry

Octahedral geometry is a common electron-domain arrangement where six electron pairs surround a central atom. This shape is characterized by 90-degree bond angles and equidistant positioning of electron pairs. For \(\mathrm{TeF}_{5}^{-}\), we start with an octahedral arrangement due to its six total electron domains (five bonds and one lone pair). However, the presence of a lone pair causes deviations from the ideal octahedral structure, leading to a square pyramidal molecular geometry instead.

Characteristics of octahedral geometry include:

• Ideal for molecules or ions with six electron domains.
• Results in symmetric distribution around the central atom in the absence of lone pairs.
• Lone pairs can distort the geometry, as seen in \\(\mathrm{TeF}_{5}^{-}\), where the lone pair compresses bond angles away from 90 degrees.