Question

The structure of \(\operatorname{Te} \mathrm{F}_{5}^{-}\) is Draw a complete Lewis structure for \(\operatorname{TeF}_{5}^{-},\) and explain the distortion from the ideal square pyramidal structure. (See Exercise 26.)

Short answer

The Lewis structure for the \(TeF_5^-\) molecule consists of a central Tellurium (Te) atom surrounded by five Fluorine (F) atoms, with one lone pair on the Te atom. The ideal square pyramidal structure has central atom-bond angles of 90° and 180°. However, due to the presence of the lone pair on Te, repulsion forces between the lone pair and the bonding electrons in the \(Te-F\) bonds cause the molecule to distort from the ideal square pyramidal structure, leading to slightly smaller bond angles.

Step-by-step solution

Count the total number of valence electrons

First, let's determine the number of valence electrons in the \(TeF_5^-\) molecule. Tellurium (Te) belongs to group 16 and has 6 valence electrons, while Fluorine (F) belongs to group 17 and has 7 valence electrons. Since there are 5 Fluorine atoms and an extra electron due to the negative charge, the total number of valence electrons in the \(TeF_5^-\) molecule is: 6 (for Te) + 5 × 7 (for F) + 1 (extra electron) = 6 + 35 + 1 = 42

Arrange the atoms

Next, we will arrange the atoms for the \(TeF_5^-\) molecule. Tellurium (Te) will be the central atom, as it is less electronegative than Fluorine. The 5 Fluorine atoms will surround the Tellurium atom.

Generate a skeleton structure

Now let's create a skeleton structure for the \(TeF_5^-\) molecule. Connect each Fluorine atom to the central Tellurium atom with a single bond. This uses 10 electrons, leaving 32 remaining electrons.

Distribute the remaining electrons as lone pairs

We will now distribute the remaining 32 electrons as lone pairs around the surrounding Fluorine atoms. Each Fluorine atom needs 3 more lone pairs (6 electrons) to satisfy the octet rule. After placing these lone pairs, all the electrons have been used, and each atom has an octet of electrons.

Check the octet rule for all atoms

Finally, we need to ensure that the octet rule is satisfied for all atoms in the \(TeF_5^-\) molecule. All 5 Fluorine atoms have an octet of electrons, and the central Tellurium atom has 10 electrons (5 shared bonds with Fluorine and one lone pair). This exceeds the octet rule, but it is allowed as Tellurium is in the third period and can have an expanded octet.

Discuss the distortion from the ideal square pyramidal structure

The ideal square pyramidal structure should have a central atom bonded to 5 surrounding atoms, with bond angles of 90° and 180°. However, due to the presence of the lone pair on the central Tellurium atom, repulsion forces between the lone pair and the bonding electrons in the \(Te-F\) bonds cause the molecule to distort from the ideal square pyramidal structure. The lone pair's repulsion forces are stronger than those between the bonding electron pairs, so the angle between the \(Te-F\) bonds slightly decrease from 90° and 180°, leading to a distorted square pyramidal structure for the \(TeF_5^-\) molecule.

Key concepts

Valence Electrons

When dealing with Lewis structures, understanding valence electrons is essential. Valence electrons are the outermost electrons of an atom and are crucial in chemical bonding. The number of valence electrons an element possesses can be determined from its group number in the periodic table.
For example, in the case of the molecule \(TeF_5^-\), tellurium (Te) belongs to Group 16, giving it six valence electrons. Each fluorine (F), from Group 17, contributes seven valence electrons.
Therefore, for \(TeF_5^-\), with five fluorines and an additional negative charge (one extra electron), the total equals:

• 6 from Te
• 5 \(\times\) 7 from F
• +1 extra electron

This adds up to a total of 42 valence electrons.

Octet Rule

The octet rule is a guideline stating that atoms tend to bond in such a way that they each have eight valence electrons, achieving a stable electronic configuration similar to the noble gases. In many molecules, particularly those of main group elements, the octet rule helps predict molecular and ionic compounds' stability.
For instance, in \(TeF_5^-\), each fluorine needs three lone pairs to satisfy its octet requirement, using up the remaining valence electrons after forming single bonds with tellurium.
This ensures that each fluorine attains a complete octet.

Expanded Octet

While the octet rule is a useful guideline, some atoms are capable of holding more than eight electrons in their valence shell. This situation is known as an expanded octet.
Elements from the third period and below in the periodic table, like tellurium, can exhibit expanded octets because they have access to d-orbitals, which can hold additional electrons.
In \(TeF_5^-\), tellurium forms bonds with five fluorine atoms and still accommodates a lone pair, resulting in ten electrons around it, thus utilizing its ability to have an expanded octet.

Molecular Geometry

The shape a molecule takes is defined by its molecular geometry, which is influenced by the arrangement of atoms and electron pairs around the central atom. These shapes are crucial for understanding a molecule's properties and reactions.
In the \(TeF_5^-\) molecule, the electron pair repulsions between the central tellurium's lone pair and the bonding pairs result in a distorted square pyramidal structure.
According to VSEPR theory (Valence Shell Electron Pair Repulsion), lone pairs exert greater repulsive forces compared to bonding pairs. Consequently, the bond angles in this molecule are slightly compressed from the ideal 90° and 180°, leading to distortions in molecular geometry.