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.)

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: \(6 + (5 \times 7) + 1 = 6 + 35 + 1 = 42 \)

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).

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.

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.