The VSEPR model was developed before any xenon compounds had been prepared. Thus, these compounds provided an excellent test of the model's predictive power. What would you have predicted for the shapes of \(\mathrm{XeF}_{2}, \mathrm{XeF}_{4},\) and \(\mathrm{XeF}_{6} ?\)

Valence electrons are the outermost electrons in an atom that can participate in chemical bonding. They are critical because they determine an element's chemical properties and reactivity. To find the number of valence electrons in an atom, look at its group number on the periodic table. For example, xenon (Xe) is in Group 18 and generally has eight valence electrons. Fluorine (F), in Group 17, has seven.
When you calculate valence electrons for a molecule, you add up the valence electrons of all the atoms involved. For instance, in \(\text{XeF}_2\), you add the valence electrons of one xenon atom (8) and two fluorine atoms (7 each), giving you a total of 22. This count helps you determine how electrons are arranged and how the molecule will shape according to the VSEPR model.

The electron pair arrangement around a central atom is influenced by the total number of valence electrons. This arrangement minimizes electron repulsion, making the molecule more stable. Using the Valence Shell Electron Pair Repulsion (VSEPR) model, you can predict this arrangement.

Electronic geometries are based on the number of electron pairs:

• 2 pairs = linear
• 3 pairs = trigonal planar
• 4 pairs = tetrahedral
• 5 pairs = trigonal bipyramidal
• 6 pairs = octahedral

For example, \(\text{XeF}_2\) with 22 valence electrons arranges into a trigonal bipyramidal shape where lone pairs take the equatorial positions to reduce repulsion.
Similarly, \(\text{XeF}_4\) has 6 pairs forming an octahedral electron arrangement.

Molecular geometry describes the three-dimensional shape of a molecule. While electron pair arrangement considers both bonded and lone pairs, molecular geometry only describes the positions of the atoms.

For example:

• \(\text{XeF}_2\) forms a linear shape in its trigonal bipyramidal arrangement because lone pairs occupy the other positions.
• In \(\text{XeF}_4\), a square planar shape arises under the octahedral arrangement since two lone pairs occupy opposite positions.
• \(\text{XeF}_6\) typically results in a distorted octahedral or square pyramidal shape due to the lone pair affecting the position of the six bonded fluorine atoms.

Uncovering molecular geometry helps predict molecule behavior in chemical reactions and physical properties.