Two different compounds have the formula XeF_2Cl_2. How do you write the Lewis structures for these two compounds, and describe how measurement of dipole moments might be used to distinguish between them?

1 Answer
Dec 4, 2016

Here's how I would do it.

Explanation:

Draw the Lewis structure.

Start with a trial structure. Put two "F" atoms and two "Cl" atoms on a central "Xe", and give every atom an octet.

Trial

The trial structure has 32 valence electrons.

"1 Xe" + "2 F" + "2 Cl" = 8 + 14 + 14 = 36 valence electrons

We have four extra electrons, so we add 2 lone pairs on the central atom.

The Lewis structure is

XeF2Cl2

This is an "AX"_4"E"_2 system.

Its electron geometry is octahedral.

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The bulky lone pairs will occupy the top and bottom axial positions, and the "F" and "Cl" atoms will occupy the equatorial positions.

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The molecular geometry is square planar.

The atoms can arrange themselves in two ways.

In the trans isomer the "F—Xe—F" and "Cl—Xe—Cl" bond angles are 180 °.

Trans

The "Xe—F" bond dipoles (red) cancel, and the "Xe—Cl" bond dipoles cancel.

There is no net dipole, so the trans isomer is nonpolar.

In the cis isomer the "Xe—F" and "Xe—Cl" bond dipoles do not cancel.

Cis

The cis isomer has a net dipole moment and the molecule is polar.