Question

Two different compounds have the formula \(\mathrm{XeF}_{2} \mathrm{Cl}_{2}\). Write Lewis structures for these two compounds, and describe how measurement of dipole moments might be used to distinguish between them.

Short answer

The two XeF₂Cl₂ compounds have different Lewis structures: 1) In the first compound, the molecular geometry is square planar, with Xe-F and Xe-Cl bond dipoles canceling each other out. The net dipole moment is zero (non-polar): ``` F | Cl - Xe - Cl | F ``` 2) In the second compound, the molecular geometry is also square planar, but the Xe-F and Xe-Cl bond dipoles are not equal and opposite. The result is a non-zero dipole moment (polar): ``` F | Cl - Xe - Cl | F ``` By measuring the dipole moments of these compounds, one can distinguish between them: the first compound (XeF₂Cl₂) is non-polar with a zero dipole moment, and the second compound (XeFClFCl) is polar with a non-zero dipole moment.

Step-by-step solution

Identify the central atom

In both compounds, the central atom is the same, which is Xenon (Xe). Xenon is a noble gas, but it can form compounds with other elements because it has empty d orbitals that can accommodate additional pairs of electrons.

Draw the possible Lewis structures

There are two possible arrangements of atoms in the XeF₂Cl₂ compounds: 1. In the first compound, both of the fluorine atoms (F) are connected to the central atom (Xe), and both of the chlorine atoms (Cl) are also attached to the central atom. The molecular geometry of this compound is square planar. The Lewis structure can be represented as: ``` F | Cl - Xe - Cl | F ``` 2. In the second compound, one fluorine atom and one chlorine atom are connected to the central atom, and the other fluorine and chlorine atoms are connected to each other instead. The molecular geometry of this compound is also square planar. The Lewis structure can be represented as: ``` F | Cl - Xe - Cl | F ```

Determine molecular polarity and dipole moment

Once we have the Lewis structures for both compounds, we have to analyze their molecular polarity and dipole moment. A molecule is polar if there is an unequal distribution of electron density, which results in a net dipole moment. 1. For the first compound (XeF₂Cl₂), the molecular geometry is square planar, and the Xe-F and Xe-Cl bond dipoles are oriented 180° to each other. The bond dipoles cancel each other out, and the net dipole moment of this molecule is zero. Therefore, this compound is non-polar. 2. For the second compound (XeFClFCl), the molecular geometry is also square planar. However, in this case, the Xe-F and Xe-Cl bond dipoles are not equal and opposite. As a result, a net dipole moment is created within the molecule. This compound is polar, with a non-zero dipole moment.

Use dipole moment measurement to distinguish between the compounds

By measuring the dipole moment of the XeF₂Cl₂ compounds, it is possible to distinguish between them. As explained in step 3: - The first compound (XeF₂Cl₂) has a net dipole moment of zero (non-polar) - The second compound (XeFClFCl) has a non-zero dipole moment (polar) By comparing the measured dipole moments of the two compounds, we can differentiate between the two compounds. If the measured dipole moment is zero, the compound is XeF₂Cl₂. If the measured dipole moment is non-zero, the compound is XeFClFCl.

Key concepts

Molecular Geometry

Understanding the shape of molecules is vital in chemistry, as the molecular geometry influences many physical and chemical properties. When determining the molecular geometry, the positions of atoms around a central atom are crucial. In the example of XeF₂Cl₂, the geometry is referred to as square planar. This specific arrangement means that the central atom, Xe, has four atoms connected to it in a way that forms a square when viewed in two dimensions, with 90 degrees between each bond.

This shape arises because of the repulsion between electron pairs as described by the VSEPR theory, which stands for Valence Shell Electron Pair Repulsion. The VSEPR theory states that electron pairs around a central atom will arrange themselves as far apart from each other as possible to minimize repulsion. In square planar molecules, two additional pairs of electrons are located on the central atom, lying above and below the plane of the four atoms to further stabilize the structure.

In the real world, this shape affects how molecules interact with each other and how they react. For instance, the specific angles can influence catalytic reactions and how molecules fit together in complex structures like biological systems.

Chemical Polarity

Chemical polarity is an important concept that tells us about the distribution of electric charge around a molecule. A polar molecule arises when there is a significant difference in electronegativity between bonded atoms, which leads to an uneven charge distribution. Electronegativity is a measure of how strongly an atom can attract electrons in a bond.

In the context of the XeF₂Cl₂ compounds, we compare the electronegativity of fluorine and chlorine atoms. Since the atoms differ in electronegativity, they may create regions of partial positive and negative charge, known as poles, making a molecule polar. However, whether or not these individual polar bonds lead to an overall polar molecule depends on their orientation – the molecular geometry.

For the first compound with a square planar shape, the polarities of the bonds cancel each other out due to their symmetry around the central xenon atom. As a result, there is no net dipole moment, and the compound is non-polar. By contrast, when the arrangement of atoms lacks symmetry in a way that causes the dipoles not to cancel out as in the second XeF₂Cl₂ compound, the entire molecule is polar. This leads to different physical properties such as a higher boiling point or dissolving better in polar solvents than non-polar molecules do.

Dipole Moments

Dipole moments are quantitative measures of the polarity of a molecule. They are a vector quantity, meaning they have both magnitude and direction. The magnitude of a dipole moment reflects the difference in electronegativity between atoms and the distance between them, while the direction indicates the orientation of the charge separation.

When molecules have a symmetrical arrangement, like our first molecule of XeF₂Cl₂, any existing individual bond dipole moments may cancel out because of their opposing directions, resulting in an overall dipole moment of zero. This is seen in molecules with a square planar geometry and identical bonds equally spaced around the central atom.

However, if the molecule's structure is such that the bonds' dipole moments don't cancel each other, the molecule will exhibit a net dipole moment, like the second version of XeF₂Cl₂. Measurements of dipole moments then become a practical tool to differentiate between isomers or conformers of a compound. Using instruments like a dipole meter, the presence or absence of a dipole moment can aid chemists in identifying substances and understanding their potential interactions with other chemical entities.