Question

In which of the following molecules does the sulfur have an expanded octet? For those that do, write the Lewis structure. (a) \(\mathrm{SO}_{2}\) (b) \(\mathrm{SF}_{4}\) (c) \(\mathrm{SO}_{2} \mathrm{Cl}_{2}\) (d) \(\mathrm{SF}_{6}\)

Short answer

Answer: Only the SF6 molecule has sulfur with an expanded octet.

Step-by-step solution

Analyze each molecule and write their Lewis structure

To analyze each molecule, we will need to write their Lewis structure and check the sulfur atom's valence electrons. The Lewis structure displays the bonding between the atoms and any lone pair electrons present around the molecule.

Determine the total number of valence electrons for each molecule

Count the total valence electrons for each molecule to distribute them according to the Lewis structure rules. (a) \(\mathrm{SO}_{2}\): O = 6 (valence electrons) x 2 = 12, S = 6, sum = 18 (b) \(\mathrm{SF}_{4}\): F = 7 (valence electrons) x 4 = 28, S = 6, sum = 34 (c) \(\mathrm{SO}_{2}\mathrm{Cl}_{2}\): O = 6 (valence electrons) x 2 = 12, Cl = 7 (valence electrons) x 2 = 14, S = 6, sum = 32 (d) \(\mathrm{SF}_{6}\): F = 7 (valence electrons) x 6 = 42, S = 6, sum = 48

Draw the Lewis structure for each molecule

(a) \(\mathrm{SO}_{2}\): The Lewis structure will have double bonds between S and each O atom. There are no lone pairs on the S atom. There are 2 lone pairs on each O atom. The total number of valence electrons used is 18 which matches the sum calculated. So the Lewis structure is accurate. (b) \(\mathrm{SF}_{4}\): The Lewis structure will have single bonds between S and each F atom. There will also be a lone pair of electrons on the S atom. There are 3 lone pairs on each F atom. The total number of valence electrons used is 34 which matches the sum calculated. So the Lewis structure is accurate. (c) \(\mathrm{SO}_{2}\mathrm{Cl}_{2}\): The Lewis structure will have single bonds between S and each O and Cl atom. There are 2 lone pairs on each O atom and 3 lone pairs on each Cl atom. There are no lone pairs on the S atom. The total number of valence electrons used is 32 which matches the sum calculated. So the Lewis structure is accurate. (d) \(\mathrm{SF}_{6}\): The Lewis structure will have single bonds between S and each F atom. There are 3 lone pairs on each F atom. There are no lone pairs on the S atom. The total number of valence electrons used is 48 which matches the sum calculated. So the Lewis structure is accurate.

Check if sulfur has an expanded octet

Now, we will check if the sulfur atom in each molecule has more than eight valence electrons. (a) \(\mathrm{SO}_{2}\): As mentioned earlier, the S atom has double bonds with each O atom. So, the total number of valence electrons around S = (2 x 2) = 4, which is not an expanded octet. (b) \(\mathrm{SF}_{4}\): The S atom has single bonds with four F atoms and a lone pair of electrons. The total number of valence electrons around S = (1 x 4) + 2 = 6, which is not an expanded octet. (c) \(\mathrm{SO}_{2}\mathrm{Cl}_{2}\): The S atom has single bonds with two O atoms and two Cl atoms. The total number of valence electrons around S = (1 x 4) = 8, which is not an expanded octet. (d) \(\mathrm{SF}_{6}\): The S atom has single bonds with six F atoms. The total number of valence electrons around S = (1 x 6) = 12, which is an expanded octet.

Conclusion

From the analysis above, only the \(\mathrm{SF}_{6}\) molecule has sulfur with an expanded octet, with 12 valence electrons around the central sulfur atom.

Key concepts

Lewis Structure

Understanding the Lewis structure is crucial when attempting to discern the electronic relationships between atoms within a molecule. This diagrammatic method indicates not only the bonding of electrons between atoms but also the lone pairs of electrons that may exist around an atom.

For molecules containing sulfur, such as SO₂, SF₄, SO₂Cl₂, and SF₆, the Lewis structure plays a significant role in identifying the presence of an expanded octet. In particular, drawing the correct Lewis structure for SF₆ allows one to see that sulfur can exceed the octet rule, facilitating the accommodation of more than eight electrons around the sulfur atom. This is in contrast with molecules like SO₂, SF₄, and SO₂Cl₂, wherein the sulfur atom does not have an expanded octet as per their Lewis structures.

Valence Electrons

Valence electrons are the electrons that reside in the outermost shell of an atom and are involved in bonding. Knowing how many valence electrons a particular element has can determine how it will interact with other elements in the formation of a compound.

In our case, sulfur typically has six valence electrons. Molecules like SF₆ present a fascinating case where the sulfur atom extends beyond the usual confines of the octet rule, utilizing twelve valence electrons. Here, the ability of sulfur to form more than four bonds takes center stage – a property that is not commonly observed but possible for elements in the third period and below of the periodic table, since they have d orbitals that can accommodate the extra electron pairs.

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. It influences the molecule's physical properties, reactivity, polarity, and even its color. When a sulfur atom forms an expanded octet, as in SF₆, understanding the resulting molecular geometry is crucial.

SF₆ displays an octahedral geometry – a direct consequence of the expanded octet allowing sulfur to form six bonds, creating a symmetric distribution of electron cloud around the sulfur. The geometry of molecules with no expanded octet, like SO₂, SF₄, and SO₂Cl₂, is determined differently, as the electron configuration adheres to the octet rule, resulting in distinct molecular shapes such as bent or seesaw configurations.