Problem 162 - Chemistry: An Atoms-Focused Approach

Question

Sulfur in the Environment Sulfur is cycled in the environment through compounds such as dimethyl sulfide \(\left(\mathrm{CH}_{3} \mathrm{SCH}_{3}\right),\) hydrogen sulfide \(\left(\mathrm{H}_{2} \mathrm{S}\right),\) and sulfite and sulfate ions. Draw Lewis structures for these four species. Are expanded valence shells needed to minimize the formal charges for any of these species?

Step-by-Step Solution

Answer

Answer: Only the sulfate ion (SO4(2-)) requires an expanded valence shell to minimize formal charges.

Step 1: Count valence electrons

First, we need to determine the number of valence electrons for each species. Dimethyl sulfide [CH3SCH3]: Carbon has 4 valence electrons. Hydrogen has 1 valence electron. Sulfur has 6 valence electrons. There are two CH3 groups, so counting valence electrons for all the atoms: 2*(4+3*1)+6=16 electrons. Hydrogen sulfide [H2S]: Hydrogen has 1 valence electron. Sulfur has 6 valence electrons. There are two Hydrogen atoms so counting valence electrons for all the atoms: 2*1+6=8 electrons. Sulfite ion [SO3(2-)]: Sulfur has 6 valence electrons. Oxygen has 6 valence electrons. For the ion we need also to add two extra electrons due to the (2-) charge: 6+3*6+2=26 electrons. Sulfate ion [SO4(2-)]: Sulfur has 6 valence electrons. Oxygen has 6 valence electrons. For the ion, we need also to add two extra electrons due to the (2-) charge: 6+4*6+2=32 electrons.

Step 2: Draw preliminary Lewis structures

Next, we will draw the preliminary Lewis structures based on the counted valence electrons. Dimethyl sulfide [CH3SCH3]: - Place Sulfur in the middle and two Carbon atoms connected to Sulfur, then add three Hydrogen atoms to each of the Carbon atoms. - Connect each Carbon with Sulfur (giving each 2 electrons) and connect each Hydrogen with a Carbon (giving each 2 electrons). Hydrogen sulfide [H2S]: - Place Sulfur in the middle and two Hydrogen atoms connected to Sulfur. - Connect each Hydrogen with Sulfur (giving each 2 electrons). Sulfite ion [SO3(2-)]: - To make the sulfite ion, place the sulfur atom in the center and the three oxygen atoms around it. - Connect each oxygen atom to the sulfur atom with single bonds (using 2 electrons each). Sulfate ion [SO4(2-)]: - Place the sulfur atom in the center and four oxygen atoms around it. - Connect each oxygen atom to the sulfur atom with single bonds (using 2 electrons each).

Step 3: Optimize Lewis structures to minimize formal charges

We need to add lone pairs on atoms and make sure every atom has the least formal charge. Dimethyl sulfide [CH3SCH3]: - Sulfur has 4 electrons from the two single bonds and requires 2 lone pairs to reach 8 electrons. - Carbon atoms have 4 electrons from their single bonds with Hydrogen and Sulfur; they do not require more electrons nor lone pairs. - Hydrogen atoms have 2 electrons from their bonds and do not need any additional electrons or lone pairs. Hydrogen sulfide [H2S]: - Sulfur has 4 electrons from the two single bonds and requires 2 lone pairs to reach 8 electrons. - Hydrogen atoms have 2 electrons from their bonds and do not need any additional electrons or lone pairs. Sulfite ion [SO3(2-)]: - Sulfur has 6 electrons from the three single bonds; it does not require extra electrons but needs a double bond instead of one of the single bonds to minimize formal charges. This means two of the oxygen atoms are bonded with single bonds and the third one has a double bond. - Oxygen atoms with single bonds will have 3 lone pairs (6 electrons), and the oxygen atom that forms a double bond has 2 lone pairs (4 electrons). Sulfate ion [SO4(2-)]: - Sulfur has 8 electrons from the four single bonds; it needs to form double bonds with two of the oxygen atoms to minimize formal charges. - Oxygen atoms with single bonds will have 3 lone pairs (6 electrons), and oxygen atoms that form a double bond have 2 lone pairs (4 electrons).

Step 4: Identify if expanded valence shells are needed

We need to identify if expanded valence shells are needed for any of the species. - Dimethyl sulfide [CH3SCH3]: Does not need an expanded valence shell. - Hydrogen sulfide [H2S]: Does not need an expanded valence shell. - Sulfite ion [SO3(2-)]: Does not need an expanded valence shell. - Sulfate ion [SO4(2-)]: Requires an expanded valence shell for Sulfur (10 electrons, including two double bonds with oxygen atoms). In conclusion, only the sulfate ion requires an expanded valence shell to minimize formal charges.

Key Concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a key role in chemical bonding. They determine how an atom will interact with other atoms. Each type of atom contributes a specific number of valence electrons. For example:

• Carbon contributes 4 valence electrons.
• Hydrogen contributes 1 valence electron.
• Sulfur contributes 6 valence electrons.
• Oxygen also contributes 6 valence electrons.

When drawing Lewis structures, it's important to count all valence electrons involved accurately. This ensures the correct representation of how atoms bond in molecules.
The total number of valence electrons determines how they are shared or transferred between atoms, directly affecting molecule formation.

Formal Charge

Formal charge is a hypothetical charge you calculate to determine the most stable Lewis structure. It's derived from thinking about how electrons are distributed among atoms in a molecule. The aim is to have a result as close as possible to zero on each atom, to suggest stability. To calculate the formal charge on an atom:

• Take the number of valence electrons the atom should have.
• Subtract the number of electrons the atom "owns" in the molecule (counting all of the lone pair electrons and half of the bonding electrons).

For complex ions like sulfate, it's often necessary to adjust bonds (like forming double bonds) to reduce atoms' formal charges, achieving a more stable structure. Sulfur in sulfate requires some of these changes for stability, which we'll explore in connection with expanded valence shells.

Expanded Valence Shell

An expanded valence shell occurs when an atom holds more than eight electrons around it. This is possible for elements in the third period or lower on the periodic table. Sulfur, for instance, can expand its valence shell. In the sulfate ion \( ext{SO}_4^{2-}\), sulfur forms double bonds with some oxygen atoms to achieve a more stable formal charge arrangement. This results in sulfur having 10 electrons around it.The expanded valence shell allows sulfur to accommodate these additional electrons, helping to minimize formal charges and making this structure more stable. Not every atom can expand its valence shell, so it's a special property of certain elements, enhancing their ability to bond.

Sulfate Ion

The sulfate ion \(\text{SO}_4^{2-}\) is a common polyatomic ion where sulfur is the central atom bonded to four oxygen atoms. A key feature of its Lewis structure is the expanded valence shell of sulfur, which facilitates bonding with additional electrons.Sulfur in the sulfate ion needs to use several double bonds to optimize the formal charges of the structure. In the most stable configuration, sulfur uses its capability to expand its electron shell to form two double bonds with oxygen, thus minimizing formal charges across the ion.
Knowing the arrangement of the sulfate ion is helpful in recognizing how expanded valence situations can lead to more stable ionic structures and is fundamental to understanding its chemical behavior in various reactions.