Problem 154 - Chemistry The Science in Context

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: Expanded valence shells are needed for sulfite ion (SO3^2-) and sulfate ion (SO4^2-) to minimize their formal charges.

Step 1: Total valence electrons for dimethyl sulfide (CH3SCH3)

To determine the total number of valence electrons, we must sum the valence electrons from each atom: Carbon (4 electrons), Hydrogen (1 electron), and Sulfur (6 electrons). In CH3SCH3, there are 2 carbon atoms, 6 hydrogen atoms, and 1 sulfur atom. Total valence electrons = 2(4) + 6(1) + 6 = 20

Step 2: Total valence electrons for hydrogen sulfide (H2S)

Hydrogen sulfide consists of 2 hydrogen atoms and 1 sulfur atom. Total valence electrons = 2(1) + 6 = 8

Step 3: Total valence electrons for sulfite ion (SO3^2-)

In sulfite ion, we have 1 sulfur atom, 3 oxygen atoms, and a -2 charge. Total valence electrons = 6 + 3(6) + 2 = 26

Step 4: Total valence electrons for sulfate ion (SO4^2-)

In sulfate ion, we have 1 sulfur atom, 4 oxygen atoms, and a -2 charge. Total valence electrons = 6 + 4(6) + 2 = 32 ##Step 2: Draw the Lewis structures for each species##

Step 5: Lewis structure for dimethyl sulfide (CH3SCH3)

Using the total valence electrons we calculated, we can create the Lewis structure for CH3SCH3. Connect each carbon atom to the sulfur atom by a single bond and arrange the hydrogen atoms around the carbon atoms, each connected by a single bond. H | H-C-S-C-H | H

Step 6: Lewis structure for hydrogen sulfide (H2S)

The Lewis structure for H2S has the sulfur atom in the center connected to the hydrogen atoms by single bonds. Add the remaining electrons to the sulfur atom as lone pairs. H | S-H | :

Step 7: Lewis structure for sulfite ion (SO3^2-)

Put the sulfur atom in the center and connect it to the three oxygen atoms by single bonds. Arrange the remaining electrons as lone pairs on the oxygen atoms. Add one double bond to minimize formal charge. O : || O-S-O | -:-

Step 8: Lewis structure for sulfate ion (SO4^2-)

Put the sulfur atom in the center and connect it to the four oxygen atoms by single bonds. Add one double bond to two oxygen atoms to minimize formal charge and assign the remaining electrons as lone pairs on the oxygen atoms. O || S | O-S-O || O ##Step 3: Determine if there is a need for expanded valence shells to minimize formal charges##

Step 9: Expanded valence shells for the four species

We can find that expanded valence shells are not necessary for dimethyl sulfide and hydrogen sulfide since their formal charges are already minimized. For sulfite and sulfate ions, the central sulfur atom has expanded its valence shell to accommodate more than 8 electrons (10 electrons for sulfite and 12 electrons for sulfate) in order to minimize formal charges. So, expanded valence shells are needed for sulfite and sulfate ions.

Key Concepts

Lewis Structure

Lewis structures are visual representations that help us understand the arrangement of atoms within a molecule. By using lines and dots, these structures depict bonds and lone pairs of electrons. In the case of dimethyl sulfide

• Steps to draw: Sum the valence electrons from all atoms involved.
• Bonding: Use single or double lines to represent shared electron pairs, i.e., covalent bonds.
• Lone pairs: Indicate electrons that are not shared between atoms as dots.

Lewis structures are fundamental in predicting the geometry, polarity, and reactivity of molecules.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial in bond formation. Here's how we determine and use them:

• Counting valence electrons: Look at an element’s group number on the periodic table. For instance, carbon is in Group 14, which means it has 4 valence electrons.
• Role in bonding: Valence electrons are involved in the formation of covalent bonds by being shared between atoms or forming ionic bonds by being transferred.
• Example calculations: For hydrogen sulfide - Hydrogen has 1 valence electron. - Sulfur has 6 valence electrons. Adding them up gives you the total number of valence electrons used in constructing the molecule's Lewis structure.

Understanding valence electrons helps you predict how atoms will interact and what bonds will form.

Formal Charge Minimization

Minimizing formal charge ensures that a molecule is in its most stable form. Formal charge involves assigning charges to individual atoms within a structure. Here’s how you manage it:

• Formula: Formal charge = (Valence electrons) - (Non-bonding electrons) - 0.5(Bonding electrons).
• Application: For the sulfite ion, adjusting the structure to distribute electrons can minimize the overall charge, often achieving a balance with zero or near-zero formal charges on all atoms.
• Reason for minimization: A molecule with minimized formal charges tends to be more stable and closer to its actual electronic configuration.

By focusing on the formal charge, you ensure the Lewis structure you draw is as realistic as possible.

Expanded Valence Shells

Expanded valence shells occur when an atom accepts more than 8 electrons. This typically happens with atoms in or beyond the third period of the periodic table. Here's why and how they matter:

• Eligibility: Atoms like sulfur, in sulfite and sulfate ions, can have more than 8 electrons, taking advantage of empty d orbitals.
• Purpose: Allowing for expanded shells can help minimize formal charges, thereby stabilizing the molecule, as seen with sulfate ions whose central sulfur has 12 electrons around it.
• When it applies: Expanded valence shells often come into play with complex molecules or ions where formal charge minimization is crucial.

When drawing Lewis structures, consider expanded shells to provide an accurate representation, especially for elements beyond the second period.