Question

A paper published in the research journal Science in 2007 (S. Vallina and R. Simo, Science, Vol. \(315,\) p. \(506,\) January 26,2007 ) reported studies of dimethylsulfide (DMS), an important greenhouse gas that is released by marine phytoplankton. This gas "represents the largest natural source of atmospheric sulfur and a major precursor of hygroscopic (i.e, cloud-forming) particles in clean air over the remote oceans, thereby acting to reduce the amount of solar radiation that crosses the atmosphere and is absorbed by the ocean." (a) Sketch the Lewis structure of dimethylsulfide, \(\mathrm{CH}_{3} \mathrm{SCH}_{3},\) and list the bond angles in the molecule. (b) Use electronegativities to decide where the positive and negative charges lie in the molecule. Is the molecule polar? (c) The mean seawater concentration of DMS in the ocean in the region between \(15^{\circ}\) north latitude and \(15^{\circ}\) south latitude is \(2.7 \mathrm{nM}\) (nanomolar). How many molecules of DMS are present in \(1.0 \mathrm{m}^{3}\) of seawater?

Short answer

(a) Lewis structure: S bonded to two CH3 groups; bonds are bent. Bond angle: ~104.5°. (b) Molecule is non-polar; dipoles cancel. (c) \(1.63 \times 10^{15}\) DMS molecules in 1.0 m³ seawater.

Step-by-step solution

Sketch the Lewis Structure

The chemical formula for dimethyl sulfide is \(\mathrm{CH}_3\mathrm{SCH}_3\). The structure consists of a sulfur atom bonded to two methyl groups (\(\mathrm{CH}_3\)). Each carbon atom forms a single bond with the sulfur atom, and each carbon atom is also bonded to three hydrogen atoms. The Lewis structure involves drawing each of these atoms with their valence electrons, ensuring that the sulfur atom is at the center connected to the two \(\mathrm{CH}_3\) groups.

Predict Bond Angles

In dimethyl sulfide, the sulfur atom is surrounded by two carbon atoms and has two lone pairs of electrons. This results in a molecular geometry analogous to that of \(\text{H}_2\text{O}\), which forms a bent shape. The typical bond angle in this bent configuration with lone pairs present is approximately \(104.5^\circ\), similar to water. However, due to the presence of the larger sulfur atom, the real bond angle can slightly vary.

Assess Polarity Using Electronegativity

For dimethyl sulfide \(\mathrm{CH}_3\mathrm{SCH}_3\), compare the electronegativities: carbon (2.55), hydrogen (2.20), sulfur (2.58). The bond between carbon and sulfur (C—S) is only slightly polar because there is only a small difference in electronegativity. However, the large difference between hydrogen and carbon/sulfur causes small dipoles. Nevertheless, due to the molecular symmetry, the C—S—C bond dipoles cancel each other out, indicating the molecule is not polar overall.

Calculate Molecules in a Given Volume

To find the number of dimethyl sulfide molecules in 1.0 \(\text{m}^3\) of seawater, use the concentration \(2.7\,\text{nM}\). First, convert nanomolar to moles per cubic meter: \(2.7 \times 10^{-9}\,\text{mol/m}^3\). Then, use Avogadro's number \(6.022\times 10^{23}\,\text{molecules/mol}\) to find the number of molecules: \(2.7 \times 10^{-9}\,\text{mol/m}^3 \times 6.022 \times 10^{23}\,\text{molecules/mol} = 1.63 \times 10^{15}\,\text{molecules/m}^3\).

Key concepts

Lewis Structure

To understand the Lewis structure of dimethylsulfide \( ext{CH}_3 ext{SCH}_3\), it's important to visualize how the atoms are arranged. A Lewis structure is a diagram that shows the arrangement of electrons in a molecule.
In the case of dimethylsulfide, the molecule consists of a sulfur atom (\( ext{S}\)) bonded to two methyl groups (\( ext{CH}_3\)). - Each carbon atom forms a single covalent bond with the sulfur atom. - Additionally, each carbon is bonded to three hydrogen atoms.The sulfur atom is at the center, demonstrating that it forms bonds with the carbon atoms of each methyl group. In drawing the Lewis structure, it's crucial to ensure the central sulfur atom is correctly represented with its valence electrons. This involves illustrating sulfur's six valence electrons paired accordingly with the surrounding atoms. By visualizing this, it becomes clear how electrons are shared and paired in the molecule, fulfilling each atom's need for a stable electronic configuration.

Electronegativity

Electronegativity is a measure of an atom's ability to attract and hold electrons. In dimethylsulfide, understanding electronegativity helps in predicting how electrons are distributed in its bonds.
For the elements in dimethylsulfide:- **Sulfur (\( ext{S}\))** has an electronegativity of 2.58.- **Carbon (\( ext{C}\))** has an electronegativity of 2.55.- **Hydrogen (\( ext{H}\))** has an electronegativity of 2.20.The bond between carbon (\( ext{C}\)) and sulfur (\( ext{S}\)) is only marginally polar since the difference in their electronegativities is small. However, the bonds between hydrogen and carbon are slightly polar due to a larger difference in electronegativity. Despite these differences, the symmetry in the dimethylsulfide molecule plays a significant role, often resulting in dipole moments that cancel each other in the larger structure.

Molecular Polarity

Molecular polarity determines how a molecule will interact with electric fields and neighboring molecules. It is a vector quantity, dependent on both the magnitude of charge differences and the molecular structure.
In dimethylsulfide (\( ext{CH}_3 ext{SCH}_3\)):- Although individual bonds between sulfur and carbon may exhibit slight polarity, the symmetrical arrangement of these methyl groups causes these dipoles to cancel each other. - As a result, dimethylsulfide is generally considered non-polar. This cancellation occurs because the molecular shape and the small differences in electronegativity don't result in a significant net dipole. Understanding molecular polarity is essential as it influences the solubility, boiling point, and interaction of DMS with other compounds in its environment.

Bond Angles

Bond angles are the angles formed between two bonds originating from a common atom. They are crucial in defining the shape and geometry of a molecule.
For dimethylsulfide (\( ext{CH}_3 ext{SCH}_3\)):- The sulfur atom is central, bonded to two carbon atoms, and it has two lone pairs of electrons. - The geometry around sulfur is similar to that of water (\( ext{H}_2 ext{O}\)), which is bent.
Due to the larger size of sulfur compared to oxygen, the bond angle in dimethylsulfide is roughly similar to the typical water angle of about 104.5°. However, because of the differences in atomic radii and masses, this angle can slightly vary. This "bent" nature is what contributes to the molecule's overall shape and influences its chemical behavior.