Question

Write electron-dot structures for each of the following molecules, and use VSEPR theory to predict the structure of each: (a) \(\mathrm{H}_{2} \mathrm{~S}\) (b) \(\mathrm{SO}_{2}\) (c) \(\mathrm{SO}_{3}\)

Short answer

H₂S is bent; SO₂ is bent; SO₃ is trigonal planar.

Step-by-step solution

Electron-Dot Structure for H2S

Start with counting the valence electrons. Hydrogen has 1 valence electron, and each sulfur atom has 6 valence electrons. Therefore, H₂S has a total of \(1 \times 2 + 6 = 8\) valence electrons. Sulfur is the central atom. Place H atoms around the S atom, and use 2 electrons to form each S-H bond. Distribute remaining electrons to give sulfur a complete octet. The electron-dot structure is ``` H-S-H```with 2 lone pairs on the S atom.

VSEPR Prediction for H2S

With two bonded pairs and two lone pairs on the central sulfur atom, VSEPR theory predicts a bent or V-shaped molecular geometry for H₂S.

Electron-Dot Structure for SO2

Sulfur has 6 valence electrons and each oxygen also has 6. Therefore, SO₂ has a total of \(6 + 6 \times 2 = 18\) valence electrons. Sulfur is the central atom. To satisfy the octet rule, double bond each O to the S and leave two lone pairs on the sulfur atom. The structure is:```O=S=O```with two lone pairs on the sulfur atom and one lone pair per oxygen atom.

VSEPR Prediction for SO2

With two bonding pairs and one lone pair on the central sulfur atom, using VSEPR theory, SO₂ has a bent geometry.

Electron-Dot Structure for SO3

Sulfur has 6 valence electrons, and each oxygen has 6, giving us a total of \(6 + 6 \times 3 = 24\) valence electrons for SO₃. Place the sulfur in the center and form double bonds with each of the oxygen atoms. All atoms achieve an octet:``` O || S || O || O```No lone pairs remain on the sulfur atom, and each oxygen atom has two lone pairs.

VSEPR Prediction for SO3

With three bonding pairs and no lone pairs on the central sulfur atom, VSEPR theory predicts a trigonal planar geometry for SO₃.

Key concepts

Electron-Dot Structures

Imagine using a simple drawing to figure out how atoms might connect in a molecule. That's what electron-dot structures, also known as Lewis structures, can help us do! They are diagrams showing the arrangement of valence electrons around atoms within a molecule. Each dot represents a valence electron, and lines represent bonds between them.
Electron-dot structures begin with identifying the number of valence electrons each atom brings. Valence electrons are found in the atom's outermost shell and are critical because they are involved in chemical bonding. You then decide which atom will be central. Typically, it's the least electronegative one because it can share electrons more easily—a common choice in molecules is sulfur or carbon.
Once you've drawn the central atom, you connect other atoms using dots or lines to form bonds. The rest of the dots are placed to ensure each atom fulfills the octet rule (or duet for hydrogen), meaning it has eight electrons in its outer shell (or two in the case of H). The goal is to have all atoms stable. For example, in the molecule \( ext{H}_2 ext{S}\), sulfur is the central atom and forms bonds with two hydrogen atoms, leaving it with extra pairs of electrons, called lone pairs. Drawing electron-dot structures requires some practice but provides insights into where bonds and electrons reside in a molecule. This picture helps predict the shape it might naturally adopt.

Molecular Geometry Prediction

Predicting the shape of a molecule involves thinking like the bonds are moving away from each other as far as possible. Visualize this as playing with balloons, where each balloon represents an electron pair.
VSEPR theory (Valence Shell Electron Pair Repulsion theory) assists in this task. It suggests that electron pairs around a central atom will arrange themselves to minimize repulsion, leading to a shape that dictates the molecule's properties.
Each molecule has a central atom strategically surrounded by other atoms or lone pairs. This distribution affects the space around the central atom, and VSEPR predicts the resulting geometry, like these examples:

• For \( ext{H}_2 ext{S}\), with two bonding pairs (to hydrogen) and two lone pairs, the repulsion from the lone pairs makes the shape bent or V-shaped.
• In \( ext{SO}_2\), there are two oxygen atom bonds and one lone pair on the sulfur, again leading to a bent shape.
• With \( ext{SO}_3\), the sulfur has no lone pairs, just three double bonds to oxygen, resulting in a trigonal planar shape.

Understanding molecular geometry helps in determining how the molecule interacts chemically and physically, showcasing the importance of spatial arrangement.

Valence Electrons

Valence electrons are the heart of chemistry! These electrons are in the outermost shell of an atom and are crucial for fostering chemical bonds. Knowing how many valence electrons each atom has can determine how a molecule forms and behaves.
In simple terms, valence electrons are like social points for atoms. They dictate how readily atoms can pair up with others—either by sharing, giving away, or borrowing electrons.
Let's take sulfur, for instance, which has six valence electrons. In molecules like \( ext{H}_2 ext{S}\), \( ext{SO}_2\), and \( ext{SO}_3\), sulfur's valence electrons play a key role in shaping multiple bonds with oxygen or hydrogen:

• \( ext{H}_2 ext{S}\) has eight total valence electrons, where each hydrogen contributes one.
• \( ext{SO}_2\) has eighteen valence electrons (three sixes: one from sulfur, two from each oxygen).
• In \( ext{SO}_3\), the count rises to twenty-four valence electrons (also from oxygen and sulfur combined).

With valence electrons, atoms form stable molecules by achieving full outer electron shells, leading to diverse molecular structures and behaviors. The understanding of valence electrons is foundational in chemistry, highlighting which elements can bond and the nature of those bonds.