Problem 4 - Chemistry and Chemical Reactivity

Question

Specify the electron-pair and molecular geometry for each of the following. Describe the hybrid orbital set used by the underlined atom in each molecule or ion. (a) \(\underline{\mathrm{CSe}}_{2}\) (b) \(\underline{\mathrm{SO}}_{2}\) (c) \(\underline{\mathrm{CH}}_{2} \mathrm{O}\) (d) \(\underline{\mathrm{NH}}_{4}^{+}\)

Step-by-Step Solution

Answer

CSe2: linear, sp; SO2: bent, sp2; CH2O: trigonal planar, sp2; NH4+: tetrahedral, sp3.

Step 1: Identify the Central Atom and Electron Groups in CSe2

For \(CSe_2\), the central atom is carbon (C). Carbon is surrounded by two selenium (Se) atoms. Since there are two double bonds and no lone pairs present on the carbon, there are two areas of electron density (bond pairs).

Step 2: Determine the Geometry and Hybridization of CSe2

The electron-pair geometry around carbon in \(CSe_2\) is linear due to the two regions of electron density. Thus, the molecular geometry is also linear. The carbon atom has \(sp\) hybridization because it forms two sigma bonds with selenium atoms.

Step 3: Identify the Central Atom and Electron Groups in SO2

For \(SO_2\), the central atom is sulphur (S). It forms two double bonded oxygens. In addition, sulphur has one lone pair, thus there are three areas of electron density.

Step 4: Determine the Geometry and Hybridization of SO2

The electron-pair geometry around sulphur in \(SO_2\) is trigonal planar due to the three regions. However, with one lone pair, the molecular geometry becomes bent. The sulphur atom uses \(sp^2\) hybridization.

Step 5: Identify the Central Atom and Electron Groups in CH2O

For \(CH_2O\), the central atom is carbon (C). Carbon is attached to two hydrogen atoms and one oxygen atom through a double bond, resulting in three areas of electron density.

Step 6: Determine the Geometry and Hybridization of CH2O

The electron-pair geometry around carbon in \(CH_2O\) is trigonal planar. Since there are no lone pairs, the molecular geometry is also trigonal planar. The carbon atom has \(sp^2\) hybridization.

Step 7: Identify the Central Atom and Electron Groups in NH4+

For \(NH_4^+\), the central atom is nitrogen (N). It is surrounded by four hydrogen atoms. The ion has no lone pairs, leading to four areas of electron density.

Step 8: Determine the Geometry and Hybridization of NH4+

The electron-pair geometry around nitrogen in \(NH_4^+\) is tetrahedral, and the molecular geometry remains tetrahedral due to the absence of lone pairs. The nitrogen atom is \(sp^3\) hybridized.

Key Concepts

Hybridization

Hybridization is a concept used in chemistry to describe how atomic orbitals mix to form new hybrid orbitals. These hybrid orbitals are essential for forming sigma bonds in molecules. For instance, carbon atom in molecules like CSe\(_2\) undergoes hybridization. When determining the hybridization state of an atom, we look at the number of regions (or areas) of electron density around it. - In CSe\(_2\), the carbon is bonded to two selenium atoms. The hybridization is determined by these two regions of electron density, resulting in sp hybridization. Here, one s orbital and one p orbital combine, forming two sp hybrid orbitals. - In other molecules such as SO\(_2\) and CH\(_2\)O, complex hybrid states like sp\(^2\) are formed due to the presence of double bonds and lone pairs on the central atom. Each state of hybridization corresponds with a particular molecular and electron pair geometry, influencing the molecule's properties and reactions.

Electron Pair Geometry

Electron pair geometry considers the spatial arrangement of all electron pairs (bonding and lone pairs) around a central atom. It's a critical concept for predicting the molecular structure and understanding the molecule's shape. Let's take a look:- In CSe\(_2\), carbon exhibits a linear electron pair geometry because it has two bonding pairs and no lone pairs. These pairs are positioned 180° apart.- For SO\(_2\), despite having three areas of electron density, the electron pair geometry is trigonal planar. Here, three regions form a planar structure even though one is a lone pair.- CH\(_2\)O shows a trigonal planar electron-pair geometry due to three regions of electron density around the central carbon.- Finally, NH\(_4^+\) displays a tetrahedral electron pair geometry, as nitrogen is surrounded by four hydrogen atoms. Understanding electron pair geometry helps in visualizing molecules' shapes and predicting their chemical behavior.

Central Atom Identification

Central atom identification is fundamental for determining a molecule's structure. It involves identifying which atom in a molecule is bonded to the most other atoms. This atom typically becomes the focal point for understanding that molecule's geometry and hybridization. Here's how it works in some examples:- For CSe\(_2\), the central atom is carbon, bonded to two selenium atoms. - In SO\(_2\), sulfur serves as the central atom, bonding with two oxygen atoms.- For CH\(_2\)O, it's again carbon, with bonds to two hydrogens and one oxygen.- NH\(_4^+\) features nitrogen at the center, surrounded by four hydrogens. Recognizing the central atom allows you to analyze how it connects to other atoms and how these connections affect the molecule's geometry and properties.

Bonding and Lone Pairs

Bonding pairs and lone pairs are key to understanding molecular shapes and reactivity. Bonding pairs are shared between atoms to form chemical bonds, while lone pairs are valence electrons localized on a single atom and not involved in bonding.Grasping these concepts involves:- In CSe\(_2\), carbon has two bonding pairs forming double bonds with selenium atoms, and no lone pairs.- In SO\(_2\), there are two double bonds between sulfur and oxygen atoms and one lone pair on sulfur, affecting its bent shape.- CH\(_2\)O exhibits three bonding pairs, with distinct pairs between carbon and hydrogen and a double bond with oxygen, with no lone pairs on carbon.- For NH\(_4^+\), all four valence electrons on nitrogen are used in bonding with hydrogen atoms, leaving no lone pairs and allowing a tetrahedral shape.Understanding the distribution of bonding and lone pairs is crucial for predicting the molecular geometry and can greatly influence the physical and chemical characteristics of a compound.