Question

Specify the electron-pair and molecular geometry for each underlined atom in the following list. Describe the hybrid orbital set used by this atom in each molecule or ion. $$\begin{aligned} &\text { (a) } \underline{\mathrm{CSe}_{2}}\\\ &\text { (b) } \underline{\mathbf{S O}_{2}} \end{aligned}$$ (c) \(\underline{\mathrm{CH}_{2} \mathrm{O}}\) (d) \(\underline{\mathrm{NH}}_{4}^{+}\)

Short answer

CSe\(_2\): linear, sp; SO\(_2\): bent, sp\(^2\); CH\(_2\)O: trigonal planar, sp\(^2\); NH\(_4^+\): tetrahedral, sp\(^3\).

Step-by-step solution

Identify the central atom in CSe2

In the molecule CSe\(_2\), the central atom is the carbon (C) atom.

Determine the electron-pair geometry for CSe2

Carbon in CSe\(_2\) has two single bonds with Se atoms and no lone pairs, giving it a linear electron-pair geometry. The line structure maintains 180-degree angles.

Assign the hybridization for C in CSe2

For carbon (C) in CSe\(_2\), with two bonding electron pairs and no lone pairs, the hybridization is sp to maximize separation and maintain linear shape.

Identify the central atom in SO2

In the molecule SO\(_2\), the central atom is sulfur (S).

Determine the electron-pair geometry for SO2

Sulfur in SO\(_2\) has two double bonds with oxygen atoms and one lone pair, resulting in a trigonal planar electron-pair geometry.

Determine the molecular geometry for SO2

The molecular geometry for SO\(_2\) is bent (or V-shaped) due to the lone pair, which reduces bond angles slightly.

Assign the hybridization for S in SO2

Sulfur in SO\(_2\) is sp\(^2\)-hybridized, accommodating three regions of electron density (2 bonds and 1 lone pair).

Identify the central atom in CH2O

In the molecule CH\(_2\)O (formaldehyde), the central atom is carbon (C).

Determine the electron-pair geometry for CH2O

Carbon in CH\(_2\)O has three bonding regions (two with H and one double-bond with O) and no lone pairs, resulting in a trigonal planar electron-pair geometry.

Determine the molecular geometry for CH2O

Since there are no lone pairs on the central carbon, the molecular geometry remains trigonal planar.

Assign the hybridization for C in CH2O

The central carbon in CH\(_2\)O is sp\(^2\)-hybridized, corresponding to three areas of electron density.

Identify the central atom in NH4+

In the molecule NH\(_4^+\) (ammonium ion), the central atom is nitrogen (N).

Determine the electron-pair geometry for NH4+

Nitrogen in NH\(_4^+\) has four bonding pairs with hydrogen atoms and no lone pairs, leading to a tetrahedral electron-pair geometry.

Determine the molecular geometry for NH4+

The molecular geometry of NH\(_4^+\) is tetrahedral as it matches the electron-pair geometry without lone pairs.

Assign the hybridization for N in NH4+

The nitrogen in NH\(_4^+\) is sp\(^3\)-hybridized, necessary for bonding with four hydrogen atoms.

Key concepts

Electron-Pair Geometry

Electron-pair geometry refers to the spatial arrangement of all pairs of electrons around a central atom, including both bonding pairs and lone pairs. This concept is crucial in understanding the 3D shape that a group of atoms will form in a molecule.
To determine electron-pair geometry, we focus on the number of electron groups surrounding a central atom. Each single, double, or triple bond counts as one electron group, as does each lone pair.

• In carbon diselenide (CSe\(_2\)), the carbon atom has two single bonds and no lone pairs, resulting in a linear electron-pair geometry.
• In sulfur dioxide (SO\(_2\)), sulfur has two double bonds with the oxygens and one lone pair, which leads to a trigonal planar electron-pair geometry overall.
• For formaldehyde (CH\(_2\)O), carbon has three bonding groups (two single bonds with hydrogens and one double bond with oxygen), producing a trigonal planar electron-pair geometry.
• In ammonium ion (NH\(_4^+\)), nitrogen has four single bonds to hydrogens, giving a tetrahedral electron-pair geometry.

Electron-pair geometry is essential for predicting molecular shapes, which include considerations of lone pairs that can slightly alter the geometry.

Hybridization

Hybridization is the concept of mixing atomic orbitals to form new hybrid orbitals, which can explain molecular shapes based on the number of electron domains.
Each type of hybridization corresponds to a different arrangement of electron pairs around a central atom. This helps in understanding the geometrical structure of a molecule.

• In CSe\(_2\), the central carbon atom is sp-hybridized, allowing it to maintain a linear shape with two electron domains.
• Sulfur in SO\(_2\) is sp\(^2\)-hybridized. This accounts for the trigonal planar electron-pair geometry due to three regions of electron density.
• In CH\(_2\)O, the carbon exhibits sp\(^2\) hybridization, with three groups of electron density keeping the trigonal planar shape.
• For NH\(_4^+\), nitrogen is sp\(^3\)-hybridized, suitable for four single bonds surrounding the central atom with a tetrahedral electron-pair geometry.

Understanding hybridization helps us predict molecular structure and reactivity as well as the angles between bonds.

Central Atom Identification

Identifying the central atom in a molecule is crucial because it determines the structure and geometry of the molecule. The central atom is typically the one with the highest bonding capacity, often the one that forms the most connections with other atoms.
In the given molecules:

• In CSe\(_2\), the central atom is carbon (C) because it connects equally with the two selenium atoms.
• In the example of SO\(_2\), sulfur (S) is the central atom, forming bonds with two oxygen atoms and holding a lone pair.
• For CH\(_2\)O, the central atom is carbon (C) due to its ability to form multiple bonds, including a double bond with oxygen and single bonds with hydrogen atoms.
• In NH\(_4^+\), nitrogen (N) serves as the central atom, connecting to four hydrogens in the tetrahedral ion.

Determining the central atom is the first step in predicting molecular geometry.

Chemical Bonding

Chemical bonding is the attraction between atoms that holds molecules together and determines their structure and properties. There are various types of bonds, such as covalent (sharing of electrons) and ionic bonds (transfer of electrons).
For our discussion about molecular geometries:

• CSe\(_2\) involves covalent bonding, with carbon sharing electrons equally with selenium, forming linear bonds due to its electron arrangement.
• In SO\(_2\), sulfur forms covalent bonds with oxygen. The molecule bends due to the lone pair effect, shifting the angle from a perfect trigonal planar.
• CH\(_2\)O's carbon forms covalent bonds with hydrogen and a double bond with oxygen. Its trigonal planar shape aligns with the sp\(^2\) hybridization.
• NH\(_4^+\) sees nitrogen form four covalent bonds with hydrogen, resulting in a symmetrical tetrahedral ion with no lone pairs.

Understanding the nature of chemical bonding helps in predicting molecular behavior and reactivity.