Problem 61 - General Chemistry: Principles and Modern Applications

Question

Each of the following is either linear, angular (bent), planar, tetrahedral, or octahedral. Indicate the correct shape of (a) \(\mathrm{H}_{2} \mathrm{S} ;\) (b) \(\mathrm{N}_{2} \mathrm{O}_{4} ;\) (c) \(\mathrm{HCN} ;\) (d) \(\mathrm{SbCl}_{6}^{-}\); (e) \(\mathrm{BF}_{4}^{-}\)

Step-by-Step Solution

Answer

(a) H2S is angular (bent), (b) N2O4 is planar, (c) HCN is linear, (d) SbCl6- is octahedral, (e) BF4- is tetrahedral.

Step 1: Analyze the Compound H2S

Sulfur (S) is the central atom. It has two bonds with Hydrogen (H) and two lone pairs of electrons. According to VSEPR theory, such an arrangement leads to a bent (or angular) molecular shape.

Step 2: Analyze the Compound N2O4

The molecule N2O4 can be described as two NO2 molecules bonded together, with Nitrogen (N) being the central atom. Each Nitrogen forms two bonds with Oxygen (O) and has one lone pair, leading to a bent shape for each NO2. The overall shape of N2O4 is therefore described as planar.

Step 3: Analyze the Compound HCN

Carbon (C) is the central atom in HCN, being bonded to both Hydrogen (H) and Nitrogen (N), with no lone pairs on the Carbon. This results in a linear molecular shape.

Step 4: Analyze the Ion SbCl6-

Antimony (Sb) is the central atom in SbCl6-, surrounded by six Chlorine (Cl) atoms. There is no lone pair on the Antimony. According to VSEPR theory, this configuration results in an octahedral molecular shape.

Step 5: Analyze the Ion BF4-

Boron (B) is the central atom in BF4-, surrounded by four Fluorine (F) atoms and no lone pairs. This results in a tetrahedral molecular shape.

Key Concepts

Molecular Geometry

Molecular geometry is a crucial concept in chemistry that describes the three-dimensional arrangement of atoms in a molecule. This arrangement influences the molecule's properties, including reactivity, polarity, phase of matter, color, magnetism, and biological activity. The Valence Shell Electron Pair Repulsion (VSEPR) theory helps predict molecular geometry by considering electron pair repulsions around a central atom, thereby shaping the molecule.
For example, in the molecule \(\text{H}_2\text{S}\), the sulfur atom is the central atom. It is bonded to two hydrogen atoms and has two additional lone pairs. The repulsion between these lone pairs and the bond pairs results in a bent molecular geometry.
Understanding molecular geometry helps chemists predict molecular behavior and interactions, forming the foundation for studying more complex chemical systems.

Molecular Shapes

Molecular shapes are essentially the visual representations of molecular geometry. The shape is determined by the positions of the atomic nuclei in a molecule as influenced by repulsions between electrons, whether they are bonded or lone pairs. Certain shapes recur often in chemistry, such as linear, angular (bent), tetrahedral, and octahedral.
For instance, in \(\text{HCN}\), the central carbon atom forms a linear shape because it is bonded to a hydrogen atom on one side and a nitrogen atom on the other without any lone pairs to alter the geometry.
Knowing the shape of a molecule allows chemists to predict how it will interact with other molecules. This is vital in areas like drug design, where the shape of a molecule can determine its effectiveness.

Lone Pairs

Lone pairs play a significant role in determining the molecular shape and geometry of compounds. They are valence electrons found on the central atom that do not participate in bonding but occupy space and exert repulsive forces on adjacent atoms and lone pairs.
In \(\text{N}_2\text{O}_4\), each central nitrogen atom has one lone pair, which affects the molecular shape of each \(\text{NO}_2\) unit, resulting in a bent geometry. When considering the overall geometry of \(\text{N}_2\text{O}_4\), these influences contribute to the planar arrangement of the molecule.
Lone pairs can distort predicted shapes based on just bonded atoms, highlighting the need to consider these electron clouds when predicting molecular geometry with VSEPR theory.

Central Atom

The central atom in a molecule is pivotal in determining the molecule’s shape and geometry. It is typically the atom with the lowest electronegativity (most often a transition metal or a heavier element) or the atom with the ability to form the most bonds. The surrounding atoms and electron pairs arrange themselves around this central atom to minimize repulsion, which defines the molecular structure.
For example, in \(\text{SbCl}_6^-\), the antimony atom acts as the central atom. Being surrounded by six chlorine atoms with no lone pairs, it adopts an octahedral shape following VSEPR theory.
Understanding the role of the central atom in molecular structure is crucial for predicting the molecular geometry, reactivity, and physical properties of a compound.