Chapter 9: Problem 88
(a) From what group must the terminal atoms come in an \(\mathrm{AB}_{x}\) molecule where the central atom is from Group \(5 \mathrm{~A},\) for both the electron-domain geometry and the molecular geometry to be trigonal bipyramidal? (b) From what group must the terminal atoms come in an \(\mathrm{AB}_{x}\) molecule where the central atom is from Group \(6 \mathrm{~A},\) for the electron-domain geometry to be tetrahedral and the molecular geometry to be bent?
Short Answer
Expert verified
(a) Group 7A; (b) Group 1A or hydrogen.
Step by step solution
Over 22 million students worldwide already upgrade their learning with Vaia!
Key Concepts
These are the key concepts you need to understand to accurately answer the question.
Group 5A elements
Group 5A elements are part of the p-block in the periodic table and include nitrogen (N), phosphorus (P), arsenic (As), antimony (Sb), and bismuth (Bi). These elements have five valence electrons, which play a key role in their ability to form bonds.
The general trend for Group 5A elements is to form three covalent bonds, resulting in trigonal pyramidal molecular geometries. However, they can expand their octet to form five covalent bonds in some cases, particularly with larger elements like phosphorus and arsenic.
In a scenario where a central atom from Group 5A forms a trigonal bipyramidal geometry, it must engage all five of its valence electrons in bonding. This is facilitated when bonded to elements that are efficient at forming single bonds. Understanding these bonding preferences is crucial to predicting molecule shapes and reactivity of the Group 5A elements.
The general trend for Group 5A elements is to form three covalent bonds, resulting in trigonal pyramidal molecular geometries. However, they can expand their octet to form five covalent bonds in some cases, particularly with larger elements like phosphorus and arsenic.
In a scenario where a central atom from Group 5A forms a trigonal bipyramidal geometry, it must engage all five of its valence electrons in bonding. This is facilitated when bonded to elements that are efficient at forming single bonds. Understanding these bonding preferences is crucial to predicting molecule shapes and reactivity of the Group 5A elements.
Group 6A elements
Group 6A elements, also known as the chalcogens, include oxygen (O), sulfur (S), selenium (Se), tellurium (Te), and polonium (Po). These elements have six valence electrons, leading them to frequently form two covalent bonds.
A common feature of Group 6A elements is their ability to form bent molecular geometries. This is particularly evident in water (H₂O), where oxygen forms two bonds with hydrogen atoms and has two lone pairs, resulting in a bent shape.
In a molecular context where Group 6A elements exhibit tetrahedral electron-domain geometry, such as when bonded with hydrogen, alkali metals, or similar atoms, two of the electron domains are occupied by lone pairs, while the others form bonds.
A common feature of Group 6A elements is their ability to form bent molecular geometries. This is particularly evident in water (H₂O), where oxygen forms two bonds with hydrogen atoms and has two lone pairs, resulting in a bent shape.
In a molecular context where Group 6A elements exhibit tetrahedral electron-domain geometry, such as when bonded with hydrogen, alkali metals, or similar atoms, two of the electron domains are occupied by lone pairs, while the others form bonds.
Electron-domain geometry
Electron-domain geometry refers to the spatial arrangement of all electron domains (bonding and non-bonding) around the central atom in a molecule. It is a key concept in understanding molecular shapes because it influences molecular geometry, which concerns the spatial arrangement of the atoms themselves.
Trigonal bipyramidal and tetrahedral are two common electron-domain geometries. For example, a trigonal bipyramidal geometry involves five electron domains arranged with three equatorial and two axial positions. This geometry is possible when there are no lone pairs on the central atom, allowing for straight lines and symmetric spatial distribution.
In the case of tetrahedral geometry, there are four electron domains arranged at 109.5° angles. The precise molecular geometry can vary if some of these domains are lone pairs, such as in the bent geometry observed in water.
Trigonal bipyramidal and tetrahedral are two common electron-domain geometries. For example, a trigonal bipyramidal geometry involves five electron domains arranged with three equatorial and two axial positions. This geometry is possible when there are no lone pairs on the central atom, allowing for straight lines and symmetric spatial distribution.
In the case of tetrahedral geometry, there are four electron domains arranged at 109.5° angles. The precise molecular geometry can vary if some of these domains are lone pairs, such as in the bent geometry observed in water.
Molecular geometry
Molecular geometry focuses on the three-dimensional arrangement of the atoms within a molecule. This arrangement is affected by the number of bonds and lone pairs of electrons surrounding a central atom. Molecular geometry can differ from electron-domain geometry when lone pairs are present, as they alter the angles between bonded atoms.
The presence of lone pairs compresses the angles between bonding pairs due to repulsion. For example, in a trigonal bipyramidal geometry, the absence of lone pairs, as in phosphorus pentachloride (\(PCl_5\)), maintains a symmetric shape. However, bent geometry occurs when two lone pairs replace two bonding pairs, such as in the case of H₂O.
Understanding molecular geometry is essential because it affects not just the physical shape of a molecule but its chemical properties and reactivity as well.
The presence of lone pairs compresses the angles between bonding pairs due to repulsion. For example, in a trigonal bipyramidal geometry, the absence of lone pairs, as in phosphorus pentachloride (\(PCl_5\)), maintains a symmetric shape. However, bent geometry occurs when two lone pairs replace two bonding pairs, such as in the case of H₂O.
Understanding molecular geometry is essential because it affects not just the physical shape of a molecule but its chemical properties and reactivity as well.
Valence electrons
Valence electrons are the outermost electrons of an atom and are primarily involved in bonding. The number of valence electrons determines how many bonds an atom can form with other atoms.
In the context of molecular shapes, valence electrons are used to predict electron-domain and molecular geometries. For Group 5A and 6A elements, managing valence electron pairs is critical for achieving the desired molecular shape. Each group behaves predictably, enabling the formation of specific geometries based on the number of available valence electrons.
In the context of molecular shapes, valence electrons are used to predict electron-domain and molecular geometries. For Group 5A and 6A elements, managing valence electron pairs is critical for achieving the desired molecular shape. Each group behaves predictably, enabling the formation of specific geometries based on the number of available valence electrons.
- For Group 5A elements: With 5 valence electrons, they can achieve a full octet by forming up to five single bonds, assuming no lone pairs alter the geometry.
- For Group 6A elements: With 6 valence electrons, typically two are used for bonding while the remaining form lone pairs, as seen in the bent geometry of H₂O.
