Question

For each molecule listed below, predict its molecular shape and bond angle, and identify the hybrid orbitals. Drawing the Lewis structure might help you. \(\begin{array}{ll}{\text { a. } \mathrm{SCl}_{2}} & {\text { c. HOF }} \\\ {\text { b. NH_ Cl }} & {\text { d. BF } 3}\end{array}\)

Short answer

a. \(\mathrm{SCl}_{2}\): Molecular shape – Bent, Bond angle – 109.5°, Hybrid orbitals – sp3. b. \(\mathrm{NH_2Cl}\): Molecular shape - Trigonal Pyramidal, Bond angle - 107°, Hybrid orbitals - sp3. c. \(\mathrm{HOF}\): Molecular shape - Bent, Bond angle - 109.5°, Hybrid orbitals - sp3. d. \(\mathrm{BF_3}\): Molecular shape - Trigonal Planar, Bond angle - 120°, Hybrid orbitals - sp2.

Step-by-step solution

Draw the Lewis structures for each molecule.

Let's first draw the Lewis structures for each molecule: a. \(\mathrm{SCl}_{2}\) S has 6 valence electrons, and each Cl atom has 7 valence electrons. The total valence electrons are 6 + 2(7) = 20. The central atom is S, and it's bonded to two Cl atoms. b. \(\mathrm{NH_2Cl}\) N has 5 valence electrons, H has 1, and Cl has 7. The total valence electrons are 5 + 2(1) + 7 = 14. The central atom is N, and it's bonded to two H atoms and one Cl atom. c. \(\mathrm{HOF}\) H has 1 valence electron, O has 6, and F has 7. The total valence electrons are 1 + 6 + 7 = 14. The central atom is O, and it's bonded to one H atom and one F atom. d. \(\mathrm{BF_3}\) B has 3 valence electrons, and each F atom has 7. The total valence electrons are 3 + 3(7) = 24. The central atom is B, and it's bonded to three F atoms.

Apply the VSEPR theory to predict the molecular shape and bond angles.

a. \(\mathrm{SCl}_{2}\): The central atom (S) has 2 bonding electron pairs and 2 lone pairs. According to the VSEPR theory, this results in a bent molecular geometry. The bond angle for a bent molecule with four electron domains is approximately 109.5°. The hybridization of the S atom is sp3. b. \(\mathrm{NH_2Cl}\): The central atom (N) has 3 bonding electron pairs and 1 lone pair. According to the VSEPR theory, this results in a trigonal pyramidal molecular geometry. The bond angle for a trigonal pyramidal molecule is approximately 107°. The hybridization of the N atom is sp3. c. \(\mathrm{HOF}\): The central atom (O) has 2 bonding electron pairs and 2 lone pairs. According to the VSEPR theory, this results in a bent molecular geometry. The bond angle for a bent molecule with four electron domains is approximately 109.5°. The hybridization of the O atom is sp3. d. \(\mathrm{BF_3}\): The central atom (B) has 3 bonding electron pairs and no lone pairs. According to the VSEPR theory, this results in a trigonal planar molecular geometry. The bond angle for a trigonal planar molecule is approximately 120°. The hybridization of the B atom is sp2.

Final answers for each molecule

a. \(\mathrm{SCl}_{2}\): Molecular shape – Bent, Bond angle – 109.5°, Hybrid orbitals – sp3. b. \(\mathrm{NH_2Cl}\): Molecular shape - Trigonal Pyramidal, Bond angle - 107°, Hybrid orbitals - sp3. c. \(\mathrm{HOF}\): Molecular shape - Bent, Bond angle - 109.5°, Hybrid orbitals - sp3. d. \(\mathrm{BF_3}\): Molecular shape - Trigonal Planar, Bond angle - 120°, Hybrid orbitals - sp2.

Key concepts

VSEPR Theory

The Valence Shell Electron Pair Repulsion (VSEPR) theory is a simple yet powerful tool used to predict the shape of molecules based on the idea that electron pairs around a central atom will position themselves as far apart as possible to minimize repulsion. According to VSEPR theory, the geometric arrangement of electron pairs is determined by the total number of bonding and lone pairs in the valence shell of the central atom.

Each pair of electrons, whether in a bond or as a lone pair, occupies a region in space known as an electron domain. For instance, in the molecule SCl₂, the central sulfur atom has four electron domains – two bonding pairs and two lone pairs. This leads to a 'bent' molecular geometry with an approximate bond angle of 109.5°, as our exercise shows.

By applying VSEPR theory to molecules such as NH₂Cl, HOF, and BF₃, we determine their molecular shapes to be trigonal pyramidal, bent, and trigonal planar respectively. These shapes are influenced by the number of bonding and lone pairs around the central atoms (N, O, and B in our examples). Simplifying complex concepts, VSEPR theory provides a fundamental understanding of why molecules adopt specific shapes and structures.

Lewis Structures

Lewis structures provide a visual representation of the valence electrons in molecules. These diagrams help in predicting the molecular structure, bonding patterns, and reaction mechanisms. The significance of Lewis structures is specifically noted when we try to fathom the foundational layout a molecule's geometry.

When drawing Lewis structures, certain rules are followed: identifying total valence electrons, determining the central atom (typically the least electronegative), and arranging electrons to satisfy the octet rule (where applicable). The Lewis structures for SCl₂, NH₂Cl, HOF, and BF₃ show different numbers of bonding pairs (lines) and lone pairs (dots).

For instance, the exercise illustrates that SCl₂ not only has single bonds with each chlorine atom but also two pairs of non-bonding electrons, which greatly influence its final molecular shape. Lewis structures are a fundamental starting point in chemistry for understanding molecular compounds, their properties, and their behaviors.

Hybrid Orbitals

The concept of hybrid orbitals is part of valence bond theory, explaining the observed shapes of molecules that cannot be described by simple atomic orbitals. Hybridization involves the mixing of atomic orbitals to form new hybrid orbitals that are energetically equivalent and oriented in specific geometries in space to minimize electron repulsion.

For example, the S atom in SCl₂ utilizes sp³ hybrid orbitals, indicating that one s orbital and three p orbitals have combined to accommodate the electron pairs and bonds the atom is part of. This hybridization corresponds with the molecule's tetrahedral electron pair geometry and bent molecular shape.

In BF₃, the B atom uses sp² hybrid orbitals, revealing that one s orbital and two p orbitals hybridize to form three equivalent orbitals, which are arranged in a trigonal planar fashion consistent with the molecule's shape. The concept of hybrid orbitals provides an in-depth understanding of how atomic orbitals come together to form the molecular geometries we observe, illustrating the three-dimensional nature of molecules.