Molecular Shape - Sigma Bonds

Molecular Shape - Sigma Bonds - Pi Bonds
TECTechnics Classroom TECTechnics Overview

Molecular Shape - Sigma Bonds - Pi Bonds

1. Define the following: (a) Resonance Structures (b) Sigma Bonds (c) Pi Bonds
(d) Localized Electrons (e) Delocalized Electrons

2. Identify the bonds (sigma, or pi, or both) that formed each of the following compounds:
(a) H₂ (Hydrogen) (b) HCl (hydrochloric acid) (c) C₂H₄ (Ethylene) (d) N₂ (Nitrogen)
(e) H₂CO (Formaldehyde) (f) C₆H₆ (Benzene).

3. What valence atomic orbitals formed the bonds of the compounds listed in problem (2)?

4. What compound listed in problem (2) has Pi (π) bonds that cannot be entirely described as localized Pi (π) bonds?

5. Write the general representative strings for Sigma (σ) bonds and Pi (π) bonds.

1(a) Resonance structures (also called resonance forms): the different Lewis structures that are equally acceptable descriptions of a single molecule.
(b) Sigma (σ) bond: a covalent bond in which the electron density is concentrated symmetrically about the line joining the nuclei (internuclear axis) of the bonded atoms. In other words, a covalent bond in which the internuclear axis passes through the middle of the region of orbital overlap (figure 9.16).
Sigma Bonds
Pi (π) bond: a covalent bond in which the electron density is concentrated above and below the internuclear axis of the bonded atoms. In other words, a covalent bond in which the line passing through the middle of the region of orbital overlap is perpendicular to the internuclear axis of the bonded atoms (figure 9.17).
Pi Bonds
Localized electrons: σ and π electrons totally associated with the atoms that form the bond. Such bonds are localized bonds.
Delocalized electrons: electrons not totally associated with the atoms that form the bond. Such bonds are delocalized bonds. Delocalized bonds are commomly encountered in molecules that have two or more resonance structures involving π bonds. Such bonds are called delocalzed π bonds. 2(a) The valence-shell configuration of H = 1s¹. Two s orbitals overlap to form H₂. The internuclear axis of the atoms passes through the middle of the orbital overlap. So, H₂ is formed by a sigma (σ) bond (figure 9.19).
Hydrogren Bonds
(b) The valence-shell configuration of H = 1s¹. The valence-shell configuration of Cl = 1s²2s²2p⁶3s²3p⁵. The s orbital of H overlaps with the p orbital of Cl to form HCl. The internuclear axis of the atoms passes through the middle of the orbital overlap. So, HCl is formed by a sigma (σ) bond (figure 9.20).
HCl Bonds
(c) Molecular geometry of C₂H₄ is trigonal planar. This geometry implies the sp² hybrid orbitals on C. The valence-shell configuration of H = 1s¹. The valence-shell configuration of C = 1s²2s²2p². One 2s electron is promoted to the third 2p orbital. This 2s orbital then merges with two 2p orbitals to form 3 sp² hybrid orbitals. Each C atom uses its 3 sp² hybrid orbitals to form σ bonds with the other C atom and two H atoms (a total of 5 σ bonds). The 2 valence electrons in the unhybridized 2p orbitals of the two C atoms form a π bond. Hence the double bond between the two C atoms (one σ bond and one π bond). In total, five σ bonds and one π bond form C₂H₄ (figure 9.21). Ethylene Bonds
(d) The valence-shell configuration of N = 1s²2s²2p³. N₂ is formed by one σ bond and two π bonds. A total of 3 pairs of 2p orbitals formed the bonds. a pair of 2p orbitals form the sigma bond and 2 pairs of 2p orbitals form the pi bonds (figure 9.22).
Nitrogen Bonds
(e) Molecular geometry of H₂CO is trigonal planar. This geometry implies the sp² hybrid orbitals on C. The valence-shell configuration of H = 1s¹. The valence-shell configuration of C = 1s²2s²2p². One 2s electron is promoted to the third 2p orbital. This 2s orbital then merges with two 2p orbitals to form 3 sp² hybrid orbitals. The C atom uses its 3 sp² hybrid orbitals to form σ bonds with two H atoms and one O atom. The remaining electron is in an unhybridized 2p orbital. The O atom has three electron domains around it which implies that it has sp² hybrid orbitals. One of the sp² hybrid orbital is used to form the C-O σ bond; the other two hold the two nonbonding elctron pairs of the O atom. The remaining electron is in an unhybridized 2p orbital. The unhybridized 2p orbital of the C atom and the unhybridized 2p orbital of the O atom formed the π bond between the C atom and the O atom. So, in H₂CO, there 3 σ bonds and 1 π bond (figure 9.23).
Formaldehyde Bonds
(f) Molecular geometry of C₆H₆ (Benzene) indicates that each carbon atom is surrounded by three atoms at 120^o angles. This geometry implies three sp² hybrid orbitals on each C atom of benzene. Consequently, six C-C σ bonds and C-H σ bonds are formed from the sp² hybrid orbital set. Each C atom has an unhybridized 2p orbital. So, benzene has a total of six unhybridized carbon 2p orbitals in a ring arrangement which form the three π bonds responsible for the resonance structures of the benzene molecule (figure 9.24).
Benzene Bonds

3(a) overlap of two s orbitals (b) overlap of an s orbital and a p orbital (c) overlap of four s orbitala and four sp² hybrid orbitals, overlap of two sp² hybrid orbitals, overlap of two unhybridized p orbitals (d) overlap of p orbitals (e) overlap of two s orbitals and two sp² hybrid orbitals, overlap of two sp² hybrid orbitals, overlap of two unhybridized p orbitals (f) overlap of six s orbitals and six sp² hybrid orbitals, overlap of twelve sp² hybrid orbitals, overlap of six unhybridized p orbitals.

4.C₆H₆ (Benzene) is the compound in the list of compounds in problem (2) whose pi (π) bonds cannot be entirely described as localized π bonds. There are three resonance structures presented by the benzene molecule. Two of the resonance structures have localized π bonds while one has delocalized π bonds (figure 9.25).
Benzene Bonds

5. Sigma (σ) and Pi (π) bonds are covalent bonds. The regions where the bondings occur are the valence orbital overlaps. So Sigma (σ) bonds and Pi (π) bonds have: S₇P₆A₆₄ (multi-criteria permutation), S₇P₃A₃₁ (force - pull) and S₇P₆A₆₆ (chemical interaction) as their general representative strings.