Problem 15 - General Chemistry: Principles and Modern Applications

Question

Write Lewis structures for the following molecules, and then label each \(\sigma\) and \(\pi\) bond. (a) \(\mathrm{HCN} ;\) (b) \(\mathrm{C}_{2} \mathrm{N}_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{CHCHCCl}_{3} ;\) (d) HONO.

Step-by-Step Solution

Answer

The Lewis structures of the molecules are: (a) H-C≡N (b) \(-C≡C-)-N=N- (c) H_3C-CHCHCCl_3 (d) H-O-N=O. In (a), there are two \(\sigma\) bonds and two \(\pi\) bonds; in (b), there are three \(\sigma\) bonds and four \(\pi\) bonds; in (c), there are nine \(\sigma\) bonds and no \(\pi\) bonds; and in (d), there are three \(\sigma\) bonds and one \(\pi\) bond.

Step 1: Draw Lewis Structures

A Lewis structure is a visual representation of the molecule that shows how the valence electrons are distributed among the atoms in the molecule. Lewis structures can be drawn for each of the molecules as follows: (a) \( \mathrm{HCN} \) (b) \(\mathrm{C}_{2} \mathrm{N}_{2}\) (c) \(\mathrm{CH}_{3} \mathrm{CHCHCCl}_{3}\) (d) HONO.

Step 2: Identify Sigma and Pi Bonds

Each molecule can now be examined for sigma (\(\sigma\)) and pi (\(\pi\)) bonds. A sigma bond (\(\sigma\)) bond is formed by the end-to-end overlapping and Pi bond (\(\pi\)) is formed by the lateral or side by side overlapping of atomic orbitals. In each molecule, single bonds represent \(\sigma\) bonds. Each double bond has one \(\sigma\) and one \(\pi\) bond while each triple bond has one \(\sigma\) and two \(\pi\) bonds.

Step 3: Label Sigma and Pi Bonds

Now each bond in the molecule is labelled as either a sigma (\(\sigma\)) or pi (\(\pi\)) bond based on the type of overlap forming the bond. Label each single bond as a \(\sigma\) bond. Label one of the bonds in a double bond as a \(\sigma\) bond and the other as a \(\pi\) bond. For a triple bond, label one of the bonds as a \(\sigma\) bond and the other two as \(\pi\) bonds.

Key Concepts

Understanding Sigma Bonds

A sigma bond, often denoted as \( \sigma \), is the strongest type of covalent bond, formed by the head-on overlap of orbitals from two atoms. This overlap occurs along the axis connecting the two nuclei, making the \( \sigma \) bond symmetrical around this axis, which provides stability, allowing for free rotation of the bonded atoms.

• Single bonds are almost always sigma bonds due to the strong overlap.
• In multiple bonds, such as double or triple bonds, there is always exactly one sigma bond, with the others being pi bonds.

To visualize a sigma bond, think of two spheres meeting directly at their surfaces. Besides providing basic linkage between atoms, \( \sigma \) bonds also determine the molecular geometry. This is why understanding them is crucial for predicting how molecules behave and interact.

The Role of Pi Bonds

Pi bonds, depicted as \( \pi \) bonds, are formed from the side-to-side overlap of orbitals. These bonds are weaker than sigma bonds, as the overlap is less extensive compared to the end-to-end overlap.

• A single pi bond is present in double bonds.
• Triple bonds contain two pi bonds along with a sigma bond.

When visualizing pi bonds, imagine two lobes of overlapping atomic orbitals placed above and below the axis that runs between two bonded atoms. This lateral overlapping prevents the rotation around the bond \( \pi \) bonds are what give multiple bonds (like double and triple bonds) their rigidity. This constraint helps determine the overall shape and properties of the molecule.

Valence Electrons Distribution Demystified

When drawing Lewis structures, it's crucial to understand how valence electrons are distributed among atoms in a molecule. Valence electrons are responsible for the bonding and chemical properties of an element.

• Lewis structures aim to show how valence electrons pair and form bonds.
• They help in visualizing the distribution of electrons and the kinds of bonds— like sigma or pi— present in the molecule.

Each atom seeks to complete its outer shell with a noble gas configuration, typically an octet for most elements. In doing so, atoms share electrons, forming bonds distinguished by their electron sharing configuration—sigma for sharing one pair of electrons head-on, or pi, for sharing above and below the bond axis. Understanding this electron distribution is key in predicting molecular geometry and reactivity.