Question

(a) Draw a picture showing how two \(p\) orbitals on two different atoms can be combined to make a sigma bond. (b) Sketch a \(\pi\) bond that is constructed from \(p\) orbitals. (c) Which is generally stronger, a \(\sigma\) bond or a \(\pi\) bond? Explain. (d) Can two \(s\) orbitals combine to form a \(\pi\) bond? Explain.

Short answer

(a) For a sigma bond between two p orbitals, the electron clouds overlap along the internuclear axis. (b) In a pi bond, p orbitals' electron clouds overlap above and below the internuclear axis. (c) Sigma bonds are generally stronger due to greater orbital overlap along the internuclear axis. (d) Two s orbitals cannot form a pi bond because their spherically symmetric electron clouds only overlap along the internuclear axis, leading to sigma bond formation.

Step-by-step solution

(a) Drawing a sigma bond from p orbitals

To form a sigma bond using two p orbitals, we need to combine the p orbitals of two adjacent atoms in such a way that their electron clouds overlap along the internuclear axis. This overlapping of electron clouds along the internuclear axis leads to the formation of a single sigma bond.

(b) Sketching a pi bond from p orbitals

To form a pi bond using two p orbitals, the electron clouds of the p orbitals must overlap above and below the internuclear axis. When the electron cloud of one p orbital overlaps with that of another p orbital perpendicular to the internuclear axis, a pi bond is formed. Sketch this by drawing the two p orbitals with their lobes pointing towards each other, overlapping above and below the internuclear axis.

(c) Comparison of sigma and pi bonds

A sigma bond is generally stronger than a pi bond. The reason behind this is that the overlapping of electron clouds in a sigma bond occurs along the internuclear axis, which allows for a greater degree of overlap between the orbitals and hence stronger electron bond formation. In contrast, the overlapping of electron clouds in a pi bond occurs above and below the internuclear axis, which results in reduced orbital overlap and weaker bond strength.

(d) Formation of pi bond from s orbitals

Two s orbitals cannot combine to form a pi bond. The reason for this is that s orbitals have a spherically symmetric electron cloud, so when they overlap, the overlap always occurs along the internuclear axis. This type of overlap forms a sigma bond, not a pi bond. In order to form a pi bond, orbitals with a directional electron cloud (like p orbitals) are needed so that the electron clouds can overlap perpendicular to the internuclear axis.

Key concepts

Sigma Bond

A sigma bond (\r\trac{\text{\raisebox{2pt}{\(\tilde{}\)\sigma\tilde{}\(}}}\)) represents the strongest type of covalent chemical bond, formed by the head-on or axial overlap of atomic orbitals. The sigma bond is a single bond, and it is the first bond formed between two atoms.

When it comes to p orbitals, which have a dumbbell shape, they can overlap end-to-end. Visualize two balloons pressing together at their tips - this is how p orbitals fuse to form a sigma bond. This direct overlap allows for a significant density of electrons between the nuclei of the bonding atoms, creating a strong bond that holds the atoms together firmly.

Understanding sigma bonds is crucial because they form the backbone of molecular structures, providing the primary connection that defines the shape of the molecule.

Pi Bond

Pi bonds (\r\trac{\text{\raisebox{2pt}{\(\tilde{}\)\pi\tilde{}\(}}}\)) are types of covalent bonds that are formed by the lateral or side-by-side overlap of atomic orbitals. Unlike sigma bonds, pi bonds involve regions of electron density that exist above and below the nuclei of the bonding atoms.

Imagine two p orbitals, each with their electron clouds shaped like elongated balloons. These p orbitals align parallel to each other so their clouds overlap side-by-side. A pi bond is less strong than a sigma bond primarily because the overlap is not as extensive.

Pi bonds always accompany a sigma bond in double and triple bonds, contributing to the reactivity and physical properties of molecules. They play a significant role in the formation of molecules with double or triple covalent bonds.

Orbital Overlap

Orbital overlap is a fundamental concept in chemical bonding, describing the process where atomic orbitals from two bonding atoms share the same region of space. This overlap allows for the pairing of electrons from each atom, leading to the formation of covalent bonds.

The greater the overlap, the stronger the bond will be. Sigma bonds benefit from the maximum orbital overlap occurring along the internuclear axis - the straight line running through the nuclei of the bonded atoms. Pi bonds, while still an overlap of orbitals, have a lesser degree of contact since their electron cloud interaction is above and below this axis.

Understanding orbital overlap is essential for predicting the strength and properties of the bonds in molecules. This concept provides insights into the molecular geometry and stability of compounds.

Bond Strength

Bond strength is a measure of how strongly two atoms are held together by a chemical bond. It is determined by several factors, including the type of bond (\r\trac{\text{\raisebox{2pt}{\(\tilde{}\)\sigma\tilde{}\( or \tilde{}\)\pi\tilde{}\(}}}\)), orbital overlap, bond length, and the difference in electronegativity of the bonding atoms.

Sigma bonds are stronger than pi bonds primarily due to their greater orbital overlap. Since sigma bonds are the first to form, they are located directly between the atoms and have a shorter bond length, which contributes to their strength. Pi bonds add extra connections between atoms but with a lesser degree of overlap, hence why they contribute to bond strength but are inherently weaker.

The bond strength is critical to the reactivity of molecules. Stronger bonds are less likely to break and participate in chemical reactions, whereas weaker bonds are more reactive and readily involved in the formation of new compounds.