Question

The number and type of bonds between two carbon atoms in \(\mathrm{CaC}_{2}\) are (a) one sigma and one \(\pi\) bond (b) one sigma and two \(\pi\) bonds (c) one sigma and one and a half \(\pi\) bonds (d) one sigma bond

Short answer

The answer is (b) one sigma and two \( \pi \) bonds.

Step-by-step solution

Understanding Calcium Carbide

Calcium carbide is represented by the chemical formula \( \text{CaC}_2 \). It is composed of calcium (Ca) and carbide ion \( \text{C}_2^{2-} \). This implies that \( \text{CaC}_2 \) consists of the \( \text{C}_2 \) unit with a charge of \( 2- \).

Analyzing the Electron Configuration of \( \text{C}_2^{2-} \)

The \( \text{C}_2^{2-} \) ion contains two carbon atoms. In this ion, each carbon atom contributes 4 valence electrons, making a total of 8 valence electrons for the \( \text{C}_2 \) unit. The additional 2 electrons due to the \( 2- \) charge result in a total of 10 electrons between the two carbon atoms.

Determining Bond Formation

Carbons generally form covalent bonds by sharing electrons. For \( \text{C}_2^{2-} \), with 10 shared electrons, 6 electrons are used in forming the carbon-carbon triple bond. This includes one sigma bond and two \( \pi \) bonds between the carbon atoms.

Identifying the Triple Bond

A triple bond consists of one sigma and two \( \pi \) bonds. In \( \text{C}_2^{2-} \), the two carbon atoms are connected by a triple bond, confirming this configuration. Triple bonds are characteristic of strong covalent interactions between two atoms.

Choosing the Correct Answer

Option (b) states there is one sigma and two \( \pi \) bonds between the two carbon atoms, which aligns with the triple bond formation. Therefore, (b) is the correct answer.

Key concepts

Sigma and Pi Bonds

In the world of chemistry, understanding how atoms bond is crucial. Two common types of bonds are sigma (σ) and pi (π) bonds. They play a critical role in determining the structure and behavior of a molecule. A sigma bond is the first bond formed between two atoms and occurs due to the head-on overlapping of orbitals. It allows for free rotation around the bond axis, leading to a single degree of covalent linkage.
Pi bonds, on the other hand, form from the side-to-side overlap of p orbitals above and below the bond axis. They generally appear when there's already a sigma bond, as they provide additional stabilization to the molecule. However, pi bonds restrict rotational freedom between bonded atoms. When combined, sigma and pi bonds determine the strength and properties of the bond.
For example, in a single bond, you typically have one sigma bond. In a double bond, there is one sigma bond and one pi bond, while in a triple bond, which is stronger and shorter, you find one sigma and two pi bonds.

Calcium Carbide (CaC2)

Calcium carbide (CaC2) is an industrially significant compound used mainly in the production of acetylene gas and in steelmaking. It comprises calcium ions (Ca²⁺) and carbide ions (\( \text{C}_2^{2-} \)). Each carbide ion is akin to a mini molecule where two carbon atoms are bonded.
In a biochemical context, calcium carbide's carbide ion is particularly interesting. The (\( \text{C}_2^{2-} \)) ion is made up of two carbon atoms sharing electrons to form stable covalent bonds. Given that these bonds are negatively charged, they signify an added electron presence due to the charge, leading to the formation of a stronger interaction between the carbon atoms.
This configuration plays a key role in identifying the types of bonds present, especially when debating the molecular structure within CaC2.

Triple Bonds

A triple bond is one of the strongest types of chemical bonds between atoms. This bond comprises one sigma bond and two pi bonds, resulting in a very strong and short linkage between the atoms involved. Due to this arrangement, atoms involved in a triple bond have restricted rotation and are closer together.
In the specific case of the carbide ion (\( \text{C}_2^{2-} \)), the bond between the two carbon atoms is a triple bond. This is determined by the sharing of six electrons: two for the sigma bond and four for the two pi bonds.

• Sigma bond: The first and strongest bond, accounting for a direct overlap that ensures maximum binding strength.
• Pi bonds: Two additional bonds that arise from the parallel overlap of p orbitals, providing extra binding stabilization without the flexibility of rotation.

These characteristics make triple bonds significant in molecular formations like those found in calcium carbide and are critical to the compound's properties and applications.