Question

Write the electron-dot structure for the covalent compound acetylene, \(\mathrm{C}_{2} \mathrm{H}_{2}\).

Short answer

The electron-dot structure for acetylene is H:C:::C:H.

Step-by-step solution

Determine the Total Number of Valence Electrons

To begin, count the total number of valence electrons available from all the atoms. Carbon (C) is in Group 14, having 4 valence electrons each, and Hydrogen (H) is in Group 1, with 1 valence electron each. Therefore, \(2 \times 4\,( ext{from 2 C atoms}) + 2 \times 1\,( ext{from 2 H atoms}) = 10\) valence electrons in total.

Arrange the Atoms and Predict Bonds

Place the carbon atoms in the center, as they are more capable of forming multiple bonds compared to hydrogen. Start with a linear structure: H-C-C-H. Carbon atoms will form a triple bond with each other to satisfy their octet and leave the remaining as single bonds with hydrogen.

Draw the Electron-Dot Structure

Place the determined bonds in the structure. Between the two carbons, draw a triple bond, which uses 6 of the valence electrons. Each hydrogen forms a single bond with a carbon atom, using 2 to more electrons (one for each bond). This means: H:C:::C:H. The triple bond is made of three pairs of electrons (dots).

Check the Total Number of Electrons Used

Initially, calculate the electrons used in the drawing. Each of the single bonds between carbon and hydrogen uses 2 electrons, and the C-C triple bond uses 6 electrons, totaling to \(2 (C-H) + 6 (C:::C) = 10\) electrons. This matches the count from Step 1, so the structure is valid.

Confirm Octet Rule Satisfaction

Confirm that each carbon atom has a 'completed' octet: the carbon atoms participate in sharing 8 electrons by accounting for the triple bond with each other and the single bond with hydrogen. Hydrogens satisfy the duet rule with 2 electrons each from their single bonds.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom and play a crucial role in chemical bonding. These electrons are important because they are involved in forming bonds between atoms. The number of valence electrons determines how an atom will interact and bond with other atoms.
For elements in the periodic table, the number of valence electrons is typically determined by the column or group that they belong to. For example:

• Carbon (C) is in Group 14, meaning it has 4 valence electrons.
• Hydrogen (H), in Group 1, has just 1 valence electron.

Understanding the valence electrons helps predict the types of bonds that can form, such as covalent bonds, and is essential in drawing electron-dot structures like the one for acetylene, \( \mathrm{C}_{2} \mathrm{H}_{2} \).

Covalent Compounds

Covalent compounds are formed when two or more atoms share electrons to achieve a full outer shell of electrons, thus adhering to the octet rule. In a covalent bond, each shared pair of electrons constitutes a bond.
This sharing allows the atoms to "borrow" electrons from each other, achieving stability. Features of covalent compounds include:

• They often involve nonmetals bonding with each other.
• The bonds can be single, double, triple, or even more complex.
• Bond strength increases with more shared electron pairs; thus, a triple bond is stronger than a double bond.
• Commonly found in organic molecules, such as acetylene.

Covalent compounds can exist in various physical states (solid, liquid, gas) at room temperature, highlighting the versatility of these bonds in different environments.

Acetylene

Acetylene, with the chemical formula \( \mathrm{C}_{2} \mathrm{H}_{2} \), is a simple alkyne known for its carbon-carbon triple bond. It is an organic molecule prominently featuring a type of covalent bond called a triple bond, where six electrons are shared between two carbon atoms.
This bond gives acetylene its unique properties and high reactivity compared to compounds with single or double bonds. Some interesting points about acetylene include:

• It is colorless and highly flammable.
• Acetylene is used in oxyacetylene welding torches because of its ability to produce extremely high temperatures when burned with oxygen.
• This molecule exemplifies the importance of the triple bond in organic chemistry, providing a basic structure for many larger, more complex molecules.

In its molecular structure, hydrogen atoms complete the linear shape, each forming a single bond with one of the carbon atoms, making its electron-dot structure an essential learning tool in chemistry.

Octet Rule

The octet rule is a chemical rule of thumb that reflects the desire of atoms to have eight electrons in their valence shell. This rule applies primarily to main group elements, dictating that atoms combine in such a way that they each have eight electrons in their valence shell, similar to a noble gas.
The role of the octet rule in covalent bonding cannot be overstated. Employing electron-dot structures helps visualize how atoms achieve this electronic configuration through bonding.Here's how the octet rule applies in acetylene:

• Each carbon atom in \( \mathrm{C}_{2} \mathrm{H}_{2} \) shares a triple bond, accounting for 6 shared electrons with another carbon atom and a single bond with hydrogen, creating a total of 8 electrons around each carbon.
• Hydrogen atoms, being an exception to the octet rule, follow the duet rule, needing only 2 electrons to be stable.

Through bond formation and sharing of electrons, the octet rule enables the prediction and understanding of molecular structures and chemical reactivity.