Question

The following species have been detected in interstellar space: (a) CH, (b) OH, (c) \(\mathrm{C}_{2},(\mathrm{~d}) \mathrm{HNC}\) (e) HCO. Draw Lewis structures for these species.

Short answer

CH: C-H; OH: O-H; C₂: C≡C; HNC: H-N≡C; HCO: H-C=O.

Step-by-step solution

Determine Total Valence Electrons

For each species, count the total number of valence electrons. For CH: Carbon has 4 and Hydrogen has 1, totaling 5 valence electrons. For OH: Oxygen has 6 and Hydrogen has 1, totaling 7 valence electrons. For \( \mathrm{C}_2 \): Each carbon has 4, totaling 8 valence electrons. For HNC: Hydrogen has 1, Nitrogen has 5, and Carbon has 4, totaling 10 valence electrons. For HCO: Hydrogen has 1, Carbon has 4, and Oxygen has 6, totaling 11 valence electrons.

Draw Basic Skeleton

Sketch the basic skeleton of each molecule. Typically, hydrogen is terminal: For CH, the structure is C-H. For OH, the structure is O-H. For \( \mathrm{C}_2 \), it is C-C. For HNC, arrange as H-N-C. For HCO, arrange as H-C-O.

Distribute Electrons Around Atoms

Place remaining valence electrons as lone pairs on atoms, starting with the most electronegative atom. For CH, distribute electrons between C and H to satisfy the octet and duet rule. For OH, form a single bond and place remaining electrons on oxygen. For \( \mathrm{C}_2 \), create a triple bond to satisfy each carbon's octet. For HNC, place electrons to bond H-N and N-C. For HCO, form single bonds and place electrons primarily on oxygen.

Check Octet/Duet Completion

Ensure that each atom (except hydrogen) satisfies the octet rule, and hydrogen satisfies the duet rule. If required, adjust electrons to form double or triple bonds.

Finalize Lewis Structures

For CH, the Lewis structure is a single bond between C and H. For OH, it is O-H with three lone pairs on O. For \( \mathrm{C}_2 \), it is a triple bond between the two C atoms. For HNC, it is H connected to N with a lone pair, and N triple-bonded to C. For HCO, it is a C-O double bond with lone pairs on O and single bond to H on C.

Key concepts

Valence Electrons

Valence electrons are the outermost electrons of an atom. They play a crucial role in determining how atoms interact and bond with each other. Imagine valence electrons as "little connectors" allowing atoms to link together to form molecules.
They are important because they determine an atom's chemical properties and its ability to bond with other atoms.

• Valence electrons are found in the outermost electron shell of an atom.
• The number of valence electrons influences whether an atom will lose, gain, or share electrons.
• Counting valence electrons is the first step in drawing a Lewis structure.

For example, carbon has 4 valence electrons. This makes it versatile in bonding, as it can form up to four covalent bonds with other elements.

Octet Rule

The octet rule is a simple guideline used to predict how atoms will bond in most molecules. According to the octet rule, atoms are most stable when they have eight electrons in their valence shell.
Think of the octet rule as an atom's goal to "feel complete" by securing a total of eight surrounding electrons.

• Each atom tends to seek a complete set of eight valence electrons.
• Exception: Hydrogen follows the "duet rule," seeking only two electrons.
• The rule helps in determining the formation of single, double, or triple bonds.

When drawing Lewis structures, ensuring that each atom fulfills the octet rule (or duet rule for hydrogen) helps inform us whether to form additional bonds, like double or triple bonds.

Triple Bond

A triple bond occurs when two atoms share three pairs of valence electrons between them. This type of bond is stronger and shorter than single or double bonds.
Consider it as a "super invisible glue" that tightly holds the atoms together.

• Triple bonds consist of one sigma bond and two pi bonds.
• They are most commonly seen in molecules containing nitrogen and carbon.
• Triple bonds significantly affect a molecule’s shape and stability.

An example of a molecule with a triple bond is acetylene (\(C_2H_2\)), where two carbon atoms form a triple bond. Triple bonds are crucial when attempting to satisfy the octet rule in certain molecules.

Interstellar Molecules

Interstellar molecules are molecules found in the space between stars, known as the interstellar medium. Despite the vastness and emptiness, this space holds a surprising number of different molecules.

• These molecules play a role in the chemical evolution of the universe.
• Common interstellar molecules include hydrogen-containing molecules like HCN and cationic forms like HCO⁺.
• Studying them helps in understanding the formation and dynamics of galaxies.

Identifying interstellar molecules involves observing the electromagnetic radiation they emit or absorb. These observations provide valuable data about the composition of distant interstellar clouds and their physical conditions.