Question

Write Lewis structures for hydrazine \(\left(\mathrm{N}_{2} \mathrm{H}_{4}\right)\) and hydrazoic acid \(\left(\mathrm{HN}_{3}\right)\).

Short answer

Hydrazine has a Lewis structure with the two nitrogen atoms bonded together by a single bond and each bonded to two hydrogen atoms. The lone pairs on the nitrogen atoms complete their octet. Hydrazoic acid has a hydrogen atom bonded to a nitrogen atom, which is triple-bonded to another nitrogen atom, which in turn is single-bonded to a third nitrogen atom with a lone pair, completing their octets.

Step-by-step solution

Understanding the Molecule Hydrazine (\text{N}_2\text{H}_4)

Hydrazine is a simple diamine consisting of two nitrogen atoms bonded to four hydrogen atoms. The nitrogen atoms in hydrazine are connected by a single bond. Each nitrogen atom will also need to form three bonds because it requires three more electrons to complete its octet.

Drawing the Lewis Structure for Hydrazine (\text{N}_2\text{H}_4)

Place the two nitrogen atoms in the center with a single bond connecting them. Then, attach two hydrogen atoms to each nitrogen atom with single bonds. Each nitrogen now has a total of three bonds—one with another nitrogen and two with hydrogen atoms. Since nitrogen prefers to have three bonds and a lone pair to complete the octet, add a lone pair of electrons to each nitrogen atom. Note that the hydrogens already have a complete outer shell with two electrons.

Understanding the Molecule Hydrazoic Acid (\text{HN}_3)

Hydrazoic acid is composed of one hydrogen atom and a chain of three nitrogen atoms. The extended structure of nitrogen atoms will involve a combination of single and triple bonds to ensure that each nitrogen atom satisfies the octet rule.

Drawing the Lewis Structure for Hydrazoic Acid (\text{HN}_3)

Place the hydrogen atom bonded to one nitrogen atom at one end of the molecule. Then, create a sequence of nitrogen atoms. The first nitrogen atom will be connected to the hydrogen with a single bond and to the second nitrogen atom with a triple bond to complete the octet. The final nitrogen atom will be connected to the second nitrogen with a single bond. Add a lone pair to this nitrogen to satisfy the octet rule. Remember to add lone pairs on the middle nitrogen atom to complete its octet as well.

Key concepts

Chemical Bonding

In the fascinating study of chemistry, chemical bonding is the force that holds atoms together to form compounds. It's a topic central to understanding molecular interactions and reactions. One of the most basic forms of chemical bonds is the covalent bond, which is when atoms share pairs of electrons.

When constructing Lewis structures, we visually represent these covalent relationships, indicating bonds with lines and non-bonding electrons with dots. For example, in hydrazine \textbf{\text{N}\(_2\)\text{H}\(_4\))}, the two nitrogen atoms are bonded with a single covalent bond, and each nitrogen is also bonded to two hydrogen atoms. In the Lewis structure, you would see lines depicting these bonding interactions and lone pairs of electrons that are not shared.

Understanding Lewis structures of molecules like hydrazine and hydrazoic acid \textbf{\text{H}N\(_3\)} helps us predict how these molecules will interact with others, their shapes, and their reactivity. It's the shared electrons in these bonds that create the glue holding the molecule together, adhering to rules like the octet rule which we explore next.

Octet Rule

The octet rule is a cornerstone of chemical bonding theory, based on the observation that atoms tend to combine in such a way that they each have eight electrons in their valence shell, achieving a noble gas configuration. This octet provides a stable, energetically favorable arrangement.

When drawing Lewis structures, such as for hydrazine \textbf{\text{N}\(_2\)\text{H}\(_4\)} and hydrazoic acid \textbf{\text{H}N\(_3\)}, the octet rule guides us to ensure that nitrogen, which typically forms three bonds and maintains a lone pair, achieves this stable configuration. Hydrogen, however, is an exception to the octet rule; it achieves stability with only two electrons in its shell.

An Exception to the Rule

In the case of hydrazoic acid \textbf{\text{H}N\(_3\)}, the octet rule helps us deduce that the middle nitrogen needs to form a triple bond with one neighbor and a single bond with another to satisfy its eight-electron requirement. Discrepancies like this are essential considerations, making the octet rule a guide rather than a strict law. Nevertheless, in most cases, following the octet rule will lead to the correct representation of a molecule's bonding.

Molecular Structure

Molecular structure refers to the three-dimensional arrangement of atoms within a molecule. The Lewis structures for \textbf{\text{N}\(_2\)\text{H}\(_4\)} and \textbf{\text{H}N\(_3\)} do more than just illustrate which atoms are bonded; they can also give clues about the molecule's shape. Although Lewis structures are two-dimensional, they are the first step in predicting three-dimensional molecular geometry using VSEPR theory (Valence Shell Electron Pair Repulsion theory).

In hydrazine, the nitrogen atoms are connected linearly with hydrogens attached to each, while in hydrazoic acid, the linear chain of nitrogen atoms presents a slightly more complex scenario due to the presence of a triple bond which affects bond angles. However, for both molecules, understanding the Lewis structure helps us begin to visualize how the molecule might look and behave in space.

Importance of Geometry

The molecular structure plays a critical role in determining a compound's properties like boiling and melting points, reactivity, polarity, and interaction with light. For students mastering topics such as Lewis structures, delving into the implications of molecular structure provides a gateway to understanding the significant impact of geometry on a substance's behavior.