Question

Under appropriate conditions, both hydrogen and nitrogen can form monatomic anions. What are the Lewis symbols for these ions? What are the Lewis structures of the compounds (a) lithium hydride; (b) calcium hydride; (c) magnesium nitride?

Short answer

The Lewis symbols for hydrogen and nitrogen ions are \(H^-\) with two dots and \(N^{3-}\) with eight dots respectively. The Lewis structures for \(LiH\) consist of \(Li^+\) with no dots and \(H^-\) with two dots. \(CaH_2\) has \(Ca^{2+}\) with no dots and two \(H^-\) with two dots each. The structure of \(Mg_3N_2\) includes three \(Mg^{2+}\) with no dots and two \(N^{3-}\) each with eight dots.

Step-by-step solution

Lewis Symbols for Hydrogen and Nitrogen Ions

In order to draw the Lewis symbols for hydrogen and nitrogen ions, we need to understand that anions are formed when an atom gains electrons. In its elemental state, hydrogen has 1 electron. When it gains an electron, it forms a monatomic anion and its Lewis symbol will be \(H^-\) with two dots around it representing the two valence electrons. Nitrogen, having 5 valence electrons, needs to gain 3 more electrons to fulfill the octet rule, forming \(N^{3-}\). The Lewis symbol will therefore be \(N^{3-}\) surrounded by eight dots symbolizing the eight valence electrons.

Lewis Structure for Lithium Hydride

Lithium hydride (\(LiH\)) is composed of lithium ions (\(Li^+\)) and hydride ions (\(H^-\)). Lithium loses one electron to form \(Li^+\) and hydrogen gains one electron to form \(H^-\). The lewis structure will therefore include Lithium with no dots (since it loses the electron it initially had) and hydride with two dots (since it gains an additional electron).

Lewis Structure for Calcium Hydride

Calcium hydride (\(CaH_2\)) is formed when calcium ions (\(Ca^{2+}\)) bonds with two hydride ions (\(2H^-\)). Calcium loses two electrons and each hydride gains one electron. In the Lewis structure, calcium will be shown without any dots (since it loses 2 valence electrons) and each hydride will have two dots around them.

Lewis Structure for Magnesium Nitride

Magnesium nitride (\(Mg_3N_2\)) consists of magnesium ions (\(Mg^{2+}\)) and nitride ions (\(N^{3-}\)). In this compound, three magnesium atoms each lose two electrons and two nitrogen atoms each gain three electrons to follow the octet rule. The Lewis structure will involve three magnesium ions with no dots and two nitride ions surrounded by eight dots each.

Key concepts

Hydrogen Anions

Hydrogen anions are negatively charged ions formed when a hydrogen atom gains an additional electron. Normally, hydrogen has one proton and one electron. When it becomes a hydride ion, represented as \( H^- \), it has two electrons.
The Lewis structure of a hydrogen anion consists of the symbol "H" with two dots around it representing its two valence electrons.
Hydrogen, in its anionic form, is essential in various chemical reactions.

• It serves as a reducing agent in reactions.
• It can form compounds like lithium hydride and calcium hydride.

Understanding the characteristics of hydrogen anions is fundamental for further delving into reactions involving metal hydrides.

Nitrogen Anions

Nitrogen, unlike hydrogen, seeks to gain more electrons because it has five electrons in its outer shell. To fulfill the octet rule, nitrogen gains three electrons forming the nitride ion \( N^{3-} \).
The Lewis structure for a nitrogen anion displays the nitrogen symbol "N" surrounded by eight dots, representing complete valence electrons.
The formation of nitride ions plays a critical role in chemistry, notably:

• Nitride ions are used to create compounds like magnesium nitride.
• They contribute to the hardness of some materials.

Recognizing these aspects of nitrogen anions can help in understanding complex reaction mechanisms.

Lithium Hydride

Lithium hydride, denoted as \( LiH \), is a binary compound consisting of lithium ions \( Li^+ \) and hydride ions \( H^- \). In this compound, lithium donates one electron to hydrogen, which then fills its outer electron shell.
In the Lewis structure, lithium is symbolized without any surrounding dots, indicating it has lost its single electron. Hydride, on the other hand, shows two dots.
Lithium hydride is notable for several reasons:

• It is often used in laboratory settings for hydrogen storage.
• It serves as a strong reducing agent.

Exploring lithium hydride's properties provides insight into its usage and safety measures needed when handling.

Calcium Hydride

Calcium hydride (\( CaH_2 \)) is made of calcium ions \( Ca^{2+} \) bonded with two hydride ions \( H^- \).
The compound's Lewis structure highlights calcium without valence dots because it has lost its two outer electrons, and each hydride ion with two surrounding dots.
Calcium hydride's appeal lies in its practical applications:

• It is extensively used in the hydrogen generation.
• It serves as a desiccant, or drying agent, in chemical processes.

These properties contribute significantly to applications, especially in industry and research labs.

Magnesium Nitride

A compound formed from magnesium ions \( Mg^{2+} \) and nitride ions \( N^{3-} \) is known as magnesium nitride (\( Mg_3N_2 \)).
In this compound, magnesium donates two electrons per atom, and nitrogen gains three electrons. The Lewis structure for magnesium shows no dots, while the nitride ions display eight dots each to represent a full set.
The remarkable aspects of magnesium nitride include:

• Its use in producing specialty ceramics.
• Its role as a precursor to important chemicals like ammonia.

Magnesium nitride's applications underline the importance of understanding its chemistry in industrial processes.