The chemical formula for lithium nitride is \(\text{Li}_3\text{N}\). This formula indicates the compound is constructed from three atoms of lithium for every single atom of nitrogen. Lithium nitride is classified as a salt, forming a crystalline solid that typically presents as a reddish-brown or ruby-red powder. It is the only stable nitride compound formed by an alkali metal, the group of elements found in the first column of the periodic table. Understanding this ratio requires examining the elements involved and their tendencies to form ions.
The Elemental Components
The compound lithium nitride is composed of the metal lithium (\(\text{Li}\)) and the nonmetal nitrogen (\(\text{N}\)). Lithium is located in Group 1 of the periodic table, meaning its atoms possess one electron in their outermost energy shell. To achieve stability, lithium readily loses this single valence electron, resulting in a positively charged ion, or cation, represented as \(\text{Li}^+\).
Nitrogen, on the other hand, is found in Group 15 and has five electrons in its outer shell. This means it requires three additional electrons to complete its stable octet. By gaining these three electrons, a nitrogen atom transforms into a negatively charged ion, or anion, known as the nitride ion (\(\text{N}^{3-}\)). This exchange of electrons forms an ionic compound, held together by a strong electrostatic attraction between the oppositely charged ions.
Balancing the Charges
The chemical formula for lithium nitride must reflect the precise ratio of ions needed to create an electrically neutral compound. The total positive charge contributed by the lithium ions must perfectly cancel out the total negative charge from the nitride ion. Each lithium ion carries a \(+1\) charge (\(\text{Li}^+\)), while each nitride ion carries a \(-3\) charge (\(\text{N}^{3-}\)).
To balance the \(-3\) charge of a single nitride ion, a corresponding positive charge of \(+3\) is required. Since each lithium ion only contributes \(+1\) of positive charge, three separate lithium ions are necessary to reach the required total positive charge (\(3 \times +1 = +3\)). This combination of three \(\text{Li}^+\) ions and one \(\text{N}^{3-}\) ion results in a net charge of zero (\(+3 + -3 = 0\)).
This necessary ratio of three lithium ions to one nitride ion dictates the subscript notation in the chemical formula. The subscript ‘3’ is placed after the symbol for lithium (\(\text{Li}\)) to indicate the presence of three lithium ions. Therefore, the formula \(\text{Li}_3\text{N}\) is derived directly from the principle of charge neutrality.
Unique Properties and Practical Uses
Lithium nitride is a compound with distinct physical and chemical characteristics, making it useful in advanced material science. It is known for its high melting point and for being a strong base. A primary chemical feature is its high reactivity with moisture, where it rapidly reacts with water to produce lithium hydroxide and ammonia gas.
One property of lithium nitride is its ability to conduct ions, specifically the movement of lithium ions, making it a superionic conductor. This high ionic conductivity is researched for use as a solid-state electrolyte in next-generation lithium batteries. Using a solid electrolyte can improve the safety and energy density of batteries compared to traditional liquid electrolytes.
Beyond energy storage, lithium nitride has been investigated for its potential in hydrogen storage applications, due to its capacity to react with and hold hydrogen. Additionally, it has been explored for its catalytic properties, being used as a source of the nitride ion in certain chemical syntheses.