Problem 152 - Introductory Chemistry Atoms First

Question

Predict the formula for lithium nitride, made from lithium, \(\mathrm{Li}\), and nitrogen, \(\mathrm{N}\). Explain your reasoning.

Step-by-Step Solution

Answer

The formula for lithium nitride is \(\text{Li}_3\text{N}\). This is because lithium forms a +1 ion and nitrogen forms a -3 ion. To create a neutral compound, we need three lithium ions to balance the charge of one nitrogen ion. This results in the formula \(\text{Li}_3\text{N}\).

Step 1: Determine the charges of the ions

Lithium, as an alkali metal, tends to lose its single valence electron to form a positive ion with a charge of +1. Following the formula: \(\text{Charge of Lithium ion} = +1\) Nitrogen, on the other hand, tends to gain 3 more electrons to complete its outer electron shell, forming a negative ion with a charge of -3. Using the formula: \(\text{Charge of Nitrogen ion} = -3\)

Step 2: Combine the ions to form a neutral compound

Now, we have the charges of each ion, we need to combine them in a ratio that results in a neutral compound. In this case, we can balance the charges by using 3 lithium ions and 1 nitrogen ion, as the total charge will be 0 in this ratio. \((\text{Charge of Lithium ion} \times 3) + (\text{Charge of Nitrogen ion} \times 1) = 0\) \((+1 \times 3) + (-3 \times 1) = 0\) \((+3) + (-3) = 0\)

Step 3: Write the formula for lithium nitride

Having found the ratio of the ions needed for a neutral charge (3 lithium ions to 1 nitrogen ion), we can write the formula for lithium nitride: \[\text{Li}_3\text{N}\] Thus, the formula for lithium nitride, made from lithium (Li) and nitrogen (N), is \(\text{Li}_3\text{N}\), with the reasoning based on achieving a neutral charge in the compound by combining the ions in a specific ratio.

Key Concepts

Ionic Compounds

Ionic compounds are chemical compounds made up of charged particles known as ions. These ions form when an atom loses or gains electrons, leading to a positive or negative charge, respectively. In an ionic compound, these ions are held together by strong electrostatic forces in a lattice structure, creating a stable compound.

Here's what happens in the case of lithium nitride:

• Positive Ions (Cations): Lithium ( \(\mathrm{Li}\) ) loses its single valence electron, resulting in a positively charged ion with a charge of +1.
• Negative Ions (Anions): Nitrogen ( \(\mathrm{N}\) ) gains electrons to fill its outer shell, becoming negatively charged with a charge of -3.

The critical aspect of ionic compounds like lithium nitride is that the total charges from the ions must balance to zero. This means that the positive and negative charges cancel each other out, creating a neutral compound overall. This balance is why three lithium ions combine with one nitrogen ion to form lithium nitride, achieving the stable neutral charge needed.

Valence Electrons

Valence electrons are the electrons in the outermost shell of an atom. These are the electrons involved in chemical bonding and determine how an atom will interact and bond with other atoms. Understanding valence electrons is key to predicting the formation of ionic compounds.

In the formation of lithium nitride:

• Lithium: Possesses one valence electron. Because it belongs to Group 1 in the periodic table, this electron is readily lost, forming a positively charged ion with a charge of +1.
• Nitrogen: Has five valence electrons, as it is part of Group 15. It needs three additional electrons to complete its outer electron shell, making it inclined to gain electrons and form an ion with a -3 charge.

The interactions between the valence electrons of lithium and nitrogen dictate how these elements form ions and subsequently bond to create a stable ionic compound. This activity is fundamental in determining the chemical formulas of compounds.

Chemical Formula

A chemical formula conveys the proportions of atoms that make up a compound. It uses element symbols and numerical subscripts to indicate the number of each kind of atom in the simplest unit of the compound.

For instance, the chemical formula for lithium nitride is \(\mathrm{Li}_3\mathrm{N}\) :

• Subscript in Li: The subscript '3' after lithium indicates there are three lithium ions for every one nitrogen ion.
• Balance of Charge: This ratio results from balancing the total positive charge from lithium \((+1 \times 3 = +3)\) with the total negative charge from nitrogen \((-3)\).
• Representation of Neutrality: The chemical formula \(\mathrm{Li}_3\mathrm{N}\) shows that the compound is neutral, with no overall charge.

Understanding the chemical formula is crucial for predicting the structure and reactivity of the compound. It provides insights into how the compound is constructed at an atomic level, ensuring the stability and balance of charges necessary for the compound's existence.