Problem 69 - Chemistry

Question

Each of the following pairs of elements will react to form a binary ionic compound. Write the formula of each compound formed, and give its name. (a) Magnesium and chlorine (b) Calcium and oxygen (c) Lithium and nitrogen (d) Aluminum and oxygen

Step-by-Step Solution

Answer

(a) MgCl₂, Magnesium Chloride (b) CaO, Calcium Oxide (c) Li₃N, Lithium Nitride (d) Al₂O₃, Aluminum Oxide

Step 1: Identify the ions for Magnesium and Chlorine

Magnesium typically forms a cation with a charge of \( \text{Mg}^{2+} \). Chlorine typically forms an anion with a charge of \( \text{Cl}^- \).

Step 2: Write the formula for Magnesium Chloride

To balance the charges, two chloride ions \( \text{Cl}^- \) are needed for each \( \text{Mg}^{2+} \) ion. Thus, the formula for magnesium chloride is \( \text{MgCl}_2 \).

Step 3: Identify the ions for Calcium and Oxygen

Calcium forms a cation \( \text{Ca}^{2+} \) and oxygen forms an anion \( \text{O}^{2-} \).

Step 4: Write the formula for Calcium Oxide

The charges are already balanced as \( \text{Ca}^{2+} \) and \( \text{O}^{2-} \), so the formula is \( \text{CaO} \).

Step 5: Identify the ions for Lithium and Nitrogen

Lithium forms a cation \( \text{Li}^+ \) and nitrogen forms an anion \( \text{N}^{3-} \).

Step 6: Write the formula for Lithium Nitride

To balance the charges, three lithium ions \( \text{Li}^+ \) are needed for each \( \text{N}^{3-} \) ion, resulting in the formula \( \text{Li}_3\text{N} \).

Step 7: Identify the ions for Aluminum and Oxygen

Aluminum forms a cation \( \text{Al}^{3+} \) and oxygen forms an anion \( \text{O}^{2-} \).

Step 8: Write the formula for Aluminum Oxide

To balance the charges, two \( \text{Al}^{3+} \) ions and three \( \text{O}^{2-} \) ions are needed, resulting in the formula \( \text{Al}_2\text{O}_3 \).

Key Concepts

Ionic Bonding

Ionic bonding is a type of chemical bonding that involves the complete transfer of valence electrons between atoms. This transfer leads to the formation of oppositely charged ions: cations and anions.
The key idea here is the electrostatic attraction between these ions, which holds them together in an ionic compound. When atoms of metallic elements (like sodium or magnesium) lose electrons, they form positively charged ions known as cations. Nonmetals, on the other hand, often gain electrons to become negatively charged anions. For example, chlorine gains an electron to form the chloride ion, \( \text{Cl}^- \).The ionic bond results in a stable compound called a binary ionic compound, which usually forms a crystalline structure. The chemical durable nature of these compounds comes from the lattice structure that forms due to the regular, repeating pattern of ions.

Chemical Formulas

Chemical formulas are a shorthand method of describing the elemental composition of a compound. They consist of the chemical symbols of the elements involved as well as numerical subscripts indicating the ratio of each type of atom in the compound.In binary ionic compounds like the ones formed in the exercise, the chemical formula reflects the balance of positive and negative charges. For example, the formula for magnesium chloride is \( \text{MgCl}_2 \). This indicates the compound contains one magnesium ion \( \text{Mg}^{2+} \) and two chloride ions \( \text{Cl}^- \).

Writing Chemical Formulas

• Identify the cation and the anion from the elements in the pair.
• Determine the charges on each ion.
• Use the charges to find the lowest whole number ratio of ions needed to achieve charge neutrality.
• Write the formula using these ratios, ensuring the cation is written first.

This process ensures the compound as a whole is electrically neutral, and the chemical formula provides a concise way to convey this information.

Cation and Anion Identification

Identifying cations and anions is crucial for predicting the formula of ionic compounds. The charge of these ions often relates directly to their position in the periodic table.

Cations

Cations are typically formed by metals, which lose electrons from their outer shell. For instance, magnesium (Mg) will lose two electrons to form \( \text{Mg}^{2+} \), and lithium (Li) loses one electron to form \( \text{Li}^+ \).

Anions

Anions, on the other hand, are usually formed by nonmetals. These elements gain electrons to fill their valence shell. For example, oxygen gains two electrons to form \( \text{O}^{2-} \), and nitrogen gains three electrons to become \( \text{N}^{3-} \).

Periodic Table Guide

• Group 1 elements form \( +1 \) cations.
• Group 2 elements form \( +2 \) cations.
• Group 17 elements form \( -1 \) anions.
• Group 16 elements form \( -2 \) anions.

By identifying these charges, you can easily determine the correct formula for common binary ionic compounds.