Question

Complete the table by filling in the formula for the ionic compound formed by each pair of cations and anions, as shown for the first pair.$$ \begin{array}{|l|l|l|l|l|} \hline \text { Ion } & \mathrm{K}^{+} & \mathrm{NH}_{4}{ }^{+} & \mathrm{Mg}^{2+} & \mathrm{Fe}^{3+} \\ \hline \mathrm{Cl}^{-} & \mathrm{KCl} & & & \\ \hline \mathrm{OH}^{-} & & & & \\ \hline \mathrm{CO}_{3}{ }^{2-} & & & & \\ \hline \mathrm{PO}_{4}{ }^{3-} & & & & \\ \hline \end{array} $$

Short answer

The completed table with the ionic compound formulas is: $$ \begin{array}{|l|l|l|l|l|} \hline \text { Ion } & \mathrm{K}^{+} & \mathrm{NH}_{4}{ }^{+} & \mathrm{Mg}^{2+} & \mathrm{Fe}^{3+} \\ \hline \mathrm{Cl}^{-} & \mathrm{KCl} & \mathrm{NH}_{4}\mathrm{Cl} & \mathrm{MgCl}_{2} & \mathrm{FeCl}_{3} \\ \hline \mathrm{OH}^{-} & \mathrm{KOH} & \mathrm{NH}_{4}\mathrm{OH} & \mathrm{Mg(OH)}_{2} & \mathrm{Fe(OH)}_{3} \\ \hline \mathrm{CO}_{3}{ }^{2-} & \mathrm{K}_{2}\mathrm{CO}_{3} & \mathrm{(NH}_{4}\mathrm{)_{2}\mathrm{CO}_{3} & \mathrm{MgCO}_{3} & \mathrm{Fe}_{2}\mathrm{(CO}_{3}\mathrm{)}_{3} \\ \hline \mathrm{PO}_{4}{ }^{3-} & \mathrm{K}_{3}\mathrm{PO}_{4} & \mathrm{(NH}_{4}\mathrm{)_{3}\mathrm{PO}_{4} & \mathrm{Mg}_{3}\mathrm{(PO}_{4}\mathrm{)}_{2} & \mathrm{FePO}_{4} \\ \hline \end{array} $$

Step-by-step solution

Determine the charges of the ions in each pair

To complete the table, we need to determine the formulas for the ionic compounds formed by each pair of cations and anions. To do this, we will first determine the charges of the ions in each pair as they are given in the table. Ion pairs: 1. K^+ and NH4^+ are cations with charges +1 2. Mg^2+ is a cation with charge +2 3. Fe^3+ is a cation with charge +3 4. Cl^- and OH^- are anions with charges -1 5. CO3^2- is an anion with charge -2 6. PO4^3- is an anion with charge -3

Combine the ions to form ionic compounds

To form ionic compounds, we need to combine the cations and anions in such a way that the charges balance out, resulting in a neutral compound. We can follow the "cross-over" rule, meaning the absolute value of the charge on one ion will be the subscript of the other ion in the formula. Make sure to use parentheses when two or more polyatomic ions are needed. Here are the combinations for the given pairs: 1. K^+ and NH4^+ with Cl^-: - KCl (already given) - NH4Cl 2. K^+ and NH4^+ with OH^-: - KOH - NH4OH 3. K^+ and NH4^+ with CO3^2-: - K2CO3 - (NH4)2CO3 4. K^+ and NH4^+ with PO4^3-: - K3PO4 - (NH4)3PO4 5. Mg^2+ with Cl^-, OH^-, CO3^2-, and PO4^3-: - MgCl2 - Mg(OH)2 - MgCO3 - Mg3(PO4)2 6. Fe^3+ with Cl^-, OH^-, CO3^2-, and PO4^3-: - FeCl3 - Fe(OH)3 - Fe2(CO3)3 - FePO4

Fill in the table with the ionic compound formulas

Now that we have determined the formulas for the ionic compounds formed by each pair of cations and anions, we can fill in the table: $$ \begin{array}{|l|l|l|l|l|} \hline \text { Ion } & \mathrm{K}^{+} & \mathrm{NH}_{4}{ }^{+} & \mathrm{Mg}^{2+} & \mathrm{Fe}^{3+} \\ \hline \mathrm{Cl}^{-} & \mathrm{KCl} & \mathrm{NH}_{4}\mathrm{Cl} & \mathrm{MgCl}_{2} & \mathrm{FeCl}_{3} \\ \hline \mathrm{OH}^{-} & \mathrm{KOH} & \mathrm{NH}_{4}\mathrm{OH} & \mathrm{Mg(OH)}_{2} & \mathrm{Fe(OH)}_{3} \\ \hline \mathrm{CO}_{3}{ }^{2-} & \mathrm{K}_{2}\mathrm{CO}_{3} & \mathrm{(NH}_{4}\mathrm{)_{2}\mathrm{CO}_{3} & \mathrm{MgCO}_{3} & \mathrm{Fe}_{2}\mathrm{(CO}_{3}\mathrm{)}_{3} \\ \hline \mathrm{PO}_{4}{ }^{3-} & \mathrm{K}_{3}\mathrm{PO}_{4} & \mathrm{(NH}_{4}\mathrm{)_{3}\mathrm{PO}_{4} & \mathrm{Mg}_{3}\mathrm{(PO}_{4}\mathrm{)}_{2} & \mathrm{FePO}_{4} \\ \hline \end{array} $$

Key concepts

Chemical Nomenclature

Chemical nomenclature is the systematic naming of chemical compounds. It provides a unique and standardized name for each chemical substance, based on its composition and structure. For students, understanding chemical nomenclature is crucial for identifying and communicating about different substances.

Let's start with ionic compounds, which consist of positively charged cations and negatively charged anions. The naming system for ionic compounds is relatively straightforward. The name of the cation is given first, followed by the name of the anion. In cases where the cation can have more than one possible charge (as with transition metals), the charge of the cation is specified in Roman numerals (e.g., Iron(III) chloride for FeCl₃).

For compounds with polyatomic ions, the ion itself might have a specific name, such as sulfate for SO₄^2- or nitrate for NO₃^-. It is essential for students to memorize common polyatomic ions, as their names and formulas are regularly used in writing and understandings chemical nomenclature.

Polyatomic Ions

Polyatomic ions are ions that consist of two or more atoms covalently bonded, that function as a single ion with a positive or negative charge. Recognizing these ions is vital for writing the formulas of ionic compounds correctly. Common polyatomic ions include hydroxide (OH^-), carbonate (CO₃^2-), and phosphate (PO₄^3-).

To write formulas correctly involving polyatomic ions, remember that the ion behaves as a unit. When more than one polyatomic ion is needed to balance the charge in an ionic compound, use parentheses before adding the subscript. For example, calcium nitrate requires two nitrate ions to balance with one calcium ion, resulting in Ca(NO₃)₂.

Cation-Anion Pairing

The pairing of cations and anions to form ionic compounds is governed by the principle of charge neutrality. The total positive charge of the cations must balance the total negative charge of the anions in the compound, leading to a neutral overall charge.

The 'cross-over' method is a popular tool for students to pair ions effectively. The absolute value of one ion's charge is used as the subscript for the other ion in the compound. If magnesium (Mg²⁺) pairs with chloride (Cl^-), the resulting formula is MgCl₂, not MgCl or Mg₂Cl₂ because two chloride ions are needed to balance the charge of one magnesium ion.

As with polyatomic ions, if multiple polyatomic ions are required, parentheses are used around the ion before the subscript is added. An example is aluminum sulfate, which is written as Al₂(SO₄)₃, since three sulfate ions are needed to balance two aluminum ions. Understanding how to pair cations with anions is critical to formulating the correct chemical formula for an ionic compound.