Question

Name each ionic compound containing a polyatomic ion. $$ \begin{array}{ll}{\text { a. } \mathrm{CuNO}_{2}} & {\text { b. } \operatorname{Mg}\left(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\right)_{2}} \\ {\text { c. } \mathrm{Ba}\left(\mathrm{NO}_{3}\right)_{2}} & {\text { d. } \mathrm{Pb}\left(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}\right)_{2}}\end{array} $$

Short answer

a. Copper(I) nitrite, b. Magnesium acetate, c. Barium nitrate, d. Lead(II) acetate

Step-by-step solution

Identify the Cation and Anion

Recognize that ionic compounds consist of a combination of cations (positive ions) and anions (negative ions). In each compound, the first element is the metal cation, and the second part is the polyatomic anion.

Name the Metal Cation

Name the metal cation by using its element name. For transition metals, indicate the ionic charge with Roman numerals in parentheses since they can have multiple oxidation states.

Name the Polyatomic Anion

Identify the polyatomic ion by referring to a list of common polyatomic ions. Use the standard name of the polyatomic ion.

Write the Full Ionic Compound Name

Combine the names of the cation and anion to form the full name of the ionic compound.

Key concepts

Polyatomic Ions

Understanding polyatomic ions is essential for naming complex ionic compounds. A polyatomic ion is a charged entity composed of two or more atoms covalently bonded together, or of a metal complex that acts as a single unit with a positive or negative charge.

Common polyatomic ions include sulfate \( \text{SO}_4^{2-} \), nitrate \( \text{NO}_3^{-} \), and acetate \( \text{C}_2\text{H}_3\text{O}_2^{-} \). It's important to memorize these ions, as they frequently occur in a variety of compounds. Polyatomic ions can combine with metals to form ionic compounds, and they have unique names that do not change regardless of the compound they are in.

For example, the ion \( \text{NO}_2^{-} \) is known as nitrite, and it forms ionic compounds by bonding with a cation, such as copper in copper(II) nitrite \( \text{CuNO}_2 \). Recognizing the polyatomic ion in a compound is the cornerstone of naming the compound correctly.

Chemical Nomenclature of Ionic Compounds

Chemical nomenclature is the systematic naming of chemical compounds based on their composition. For ionic compounds containing polyatomic ions, it is essential to name the cation first and the anion second.

Metal Cations

With metal cations, you often use the element name directly. In the case of transition metals like copper \( \text{Cu} \) or lead \( \text{Pb} \), which can have more than one possible ionic charge, Roman numerals indicate the specific charge in the name. For example, copper can exist as \( \text{Cu}^{+} \) or \( \text{Cu}^{2+} \), leading to names such as copper(I) or copper(II).

Polyatomic Anions

After naming the metal cation, focus on the polyatomic anion. Each polyatomic ion has a unique name that you must learn to identify and use. For example, \( \mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2}^{-} \) is named acetate, not to be confused with the similar-sounding acetyl group, which is a different chemical entity.

By combining the names of the cation and anion, you create the full name of the compound, such as magnesium acetate for \( \text{Mg}\left(\text{C}_2\text{H}_3\text{O}_2\right)_2 \) and lead(II) acetate for \( \text{Pb}\left(\text{C}_2\text{H}_3\text{O}_2\right)_2\). The parentheses around the polyatomic ion and subscript indicate how many of these ions are present in the compound.

Ionic Charge

Ionic charge is the electric charge an atom or molecule has when it has lost or gained one or more electrons. In an ionic compound, the total charge must be balanced, meaning the sum of the charges of cations and anions equals zero.

Ions are represented with their elemental symbol followed by a superscript indicating the magnitude and sign of the charge. For instance, a sodium ion with a single positive charge is written as \( \text{Na}^{+} \), while a chloride ion with a single negative charge is \( \text{Cl}^{-} \). The balancing of charge is crucial when forming ionic compounds. If a barium ion \( \text{Ba}^{2+} \) combines with nitrate \( \text{NO}_3^{-} \) to form barium nitrate, you need two nitrate ions to balance the charge of one barium ion, resulting in \( \text{Ba}\left(\text{NO}_3\right)_2 \).

When naming ionic compounds with transition metals, indicating the ionic charge is vital since these metals can have multiple oxidation states, as seen with the lead(II) ion in lead(II) acetate. Recognizing and calculating ionic charge helps ensure you write and name ionic compounds correctly.