Problem 69 - Chemistry The Central Science

Question

Give the names and charges of the cation and anion in each of the following compounds: (a) \(\mathrm{CaO},(\mathbf{b}) \mathrm{Na}_{2} \mathrm{SO}_{4},(\mathbf{c}) \mathrm{KClO}_{4}\) (\mathbf{d} ) ~ \(\mathrm{Fe}\left(\mathrm{NO}_{3}\right)_{2},(\mathbf{e}) \operatorname{Cr}(\mathrm{OH})_{3}\)

Step-by-Step Solution

Answer

The cations and anions, along with their charges, for each compound are: (a) CaO: Calcium (2+) and Oxide (2-) (b) Na2SO4: Sodium (1+) and Sulfate (2-) (c) KClO4: Potassium (1+) and Perchlorate (1-) (d) Fe(NO3)2: Iron(II) (2+) and Nitrate (1-) (e) Cr(OH)3: Chromium(III) (3+) and Hydroxide (1-)

Step 1: Identify cations and anions for each compound

To find the charges of the ions in each compound, we must identify the cation (positive ion) and the anion (negative ion) in each formula. (a) CaO: Ca is the cation and O is the anion. (b) Na2SO4: Na is the cation and SO4 is the anion. (c) KClO4: K is the cation and ClO4 is the anion. (d) Fe(NO3)2: Fe is the cation and NO3 is the anion. (e) Cr(OH)3: Cr is the cation and OH is the anion.

Step 2: Determine the charges of the cations and anions based on the periodic table and common polyatomic ions

Consult the periodic table and a list of common polyatomic ions to determine the charges of the ions in each compound. (a) CaO: Ca is in Group 2 and has a charge of 2+, and O is in Group 16 and has a charge of 2-. (b) Na2SO4: Na is in Group 1 and has a charge of 1+, and SO4 is the polyatomic ion sulfate, with a charge of 2-. (c) KClO4: K is in Group 1 and has a charge of 1+, and ClO4 is the polyatomic ion perchlorate, with a charge of 1-. (d) Fe(NO3)2: Fe is iron and has a charge of 2+ in this compound (as indicated by the subscript of NO3), and NO3 is the polyatomic ion nitrate, with a charge of 1-. (e) Cr(OH)3: Cr is chromium and has a charge of 3+ in this compound (as indicated by the subscript of OH), and OH is the polyatomic ion hydroxide, with a charge of 1-.

Step 3: Give the names and charges of the cation and anion in each compound

After determining the charges for the cations and anions involved in each compound, we can now write the names and charges for each as follows: (a) CaO: Calcium (2+) and Oxide (2-) (b) Na2SO4: Sodium (1+) and Sulfate (2-) (c) KClO4: Potassium (1+) and Perchlorate (1-) (d) Fe(NO3)2: Iron(II) (2+) and Nitrate (1-) [Iron has multiple possible charges, so we must use Roman numerals to indicate the specific charge]. (e) Cr(OH)3: Chromium(III) (3+) and Hydroxide (1-) [Similarly, we must use Roman numerals for chromium to indicate the specific charge].

Key Concepts

Cation and Anion Identification

In ionic compounds, identifying the cation (positive ion) and the anion (negative ion) is crucial. By looking at the chemical formula, you can determine which elements or groups are involved. For example, in the compound \( \mathrm{CaO} \), \( \mathrm{Ca} \) is the cation, and \( \mathrm{O} \) is the anion. This is because calcium has a tendency to lose electrons and form positive ions, while oxygen gains electrons to become a negative ion.

• Cations are generally metals or groups of atoms that carry a positive charge.
• Anions are typically non-metals or groups of atoms that carry a negative charge.

Thus, a good understanding of the components of the formula allows you to identify the ions present.

Polyatomic Ions

Polyatomic ions are composed of more than one atom bonded together, acting as a single charged entity. Recognizing these ions within compounds is essential for understanding ionic formulas.
For instance, in \( \mathrm{Na_2SO_4} \), \( \mathrm{SO_4} \) is a polyatomic ion known as sulfate. It consists of sulfur and oxygen atoms bonded together, carrying an overall negative charge.

• Sulfate (\( \mathrm{SO_4}^{2-} \))
• Perchlorate (\( \mathrm{ClO_4}^{-} \))
• Nitrate (\( \mathrm{NO_3}^{-} \))
• Hydroxide (\( \mathrm{OH}^{-} \))

Being familiar with common polyatomic ions helps in identifying them in various compounds.

Charge Determination

Determining the charge of ions relies on their position in the periodic table and the nature of polyatomic ions. Elements tend to form predictable charges based on their group numbers.
Metals commonly lose electrons to become positively charged cations. For example, \( \mathrm{Na} \) from Group 1 forms a \( 1^+ \) charge, while \( \mathrm{Ca} \) from Group 2 forms a \( 2^+ \) charge. Non-metals, on the other hand, gain electrons to become negatively charged anions, such as \( \mathrm{O}^{2-} \).

• The charge of a polyatomic ion is a fixed value, based on the sum of the oxidation states of its atoms. For example, sulfate (\( \mathrm{SO_4} \)) has a \( 2^- \) charge.

Knowing this can help you determine the charges in any compound accurately.

Periodic Table Groups

The periodic table groups provide insight into the number and charge of electrons that an element gains or loses.
Each vertical column is a group, and elements in a group often share the same charge characteristics due to having a similar number of valence electrons. Metals in Group 1 form \( 1^+ \) ions, while those in Group 2 form \( 2^+ \) ions.
Non-metals in Groups 16 and 17 typically form \( 2^- \) and \( 1^- \) ions, respectively.

• Group 1: \( 1^+ \) charge (e.g., \( \mathrm{Na} \) and \( \mathrm{K} \))
• Group 2: \( 2^+ \) charge (e.g., \( \mathrm{Ca} \))
• Group 16: \( 2^- \) charge (e.g., \( \mathrm{O} \))
• Group 17: \( 1^- \) charge (e.g., \( \mathrm{Cl} \))

Understanding the concept of groups makes predicting ionic charges straightforward.

Nomenclature of Ions

Naming ions involves a clear set of rules that reflect their charge and composition. For simple cations, the element's name remains unchanged, like \( \mathrm{Ca}^{2+} \) being called calcium.
For transition metals, which can have multiple charges, Roman numerals indicate the specific charge—such as iron(II) for \( \mathrm{Fe}^{2+} \). Anions often have their names changed to end in "-ide," such as \( \mathrm{O}^{2-} \) (oxide).

• Polyatomic ions retain unique names, like sulfate for \( \mathrm{SO_4}^{2-} \) and nitrate for \( \mathrm{NO_3}^{-} \).

Mastery of these naming conventions is key to accurately communicating the types and charges of ions in a compound.