Question

Write the formula of the following ionic compounds: a. nickel(III) oxide b. iron(III) sulfide c. lead(II) sulfate d. chromium(III) iodide e. lithium nitride f. gold(I) oxide

Short answer

Nickel(III) oxide: Ni2O3, Iron(III) sulfide: Fe2S3, Lead(II) sulfate: PbSO4, Chromium(III) iodide: CrI3, Lithium nitride: Li3N, Gold(I) oxide: Au2O.

Step-by-step solution

Identify the Ion Charges

Determine the charge of each cation and anion in the compounds. For example, nickel(III) indicates a charge of +3 for nickel ions (Ni^3+), and oxide indicates a charge of -2 for oxide ions (O^2-).

Use the Criss-Cross Method

Use the criss-cross method to balance the charges. Swap the absolute values of the charges to use as subscripts for the opposite ion. Reduce subscripts to their simplest form if necessary. For Ni^3+ and O^2-, the formula becomes Ni2O3 because 3 and 2 are swapped and no further reduction is needed.

Write the Formula for Nickel(III) Oxide

Following the criss-cross method, the formula for nickel(III) oxide is Ni2O3.

Write the Formula for Iron(III) Sulfide

Iron(III) indicates Fe^3+ and sulfide indicates S^2-. Using the criss-cross method, the formula is Fe2S3.

Write the Formula for Lead(II) Sulfate

Lead(II) indicates Pb^2+ and sulfate is SO4^2-. When the charges are equal, they balance each other, so the formula is PbSO4.

Write the Formula for Chromium(III) Iodide

Chromium(III) indicates Cr^3+ and iodide is I^-. Using the criss-cross method, the formula is CrI3 as the 3 charges swap.

Write the Formula for Lithium Nitride

Lithium has a charge of +1 (Li^+) and nitride has a charge of -3 (N^3-). Using the criss-cross method, the formula is Li3N.

Write the Formula for Gold(I) Oxide

Gold(I) indicates Au^+ and oxide is O^2-. Using the criss-cross method, the formula is Au2O.

Key concepts

Ion Charges

Ion charges are essential for understanding ionic compounds. An ion is an atom that has gained or lost electrons, giving it a positive or negative charge.

• **Positive ions (cations)**: Formed when an atom loses electrons. For example, nickel(III) has lost three electrons, making it Ni³⁺.
• **Negative ions (anions)**: Formed when an atom gains electrons. For instance, an oxide ion has gained two electrons to become O^2-.

Understanding the charge on ions helps predict the composition of ionic compounds. Each cation and anion's charge must balance to create a stable compound. For the exercise:

• Nickel(III) oxide: Ni³⁺ and O^2-
• Iron(III) sulfide: Fe³⁺ and S^2-
• Lead(II) sulfate: Pb²⁺ and SO₄^2-
• Chromium(III) iodide: Cr³⁺ and I^-
• Lithium nitride: Li⁺ and N^3-
• Gold(I) oxide: Au⁺ and O^2-

Knowing these charges will help use methods like the 'criss-cross method' to write correct formulas.

Criss-Cross Method

The criss-cross method is a simple way to balance the charges of ions when writing formulas for ionic compounds. Here’s how it works:
1. Write down the charges of the cation and the anion.
2. Criss-cross the absolute values of these charges to become the subscripts of the opposite ions. For example, for nickel(III) oxide (Ni³⁺ and O^2-), you swap the charges to get Ni₂O₃.
Here’s how the method applies to the exercise compounds:

• **Nickel(III) oxide (Ni³⁺ and O^2-)**: Criss-cross the charges to get Ni₂O₃.
• **Iron(III) sulfide (Fe³⁺ and S^2-)**: Criss-cross the charges to get Fe₂S₃.
• **Lead(II) sulfate (Pb²⁺ and SO₄^2-)**: When the charges are the same, they balance out to form PbSO₄.
• **Chromium(III) iodide (Cr³⁺ and I^-)**: Criss-cross the charges to get CrI₃.
• **Lithium nitride (Li⁺ and N^3-)**: Criss-cross the charges to get Li₃N.
• **Gold(I) oxide (Au⁺ and O^2-)**: Criss-cross the charges to get Au₂O.

It's a simple yet powerful technique to ensure the overall neutrality of ionic compounds.

Ionic Compounds

Ionic compounds form when metals transfer electrons to non-metals, creating positively and negatively charged ions that attract each other. Here's what you should know:

• **Consistency in charges**: Each ion in an ionic compound must combine in a ratio that balances the overall charge.
• **Formula representation**: Ionic compounds are represented by formulas that reflect the balanced ratio of cations and anions. For example, sodium chloride (table salt) is NaCl, meaning each sodium ion (Na⁺) pairs with one chloride ion (Cl^-).

In the exercise, each compound's formula is derived from balancing the ion charges. For example:
- **Nickel(III) oxide** is Ni₂O₃ due to the need for two Ni³⁺ ions to balance three O^2- ions.
- **Iron(III) sulfide** is Fe₂S₃ as two Fe³⁺ ions balance with three S^2- ions.
Understanding the nature of ionic bonds and how they form helps in predicting and writing accurate chemical formulas for ionic compounds.