Question

Write formulas for the following ionic compounds. (a) cobalt(II) acetate (b) barium oxide (c) aluminum sulfide (d) potassium permanganate (e) sodium hydrogen carbonate

Short answer

Question: Write the chemical formulas for the following ionic compounds: (a) Cobalt(II) acetate, (b) Barium oxide, (c) Aluminum sulfide, (d) Potassium permanganate, and (e) Sodium hydrogen carbonate. Answer: (a) Co(CH3COO)2, (b) BaO, (c) Al2S3, (d) KMnO4, (e) NaHCO3.

Step-by-step solution

(a) Cobalt(II) acetate

First, we need to determine the charges of the ions involved. Cobalt(II) means that cobalt has a +2 charge. Acetate is a polyatomic ion with the formula CH3COO- and a charge of -1. Since cobalt has a +2 charge and acetate has a -1 charge, we need two acetate ions to balance the charges, giving us the final formula of Co(CH3COO)2.

(b) Barium oxide

In this compound, we have barium (Ba) and oxygen (O). Barium is in group 2 of the periodic table and has a +2 charge, while oxygen is in group 6 and has a -2 charge. Since both ions have charges of equal magnitude but opposite signs, the compound's overall charge is neutral. The formula for barium oxide is BaO.

(c) Aluminum sulfide

In this compound, we have aluminum (Al) and sulfur (S). Aluminum is in group 3 of the periodic table and has a +3 charge, while sulfur is in group 6 and has a -2 charge. We need to find the smallest common multiple of their charges to achieve a neutral compound. The smallest common multiple of 3 and 2 is 6, so we need two aluminum ions (2 × 3 = 6) and three sulfur ions (3 × 2 = 6) to cancel out each other's charges. The formula for aluminum sulfide is Al2S3.

(d) Potassium permanganate

In this compound, we have potassium (K) and the polyatomic ion permanganate (MnO4-). Potassium is in group 1 of the periodic table and has a +1 charge, while permanganate has a -1 charge. Since both ions have charges of equal magnitude but opposite signs, the compound's overall charge is neutral. The formula for potassium permanganate is KMnO4.

(e) Sodium hydrogen carbonate

In this compound, we have sodium (Na) and the polyatomic ion hydrogen carbonate (HCO3-). Sodium is in group 1 of the periodic table and has a +1 charge, while hydrogen carbonate has a -1 charge. Since both ions have charges of equal magnitude but opposite signs, the compound's overall charge is neutral. The formula for sodium hydrogen carbonate is NaHCO3.

Key concepts

Chemical Formulas

Chemical formulas are the basic way to represent chemical compounds using symbols for the elements and numbers to show the ratios in which the elements combine. Each element in the chemical formula is denoted by its symbol from the periodic table, and a subscript number following each symbol indicates the number of atoms of that element in the compound.

For ionic compounds, these formulas reflect the smallest whole number ratio of the ions involved. Here are some key steps to writing chemical formulas for ionic compounds:

• Identify the cation (positive ion) and anion (negative ion) involved.
• Determine the charges on the ions based on their positions on the periodic table or known charges of polyatomic ions.
• Use the charge balancing concept to figure out the ratio of ions that results in a neutral compound.

Ionic compound formulas can vary because different ions combine in different ways to achieve electrical neutrality.

Charge Balancing

Charge balancing is a critical concept when writing chemical formulas for ionic compounds. Since the overall charge of a compound must be zero, the charges of the cations and anions have to balance out. If the charge on the cation is not equal to the charge on the anion, multiple ions may be needed.

Here's how charge balancing works in practice:

• Write down the charges on the cation and anion.
• Make sure the total positive charge equals the total negative charge by using the smallest whole number ratio of ions that will achieve this balance.

For example, if you have aluminum with a charge of +3 and sulfur with a charge of -2, you can achieve charge balance using two aluminum ions (total charge +6) and three sulfur ions (total charge -6), leading to the formula \( \text{Al}_2\text{S}_3 \).

Polyatomic Ions

Polyatomic ions are ions consisting of two or more atoms bonded together, carrying an overall charge. These ions often behave as a single unit in chemical reactions and are fundamental in the formation of ionic compounds.

When dealing with polyatomic ions, the rules for charge balancing still apply, and the polyatomic ion is treated as a single entity:

• The charge on the polyatomic ion must be counterbalanced by the charges of the other ions in the compound.
• Polyatomic ions have specific formulas and names, which are typically memorized for reference. For example, acetate is \( \text{CH}_3\text{COO}^- \) and permanganate is \( \text{MnO}_4^- \).

In formulas, if more than one polyatomic ion is needed, parentheses are used to indicate the repetition of the entire ion, such as \( \text{Co(CH}_3\text{COO)}_2 \).

Periodic Table

The periodic table serves as an essential tool for predicting the charges of ions, especially for many main group elements. It organizes elements based on their atomic structure and properties, helping us understand their common oxidation states, which aid in writing chemical formulas.

Elements in the same group (vertical column) of the periodic table often share similar properties and typically form ions with the same charge:

• Group 1 elements (like sodium and potassium) typically have a +1 charge.
• Group 2 elements (like barium) typically have a +2 charge.
• Group 17 elements (halogens) typically form -1 ions.

Understanding these patterns helps you anticipate the charges and correctly balance the formulas of ionic compounds. For transition metals, the charge might vary, so additional information such as roman numerals may be needed to identify their charge, as seen in cobalt(II).