Problem 11 - Chemical Principles: The Quest for Insight

Question

Write formulas for the ionic compounds formed from (a) sodium and oxide ions; (b) potassium and sulfate ions; (c) silver and fluonide ions; (d) zinc and nitrate ions; (e) aluminum and sulfide ions.

Step-by-Step Solution

Answer

The formulas for the ionic compounds are (a) Na2O, (b) K2SO4, (c) AgF, (d) Zn(NO3)2, (e) Al2S3.

Step 1: Identify the Charge of Each Ion

For each pair of ions, identify the charge of the cation (positive ion) and the anion (negative ion). Sodium forms a +1 cation (Na+), oxide forms a -2 anion (O^2-). Potassium forms a +1 cation (K+), sulfate forms a -2 anion (SO4^2-). Silver forms a +1 cation (Ag+), fluoride forms a -1 anion (F-). Zinc forms a +2 cation (Zn^2+), nitrate forms a -1 anion (NO3-). Aluminum forms a +3 cation (Al^3+), sulfide forms a -2 anion (S^2-).

Step 2: Determine the Ratio of Ions for a Neutral Compound

The total positive charge must balance the total negative charge. For sodium and oxide, the ratio is 2:1. For potassium and sulfate, it is 2:1. For silver and fluoride, it is 1:1. For zinc and nitrate, it is 1:2. For aluminum and sulfide, it is 2:3.

Step 3: Write the Formulas for the Ionic Compounds

Use the determined ratios to write the formula. For sodium and oxide, the formula is Na2O. For potassium and sulfate, the formula is K2SO4. For silver and fluoride, the formula is AgF. For zinc and nitrate, the formula is Zn(NO3)2. For aluminum and sulfide, the formula is Al2S3.

Key Concepts

Chemical Formula Writing

Understanding how to write chemical formulas for ionic compounds is essential in mastering chemistry. An ionic compound is composed of a cation, which is a positively charged ion, and an anion, which is a negatively charged ion. These ions bond together to form a neutral compound.

In writing the formula of an ionic compound, the goal is to combine the ions in proportions such that the positive and negative charges balance out. The sum of the charges must equal zero. When you look at the provided exercise, you can see that this balance is achieved by adjusting the ratio of cations to anions. For example, to balance the charges for sodium (Na+) and oxide (O^2-), we need two sodium ions for every oxide ion, resulting in the formula Na2O.

Ionic Charges and Ratios

Identifying Ionic Charges

Each ion has a characteristic charge based on its position in the periodic table. In our exercise, sodium (Na) loses one electron to form a +1 cation, while oxide (O) gains two electrons to form a -2 anion.

Finding the Correct Ratios

To determine the ratio of ions, compare their charges. The number of ions needed from either side will depend on the necessity to balance the overall charge. In the exercise with zinc (Zn^2+) and nitrate (NO3-), one zinc cation, which has a +2 charge, balances with two nitrate anions each with a -1 charge, leading to the formula Zn(NO3)2.

Cation-Anion Pairing

Pairing cations and anions to form ionic compounds can be visualized like solving a puzzle; each piece must fit together perfectly. The cation, a 'puzzle piece' with one or more positive charges, must be matched with an anion 'piece' carrying negative charges. Their charges must cancel each other out for the compound to be stable. In our exercise, aluminum (Al^3+) requires three positive charges to be neutralized, which are balanced by two sulfide anions (S^2-), hence the resulting chemical formula is Al2S3. This pairing indicates that two aluminum ions will pair with three sulfide ions to form a stable ionic compound.

Chemical Nomenclature

The naming of chemical compounds, known as chemical nomenclature, adheres to a set of rules and conventions to ensure clear and concise communication. Ionic compounds are typically named by stating the cation first, followed by the anion. For monatomic anions, the ending '-ide' is added to the root of the element's name. The exercise includes potassium (K+) and sulfate (SO4^2-), where potassium is the cation and sulfate is the polyatomic anion. Following the rules, the name of the compound is 'potassium sulfate,' and the formula, as determined by the ion charges and ratios, is K2SO4.