Question

For each of the following positive ions, use the concept that a chemical compound must have a net charge of zero to predict the formula of the simple compounds that the positive ions would form with the \(\mathrm{Cl}^{-}, \mathrm{S}^{2-},\) and \(\mathrm{N}^{3-}\) ions. a. \(\mathrm{K}^{+}\) b. \(\mathrm{Mg}^{2+}\) c. \(\mathrm{Al}^{3+}\) d. \(\mathrm{Ca}^{2+}\) e. \(\mathrm{Li}^{+}\)

Short answer

The formulas for the simple compounds formed with the given ions are as follows: a. KCl, K₂S, K₃N b. MgCl₂, MgS, Mg₃N₂ c. AlCl₃, Al₂S₃, AlN d. CaCl₂, CaS, Ca₃N₂ e. LiCl, Li₂S, Li₃N

Step-by-step solution

a. K+ with Cl-, S²-, and N³- ions

To form compounds with K+ ion, find the least common multiple (LCM) of their charge values, so that the charges can cancel out and yield a neutral compound. 1. K+ and Cl-: LCM of 1 and 1 is 1, so we have KCl. 2. K+ and S²-: LCM of 1 and 2 is 2, so we have K₂S. 3. K+ and N³-: LCM of 1 and 3 is 3, so we have K₃N.

b. Mg²+ with Cl-, S²-, and N³- ions

Find the LCM of the charges for each combination. 1. Mg²+ and Cl-: LCM of 2 and 1 is 2, so we have MgCl₂. 2. Mg²+ and S²-: LCM of 2 and 2 is 2, so we have MgS. 3. Mg²+ and N³-: LCM of 2 and 3 is 6, so we have Mg₃N₂.

c. Al³+ with Cl-, S²-, and N³- ions

Find the LCM of the charges for each combination. 1. Al³+ and Cl-: LCM of 3 and 1 is 3, so we have AlCl₃. 2. Al³+ and S²-: LCM of 3 and 2 is 6, so we have Al₂S₃. 3. Al³+ and N³-: LCM of 3 and 3 is 3, so we have AlN.

d. Ca²+ with Cl-, S²-, and N³- ions

Find the LCM of the charges for each combination. 1. Ca²+ and Cl-: LCM of 2 and 1 is 2, so we have CaCl₂. 2. Ca²+ and S²-: LCM of 2 and 2 is 2, so we have CaS. 3. Ca²+ and N³-: LCM of 2 and 3 is 6, so we have Ca₃N₂.

e. Li+ with Cl-, S²-, and N³- ions

Find the LCM of the charges for each combination. 1. Li+ and Cl-: LCM of 1 and 1 is 1, so we have LiCl. 2. Li+ and S²-: LCM of 1 and 2 is 2, so we have Li₂S. 3. Li+ and N³-: LCM of 1 and 3 is 3, so we have Li₃N.

Key concepts

Ion Charge Balance

Understanding the ion charge balance is crucial for predicting the formulas of chemical compounds. At its core, the concept revolves around the idea that the total positive charge in a compound must equal the total negative charge. This ensures that the compound is electrically neutral.

For instance, consider a potassium ion (\r\(\r\mathrm{K}^{+}\r\)\r)) which carries a single positive charge. To achieve neutrality, it must be paired with an ion that has an equivalent but opposite charge. In the case of a chloride ion (\r\(\r\mathrm{Cl}^{-}\r\)\r)), which has a single negative charge, one potassium ion can neatly balance one chloride ion, resulting in the formula \r\(\r\mathrm{KCl}\r\)\r. However, when we look at a sulfide ion (\r\(\r\mathrm{S}^{2-}\r\)\r)), with a double negative charge, two potassium ions will be needed to balance the charge, giving us the compound formula \r\(\r\mathrm{K}_2\mathrm{S}\r\)\r.

In textbook exercises, the charge balance concept is applied by finding the least common multiple (LCM) of the ionic charges to determine the ratio of ions needed to form a neutral compound. This method ensures the overall charge adds up to zero, adhering to the principle of charge neutrality in chemical compounds.

Compound Neutrality

Compound neutrality is a fundamental principle leading us to correctly formulate chemical compounds. This principle states that in a stable, electrically neutral compound, the total positive charge from cations (positively charged ions) must be equal to the total negative charge from anions (negatively charged ions).

Considering an example with magnesium ions (\r\(\r\mathrm{Mg}^{2+}\r\)\r)) and nitride ions (\r\(\r\mathrm{N}^{3-}\r\)\r)), the principle guides us toward finding the correct stoichiometric ratio to form a compound with no net charge. To create a neutral compound, we can't simply combine one \r\(\r\mathrm{Mg}^{2+}\r\)\r and one \r\(\r\mathrm{N}^{3-}\r\)\r; instead, we need to find a ratio that balances the charges completely. This leads to the formation of magnesium nitride with the formula \r\(\r\mathrm{Mg}_3\mathrm{N}_2\r\)\r, where three magnesium ions with a total charge of +6 balance two nitride ions with a total charge of -6, ensuring compound neutrality.

The exercise of compound formulation is less about memorization and more about understanding how to balance charges to achieve neutrality. This concept is closely related to the ion charge balance and is central to chemical nomenclature and formula writing exercises.

Ionic Compound Nomenclature

Ionic compound nomenclature follows a system of rules devised to name compounds systematically and predictably. This includes the use of prefixes and suffixes to indicate the type and number of ions in the compound.

The rules are straightforward but require attention to detail. The name of an ionic compound consists of the cation's name followed by the anion's name. For simple metal cations, the element name is used, such as potassium for \r\(\r\mathrm{K}^{+}\r\)\r. For anions, the root of the element's name is taken with an '-ide' suffix, such as chloride for \r\(\r\mathrm{Cl}^{-}\r\)\r. When combining \r\(\r\mathrm{K}^{+}\r\)\r and \r\(\r\mathrm{Cl}^{-}\r\)\r, the result is potassium chloride, corresponding to the compound's formula \r\(\r\mathrm{KCl}\r\)\r.

When the metal has multiple ionic forms, Roman numerals are used to denote the charge. Also, for polyatomic ions, specific names are given, for instance, sulfate for \r\(\r\mathrm{SO}_4^{2-}\r\)\r and phosphate for \r\(\r\mathrm{PO}_4^{3-}\r\)\r. Understanding these naming conventions is essential for students as it helps them both decipher and construct the chemical formulas from the names of ionic compounds. Learning ionic compound nomenclature can be challenging, but with practice, students can quickly become adept at naming compounds and understanding their composition.