Problem 26 - Chemical Principles: The Quest for Insight

Question

On the basis of the expected charges on the monatomic ions, give the chemical formula of each of the following compounds: (a) sodium sulfide;(b) lithium nitride; (c) calcium chloride; (d) gallium arsenide; (c) cobalt(III) oxide.

Step-by-Step Solution

Answer

\( \text{(a) Na}_2\text{S, (b) Li}_3\text{N, (c) CaCl}_2\text{, (d) GaAs, (e) Co}_2\text{O}_3 \)

Step 1: Sodium Sulfide

Identify the ions: Sodium (Na) forms a +1 charge, and Sulfur (S) forms a -2 charge. To neutralize the charges, we need two sodium ions for every sulfide ion. Formula: \( \text{Na}_2S \).

Step 2: Lithium Nitride

Identify the ions: Lithium (Li) forms a +1 charge, and Nitrogen (N) forms a -3 charge. To balance the charges, we need three lithium ions for every nitride ion. Formula: \( \text{Li}_3N \).

Step 3: Calcium Chloride

Identify the ions: Calcium (Ca) forms a +2 charge, and Chlorine (Cl) forms a -1 charge. To balance the charges, we need one calcium ion for every two chloride ions. Formula: \( \text{CaCl}_2 \).

Step 4: Gallium Arsenide

Identify the ions: Gallium (Ga) typically forms a +3 charge, and Arsenic (As) forms a -3 charge. To balance the charges, we need one gallium ion for every arsenide ion. Formula: \( \text{GaAs} \).

Step 5: Cobalt(III) Oxide

Identify the ions: Cobalt (III) indicates a +3 charge, and Oxygen (O) forms a -2 charge. To balance the charges, we need two cobalt ions for every three oxide ions. Formula: \( \text{Co}_2\text{O}_3 \).

Key Concepts

Ion Charge Balance

Understanding ion charge balance is crucial when it comes to writing chemical formulas for ionic compounds. An ionic compound consists of positive and negative ions that are held together by the strong electrostatic forces of attraction known as ionic bonds.

For the compound to be stable, the total positive charge must equal the total negative charge. In other words, the number of positive charges contributed by the metal or positive ion should balance out the negative charges contributed by the nonmetal or negative ion. This is what we refer to as 'charge neutrality'.

To achieve charge neutrality, we often use subscripts in the chemical formula to indicate the number of ions needed to balance the charges. For instance, sodium (Na) has a +1 charge and sulfur (S) has a -2 charge. By bringing in two sodium ions, we counteract the two negative charges from one sulfide ion resulting in the formula \( \text{Na}_2S \). This indicates that the charges are balanced and the ionic compound is neutral.

Ionic Compound Naming

The naming of ionic compounds follows specific rules that make it possible for scientists and students alike to understand the composition of the compound from its name.

The name of an ionic compound is composed of the cation name, which is typically a metal, followed by the anion name, which is often a nonmetal. The cation retains its element name whereas the anion is named by taking the root of the element and adding the suffix '-ide'.

For example, \( \text{Na}_2S \) is named sodium sulfide, not sodium sulfur, because sulfur as an anion becomes sulfide. Similarly, \( \text{CaCl}_2 \) is named calcium chloride instead of 'calcium chlorine'. Special attention is needed when the cation can have different charges; this is indicated by Roman numerals in parentheses. For instance, \( \text{Co}_2\text{O}_3 \), is named cobalt(III) oxide to denote the +3 charge on cobalt.

Chemical Nomenclature

Chemical nomenclature is a systematic way of naming chemical compounds to give unambiguous information about the chemical species involved. It involves rules and conventions set by the International Union of Pure and Applied Chemistry (IUPAC).

Chemical formulas reflect the types and numbers of atoms within a compound. For ionic compounds like calcium chloride, we would write \( \text{CaCl}_2 \) where 'Ca' represents the calcium cation and 'Cl' represents the chloride anion. The subscript '2' indicates that two chloride ions pair with each calcium ion to balance the opposite charges.

When it comes to molecular compounds, which are made of nonmetals, we use prefixes (like mono-, di-, tri-) to show the number of atoms of each element in the compound. Chemical nomenclature may seem daunting, but it becomes manageable once you understand the basic principles and apply them systematically.