Problem 210 - Chemistry Matter and Change

Question

Write the formula and name the compound formed when each pair of elements combine. (Chapter 7) \begin{equation} \begin{array}{l}{\text { a. barium and chlorine }} \\ {\text { b. aluminum and selenium }} \\ {\text { c. calcium and phosphorus }}\end{array} \end{equation}

Step-by-Step Solution

Answer

Barium chloride (\( \text{BaCl}_2 \)), aluminum selenide (\( \text{Al}_2\text{Se}_3 \)), and calcium phosphide (\( \text{Ca}_3\text{P}_2 \)).

Step 1: Determine the Ions

Firstly, determine the ions each element forms in a compound. Barium forms a cation, \( \text{Ba}^{2+} \), and chlorine forms an anion, \( \text{Cl}^- \). Aluminum forms \( \text{Al}^{3+} \), and selenium forms \( \text{Se}^{2-} \). Calcium forms \( \text{Ca}^{2+} \), and phosphorus forms \( \text{P}^{3-} \).

Step 2: Balance the Charges in the Compounds

To form a neutral compound, the total positive charge must balance the total negative charge. For barium chloride, combine ions as \( \text{BaCl}_2 \) to balance the charges. For aluminum selenide, use \( \text{Al}_2\text{Se}_3 \). For calcium phosphide, use \( \text{Ca}_3\text{P}_2 \).

Step 3: Name the Compounds

Now, assign names to each compound. \( \text{BaCl}_2 \) is named barium chloride, \( \text{Al}_2\text{Se}_3 \) is aluminum selenide, and \( \text{Ca}_3\text{P}_2 \) is calcium phosphide.

Key Concepts

Ionic Compounds

Ionic compounds are a type of chemical compound formed when atoms transfer electrons from a metal to a non-metal. This electron transfer results in the formation of positively charged ions called cations and negatively charged ions known as anions. The electrostatic attraction between these oppositely charged ions holds the compound together. This process is fundamental in forming stable compounds from elements.
To provide an example, consider barium and chlorine from the exercise. When they react, barium, which is a metal, loses two electrons and forms a cation, represented as \( \text{Ba}^{2+} \). Chlorine, a non-metal, gains one electron for each atom to form an anion, represented as \( \text{Cl}^- \).
The combination of barium ions and chloride ions results in the formation of an ionic compound called barium chloride, abbreviated as \( \text{BaCl}_2 \). This notation indicates two chloride ions are needed to balance the charge of one barium cation.
Common properties of ionic compounds include high melting points, electrical conductivity when melted or dissolved in water, and the formation of crystalline solids.

Chemical Formulas

Chemical formulas are a way of expressing the composition of chemical compounds using symbols for the elements and numbers indicating the ratios in which the atoms combine. These formulas show the exact numbers of each type of atom present in a molecule and provide important information regarding the structure of the compound.
For example, in the compound aluminum selenide, the chemical formula is \( \text{Al}_2\text{Se}_3 \). This indicates that for every two aluminum (\( \text{Al}^{3+} \)) cations, three selenium (\( \text{Se}^{2-} \)) anions are present to maintain the compound's neutrality. This fixed ratio is determined by the charges on the ions and is reflected in the subscripts used in the chemical formula.
The chemical formula of an ionic compound not only informs about the number of each type of atom, but it also represents the smallest whole-number ratio of ions in the compound, which is known as the empirical formula. Understanding chemical formulas is essential for interpreting the composition and thus the potential reactivity and properties of compounds.

Balancing Charges

In the formation of any compound, balancing charges is crucial. This concept ensures that the resulting compound is electrically neutral, meaning the total positive charge must equal the total negative charge.
Let's consider calcium and phosphorus from the exercise. Calcium forms a \( \text{Ca}^{2+} \) ion, while phosphorus forms a \( \text{P}^{3-} \) ion. To achieve neutrality in the resulting compound, named calcium phosphide, the combined positive and negative charges must balance.

• Three calcium ions (\( 3 \times \text{Ca}^{2+} \)) contribute a total charge of \(+6\).
• This requires two phosphorus ions (\( 2 \times \text{P}^{3-} \)) to contribute a total charge of \(-6\).

This balance results in the chemical formula \( \text{Ca}_3\text{P}_2 \). By aligning the total positive and negative charges, the compound remains neutral.
Balancing charges is vital in writing accurate chemical formulas and understanding the interactions between different ions, thus allowing for the correct naming and prediction of compound properties.