Problem 18 - Chemistry The Central Science In Si Units

Question

Which ionic compound is expected to form from combining the following pairs of elements? (a) calcium and nitrogen, (b) cesium and bromine, (c) strontium and sulfur, (d) aluminum and selenium.

Step-by-Step Solution

Answer

(a) Ca_3N_2, (b) CsBr, (c) SrS, (d) Al_2Se_3

Step 1: Determine the Ion Charges

First, determine the common ionic charges for the elements involved. For calcium (Ca), the common ion is Ca^{2+}. For nitrogen (N), it often forms N^{3-} ions. Cesium (Cs) typically forms Cs^{+} ions, while bromine (Br) forms Br^{-} ions. Strontium (Sr) forms Sr^{2+}, and sulfur (S) forms S^{2-}. Aluminum (Al) forms Al^{3+}, and selenium (Se) often forms Se^{2-}.

Step 2: Balance the Charges to form Neutral Compounds

For each pair, combine the ions in such a way that the total charge equals zero, forming a neutral compound: (a) Calcium (Ca^{2+}) and Nitrogen (N^{3-}): To balance, use two Ca^{2+} ions with three N^{3-} ions to form Ca_3N_2. (b) Cesium (Cs^{+}) and Bromine (Br^{-}): Use one Cs^{+} and one Br^{-} to form CsBr. (c) Strontium (Sr^{2+}) and Sulfur (S^{2-}): Use one Sr^{2+} and one S^{2-} to form SrS. (d) Aluminum (Al^{3+}) and Selenium (Se^{2-}): Use two Al^{3+} ions with three Se^{2-} ions to form Al_2Se_3.

Step 3: Write the Chemical Formulas

List the chemical formulas for the compounds formed from each element pair: (a) Calcium and Nitrogen form Ca_3N_2. (b) Cesium and Bromine form CsBr. (c) Strontium and Sulfur form SrS. (d) Aluminum and Selenium form Al_2Se_3.

Key Concepts

Ion Charges

Understanding ion charges is crucial when forming ionic compounds. When elements lose or gain electrons, they form ions with positive or negative charges, respectively. These charges are predictable based on the element’s position in the periodic table. For instance:

• Calcium (Ca) is an alkaline earth metal commonly found in Group 2A, usually forming a ion of \( \text{Ca}^{2+} \).
• Nitrogen (N)
• Similarly, Cesium (Cs) in Group 1A forms \( \text{Cs}^{+} \) and Bromine (Br)

These charges determine how elements combine to form stable ionic compounds. Positive ions (cations) attract negative ions (anions), leading to neutral compounds. This attraction is the essence of ionic bonding, where the compound's overall charge remains zero by balancing charges.

Chemical Formulas

Once ion charges are known, we can determine the chemical formulas of ionic compounds. Formulas are written to reflect the smallest neutral constant ratio of ions. For instance:

• For Calcium and Nitrogen: Calcium forms \( \text{Ca}^{2+} \) and Nitrogen \( \text{N}^{3-} \). To create a neutral compound, three calcium ions bond with two nitrogen ions to yield \( \text{Ca}_3\text{N}_2 \).
• For Cesium and Bromine: Cesium forms \( \text{Cs}^{+} \) and Bromine \( \text{Br}^{-} \). The ions combine in a 1:1 ratio, forming \( \text{CsBr} \).

The chemical formula reflects the exact ratio needed to balance charges, ensuring the compound is neutral. This vital process underscores the elegance of chemical interactions, guided by simple numeric rules of ion balance.

Balancing Charges

Balancing charges is a key step in forming ionic compounds. To achieve charge neutrality, the positives and negatives must cancel each other. Let's explore this with examples:

• Calcium and Nitrogen: With \( \text{Ca}^{2+} \) and \( \text{N}^{3-} \), one needs two \( \text{Ca}^{2+} \) ions (total +4) and three \( \text{N}^{3-} \) ions (total -6) to achieve neutrality. This results in the compound \( \text{Ca}_3\text{N}_2 \).
• Strontium and Sulfur: Both \( \text{Sr}^{2+} \) and \( \text{S}^{2-} \) bind in a simple 1:1 ratio, forming \( \text{SrS} \), perfectly balanced at zero charge.

These balanced equations demonstrate how elements combine based on their natural tendencies, driven by the quest for chemical stability. Through balancing, compounds achieve a stable electronic configuration, mirroring the inert nature of noble gases.