Writing Formula Units of Ionic Compounds: Videos & Practice Problems

The concept of a formula unit is essential in understanding ionic compounds, as it represents the empirical formula, or the simplest ratio of ions that combine to achieve a neutral charge. However, it's important to note that ionic solids do not exist merely as isolated ionic pairs; rather, they form a structured arrangement known as a crystal lattice. This three-dimensional structure consists of numerous cations and anions that are organized in a stable pattern.

For instance, in sodium chloride (NaCl), the sodium ions (Na⁺) and chloride ions (Cl^-) are attracted to each other due to their opposing charges. This attraction leads to the formation of a crystal lattice, which is a collective arrangement of many positive and negative ions rather than just a single pair. The formula unit, represented as NaCl, serves as a simplified depiction of this ionic compound, illustrating the ratio of sodium to chloride ions.

In summary, while the formula unit provides a basic understanding of the ionic compound's composition, the true nature of an ionic solid is best represented by the extensive network of cations and anions that create the crystal lattice structure. This distinction is crucial for grasping the properties and behaviors of ionic compounds in various contexts.

Understanding how to write ionic compounds involves a systematic approach to identifying the ions and their respective charges. The first step is to determine the ions from the name of the compound. For example, in the case of aluminum nitride and barium phosphate, we start by identifying the ions involved.

Aluminum, which is located in group 3A of the periodic table, has a charge of +3, represented as \( \text{Al}^{3+} \). Nitride, derived from nitrogen in group 5A, has a charge of -3, denoted as \( \text{N}^{3-} \). When the charges of the ions are equal, as in this case (both 3), they cancel each other out, resulting in the formula for aluminum nitride being \( \text{AlN} \).

Next, we analyze barium phosphate. Barium, found in group 2A, carries a charge of +2, represented as \( \text{Ba}^{2+} \). Phosphate is a polyatomic ion with the formula \( \text{PO}_4^{3-} \). When the charges differ, as they do here (2 and 3), we employ the crisscross method. This means the +2 charge from barium becomes the subscript for phosphate, and the -3 charge from phosphate becomes the subscript for barium. Thus, we write the formula as \( \text{Ba}_3(\text{PO}_4)_2 \), where the parentheses indicate that the subscript applies to the entire phosphate ion.

In summary, when writing ionic compounds, first identify the ions and their charges. If the charges are the same, they cancel out; if they differ, use the crisscross method to determine the subscripts. This systematic approach ensures accurate representation of ionic compounds.

To determine the formula unit for the compound formed by magnesium ions and sulfate ions, we start with the respective ions: magnesium ion (Mg²⁺) and sulfate ion (SO₄^2-). The charges of these ions are both 2, which means they can combine in a 1:1 ratio. When the charges are equal, they effectively cancel each other out.

Thus, when we combine the magnesium ion with the sulfate ion, we simply write the formula by combining the symbols of the ions without the charges. This results in the formula unit MgSO₄, representing magnesium sulfate, an ionic compound.