What is the formula unit for a compound made from $Fe ^{2+}$ and oxygen?
To determine the formula unit for a compound formed by iron(II) and oxygen, we analyze the charges of the constituent ions. Iron(II) is provided as the cation \(Fe^{2+}\). Oxygen, when forming an ionic bond in an oxide compound, gains two electrons to achieve a stable noble gas configuration, resulting in the oxide anion \(O^{2-}\). An ionic compound must be electrically neutral, meaning the sum of the positive and negative charges in the formula unit must equal zero. If we let \(x\) be the number of \(Fe^{2+}\) ions and \(y\) be the number of \(O^{2-}\) ions, the total charge equation is \(x(+2) + y(-2) = 0\), which simplifies to \(2x - 2y = 0\) or \(x = y\). Therefore, the smallest positive integer ratio of iron ions to oxygen ions is 1:1, leading to the formula unit \(FeO\).
Comparing the derived formula unit \(FeO\) with the provided options:
Thus, the correct formula unit for the compound made from \(Fe^{2+}\) and oxygen is \(FeO\). The final answer is \(\boxed{FeO}\).
In laboratory settings, chemists determine the formulas of ionic compounds by understanding the valency of the ions involved. This principle is vital when synthesizing materials like catalysts or pigments. For instance, knowing that iron(II) forms an oxide as \(FeO\) allows for precise measurement of reactants when creating specific iron-based compounds, ensuring that the resulting product is pure and chemically balanced. Understanding stoichiometry ensures that resources are used efficiently without leaving excess unreacted material.