Question

Predict the chemical formula for the ionic compound formed by (a) \(\mathrm{Ca}^{2+}\) and \(\mathrm{Br}^{-}\), (b) \(\mathrm{K}^{+}\) and \(\mathrm{CO}_{3}^{2-}\), (c) \(\mathrm{Al}^{3+}\) and \(\mathrm{CH}_{3} \mathrm{COO}^{-}\), (d) \(\mathrm{NH}_{4}{ }^{+}\) and \(\mathrm{SO}_{4}{ }^{2-}\), (e) \(\mathrm{Mg}^{2+}\) and \(\mathrm{PO}_{4}^{3-}\)

Short answer

The chemical formulas for the ionic compounds formed are: (a) $\mathrm{CaBr}_2$, (b) $\mathrm{K}_2\mathrm{CO}_3$, (c) $\mathrm{Al}\left(\mathrm{CH}_3\mathrm{COO}\right)_3$, (d) $\left(\mathrm{NH}_4\right)_2\mathrm{SO}_4$, and (e) $\mathrm{Mg}_3\left(\mathrm{PO}_4\right)_2$.

Step-by-step solution

(a) Ca²⁺ and Br⁻

: First, let's analyze the charges of the ions given: Ca²⁺ has a charge of +2 and Br⁻ has a charge of -1. To create a neutral compound, we need to balance these charges. In this case, it's easy to see that by having two Br⁻ ions, we get a total charge of -2, which balances the Ca²⁺ ion. Therefore the resulting formula for this ionic compound is: \[ \mathrm{CaBr}_2 \]

(b) K⁺ and CO₃²⁻

: The ions given are K⁺ with a charge of +1 and CO₃²⁻ with a charge of -2. To balance the charges, we'll need two K⁺ ions each with a charge of +1, in order to equal the -2 charge from CO₃²⁻. Thus, the chemical formula for this ionic compound is: \[ \mathrm{K}_2\mathrm{CO}_3 \]

(c) Al³⁺ and CH₃COO⁻

: In this case, the ions are Al³⁺ with a charge of +3 and CH₃COO⁻ with a charge of -1. To balance the charges, we'll need three CH₃COO⁻ ions, one for each of the Al³⁺ ion's charges. Therefore, the chemical formula for this ionic compound is: \[ \mathrm{Al}\left(\mathrm{CH}_3\mathrm{COO}\right)_3 \]

(d) NH₄⁺ and SO₄²⁻

: The ions in this instance are NH₄⁺ with a charge of +1 and SO₄²⁻ with a charge of -2. To balance these charges, two NH₄⁺ ions will be necessary to match the -2 charge from the SO₄²⁻. The chemical formula for this ionic compound is: \[ \left(\mathrm{NH}_4\right)_2\mathrm{SO}_4 \]

(e) Mg²⁺ and PO₄³⁻

: Finally, the ions given are Mg²⁺ with a charge of +2 and PO₄³⁻ with a charge of -3. In order to balance these charges, we'll need 2 PO₄³⁻ ions and 3 Mg²⁺ ions, which means a total charge of -6 and +6, respectively. Thus, the chemical formula for this ionic compound is: \[ \mathrm{Mg}_3\left(\mathrm{PO}_4\right)_2 \]

Key concepts

Balancing Ionic Charges

Understanding how to balance ionic charges is vital when predicting the chemical formula of ionic compounds. Each ion brings a specific charge, with cations having a positive charge and anions carrying a negative charge. To obtain a neutral compound, the total positive charge must equal the total negative charge.

Consider the example of calcium (Ca²⁺) and bromine (Br⁻). Calcium has a +2 charge, and bromine has a -1 charge. To balance these, we need two bromine ions, resulting in a neutral compound and the chemical formula \( \mathrm{CaBr}_2 \). This step is crucial as it ensures the stability of the resulting compound by achieving electrical neutrality.

Writing Chemical Formulas

Writing chemical formulas may seem daunting, but with practice, it becomes simpler. Once the charges are balanced, the formula is written by placing the cation first followed by the anion. Subscripts are used to indicate the number of ions needed to balance the charges.

For potassium (K⁺) and carbonate (CO₃²⁻), we bring in two potassium ions to offset the -2 charge of carbonate, leading to the formula \( \mathrm{K}_2\mathrm{CO}_3 \). An important point to remember is that when dealing with polyatomic ions like carbonate, parentheses are used only when multiple copies of the polyatomic ion are needed, as seen in \( \mathrm{Mg}_3(\mathrm{PO}_4)_2 \) where two phosphate ions are required.

Ionic Compound Nomenclature

Naming ionic compounds, or ionic compound nomenclature, involves a straightforward set of rules. The name of the cation is given first followed by the name of the anion. For monatomic anions, the end of the element's name is replaced with '-ide', as seen in calcium bromide for \( \mathrm{CaBr}_2 \).

Compounds with polyatomic ions follow the same rule, with the ion's common name being used, such as aluminum acetate for \( \mathrm{Al}(\mathrm{CH}_3\mathrm{COO})_3 \). It's essential to become familiar with common polyatomic ions to accurately name compounds containing them, and to remember that cations with more than one possible charge often have a Roman numeral indicating the charge in the name.

Polyatomic Ions

Polyatomic ions are ions composed of more than one atom, but they act as a single charged particle. These are widespread in ionic compounds and must be understood for accurate formula writing and naming. For example, the ammonium ion (NH₄⁺) and the sulfate ion (SO₄²⁻) combine to form ammonium sulfate \( (\mathrm{NH}_4)_2\mathrm{SO}_4 \).

The polyatomic ions often have unique names and formulas that must be memorized for correct usage. Importantly, these ions can have multiple charges, and it is the combination of these charges that determines how they combine with other ions to form ionic compounds, as seen with the phosphate ion (PO₄³⁻) in magnesium phosphate \( \mathrm{Mg}_3(\mathrm{PO}_4)_2 \). Understanding polyatomic ions is critical for predicting the chemical formula for more complex ionic compounds.