Question

Complete the table by filling in the formula for the ionic compound formed by each pair of cations and anions, as shown for the first pair. $$ \begin{array}{c|c|c|c|c} \hline \text { Ion } & \mathrm{Na}^{+} & \mathrm{Ca}^{2+} & \mathrm{Fe}^{2+} & \mathrm{Al}^{3+} \\ \hline \mathrm{O}^{2-} & \mathrm{Na}_{2} \mathrm{O} & & & \\ \mathrm{NO}_{3}^{-} & & & & \\ \mathrm{SO}_{4}^{2-} & & & & \\ \mathrm{AsO}_{4}{ }^{3-} & & & & \\ \hline \end{array} $$

Short answer

Fill the table with: NaNO3, Na2SO4, Na3AsO4, CaO, Ca(NO3)2, CaSO4, Ca3(AsO4)2, FeO, Fe(NO3)2, FeSO4, Fe3(AsO4)2, Al2O3, Al(NO3)3, Al2(SO4)3, AlAsO4.

Step-by-step solution

Understanding Ionic Charges

Ionic compounds are formed when cations and anions combine in such a way that the overall charge of the compound is neutral. This means that the total positive charge from the cations must equal the total negative charge from the anions.

Pair Na+ with Different Anions

For each anion, determine the formula for the compound formed with Na+. Use the charges to find the correct ratio. For example, Na+ pairs with O2- in the ratio of 2:1 to form Na2O. Apply this concept to other anions: Na+ and NO3- forms NaNO3, Na+ and SO4^2- forms Na2SO4, Na+ and AsO4^3- forms Na3AsO4.

Pair Ca2+ with Different Anions

For each anion, determine the formula for the compound formed with Ca2+. Ca2+ pairs with O2- in the ratio of 1:1 to form CaO. Pair these ions with NO3-: Ca2+ with NO3- forms Ca(NO3)2 due to the need for two NO3- ions. With SO4^2-, the ratio is 1:1, forming CaSO4. With AsO4^3-, the ratio is 3:2, forming Ca3(AsO4)2.

Pair Fe2+ with Different Anions

For each anion, determine the formula for the compound formed with Fe2+. Fe2+ pairs with O2- in the ratio of 1:1 to form FeO. With NO3-, the ratio is 2:1, forming Fe(NO3)2. With SO4^2-, the ratio is 1:1, forming FeSO4. With AsO4^3-, a 3:2 ratio forms Fe3(AsO4)2.

Pair Al3+ with Different Anions

For each anion, determine the formula for the compound formed with Al3+. Al3+ pairs with O2- in the ratio of 2:3 to form Al2O3. With NO3-, the ratio is 1:3, forming Al(NO3)3. With SO4^2-, the ratio is 2:3, forming Al2(SO4)3. With AsO4^3-, the ratio is 1:1, forming AlAsO4.

Key concepts

Cation-Anion Pairing

In the world of ionic compounds, cations and anions team up in special ways to create a neutral charge. Cations are positively charged ions, and anions are negatively charged. When they pair together, the goal is to balance these charges.
Imagine the pairing as a puzzle, where pieces need to fit perfectly. Each cation wants to find an anion partner in the right amount to make sure the whole puzzle stays balanced. The attraction between these oppositely charged ions is what holds the compound together.

• **Example:** Sodium cation, Na\(^+\), pairs with oxygen anion, O\(^{2-}\). Two sodium ions are needed for each oxygen ion to maintain balance, forming Na\(_2\)O. This 2:1 ratio ensures no leftover charges.

This careful pairing is key to forming the correct ionic compound, ensuring its neutrality and stability.

Charge Neutrality

Charge neutrality is a fundamental rule for forming ionic compounds. An ionic compound must have a net charge of zero. This means the total positive charge (from cations) equals the total negative charge (from anions).
Charge balancing is like budgeting: making sure what goes out is equal to what comes in. This ensures the compound is stable and inert, not seeking other ions to bond with.

• **Example:** Calcium ions, Ca\(^{2+}\), when pairing with sulfate ions, SO\(_4\)^{2-}\, do so in a 1:1 ratio. Both ions contribute an equal but opposite charge, resulting in a neutral compound, CaSO\(_4\).

Maintaining charge neutrality is crucial for the compound to exist overall in nature, keeping it from attracting additional free ions and maintaining its structure.

Chemical Formula Determination

The chemical formula of an ionic compound reveals the ratio of cations to anions needed to maintain charge neutrality. Determining this formula involves understanding the charge balance between the ions.
Writing the correct chemical formula is like decoding a recipe: the ingredients must be mixed in the right proportions. By knowing the charges of the cations and anions, you can determine the smallest whole number ratio required to neutralize the charges.

• **Approach:** Start by identifying the charges of the ions. For instance, an aluminum ion, Al\(^{3+}\), pairing with a phosphate ion, PO\(_4\)^{3-}\, forms a 1:1 ratio resulting in AlPO\(_4\).
• **Strategy:** For ions like nitrate, NO\(_3\)^{-}\, pairing with calcium, Ca\(^{2+}\), you need two nitrate ions for every calcium ion because Ca needs two negatives to counteract its double positive charge, forming Ca(NO\(_3\)_2).

Understanding these formulas helps in predicting the properties of the compound, including solubility and reactivity.