Question

Write formula units by combining the cations and anions in each of the following pairs: (a) \(\mathrm{Sc}^{3+}\) and \(\mathrm{N}^{3-}\) (b) \(\mathrm{Ti}^{4+}\) and \(\mathrm{O}^{2-}\) (c) \(\mathrm{NH}_{4}{ }^{+}\) and \(\mathrm{CO}_{3}{ }^{2-}\) (d) \(\mathrm{Hg}_{2}{ }^{2+}\) and \(\mathrm{PO}_{4}{ }^{3-}\)

Short answer

(a) \(\mathrm{ScN}\), (b) \(\mathrm{TiO}_{2}\), (c) \((\mathrm{NH}_{4})_{2}\mathrm{CO}_{3}\), (d) \((\mathrm{Hg}_{2})_{3}\mathrm{(PO}_{4})_{2}\)."}

Step-by-step solution

Understanding the Charges

The first step in combining the ions is to determine the charge of each ion in the pair. Each ion is given with its respective charge: (a) \( \mathrm{Sc}^{3+} \) and \( \mathrm{N}^{3-} \), (b) \( \mathrm{Ti}^{4+} \) and \( \mathrm{O}^{2-} \), (c) \( \mathrm{NH}_{4}{ }^{+} \) and \( \mathrm{CO}_{3}{ }^{2-} \), (d) \( \mathrm{Hg}_{2}{ }^{2+} \) and \( \mathrm{PO}_{4}{ }^{3-} \).

Balancing the Ion Charges

The next step is to ensure that the total positive charges from the cations equal the total negative charges from the anions to form neutral compounds. This can be achieved by adjusting the number of each ion used.(a) Both ions have charges of 3 (\(+3\) and \(-3\)), so one \( \mathrm{Sc}^{3+} \) ion combines with one \( \mathrm{N}^{3-} \) ion.(b) To balance \( \mathrm{Ti}^{4+} \) and \( \mathrm{O}^{2-} \), two \( \mathrm{O}^{2-} \) ions are needed for each \( \mathrm{Ti}^{4+} \) ion (total charge \(+4 = 2 \times -2\)).(c) Two \( \mathrm{NH}_{4}^{+} \) ions are needed to balance one \( \mathrm{CO}_{3}^{2-} \) ion (total charge \(+2 = -2\)).(d) Three \( \mathrm{Hg}_{2}^{2+} \) ions are needed to balance two \( \mathrm{PO}_{4}^{3-} \) ions (total charge \(+6 = -6\)).

Writing the Formula Units

Combine the ions in the appropriate ratios determined in Step 2, ensuring charge neutrality:(a) Combine as \( \mathrm{ScN} \).(b) Combine as \( \mathrm{TiO}_{2} \).(c) Combine as \( \mathrm{(NH}_{4})_{2}\mathrm{CO}_{3} \).(d) Combine as \( \mathrm{(Hg}_{2})_{3}\mathrm{(PO}_{4})_{2} \).

Key concepts

Ionic Compounds

Ionic compounds are fascinating structures formed when metals and nonmetals exchange electrons to create a chemical bond. This exchange results in the formation of ions: cations and anions. Cations are positively charged because they lose electrons, typically seen in metals like sodium (Na) or calcium (Ca). On the other hand, anions are negatively charged as they gain electrons, common in nonmetals like chlorine (Cl) or oxygen (O).

When these ions come together, they form a crystal lattice structure. This structure is highly ordered, giving ionic compounds their unique properties, such as high melting and boiling points. One classic example is table salt, or sodium chloride (NaCl), which consists of sodium ions (Na⁺) and chloride ions (Cl^-) arranged in a repeating pattern.

To summarize, ionic compounds are held together by the electrostatic attraction between oppositely charged ions. This bond creates substances that are typically solid at room temperature, and dissolve well in water to conduct electricity, due to the mobility of the ions.

Cation and Anion Charges

In ionic compounds, understanding the charges of cations and anions is crucial for predicting how elements combine to form stable compounds. Each ion in an ionic compound carries a specific charge, which is determined by the number of electrons lost or gained to achieve a stable electron configuration.

• Cations: These are positively charged ions formed when an atom loses one or more electrons. Metals, such as magnesium (Mg²⁺), often form cations.
• Anions: These are negatively charged ions formed when an atom gains one or more electrons. Nonmetals, like sulfur (S^2-), typically form anions.

The key to forming ionic compounds is ensuring the total positive charge of the cations equals the total negative charge of the anions. This balance creates a neutral compound overall. For instance, in calcium oxide (CaO), calcium (Ca²⁺) and oxide (O^2-) combine in a 1:1 ratio to balance their charges. Remember, the sum of the charges in the compound must always be zero for the compound to be stable and neutral.

Balancing Chemical Equations

Balancing chemical equations is a fundamental concept in chemistry, vital for understanding chemical reactions and the distribution of reactants and products. In an equation, reactants on the left side transform into products on the right. To balance an equation, each element must have the same number of atoms on both sides.

Let's consider the reaction forming water:\[ 2H_2 + O_2 \rightarrow 2H_2O \]

Here, 2 molecules of hydrogen (H₂) react with 1 molecule of oxygen (O₂) to produce 2 molecules of water (H₂O). Each side has 4 hydrogen atoms and 2 oxygen atoms, illustrating a balanced equation.

Balancing equations involves adjusting coefficients, which are numbers placed before formulas to indicate the number of units involved. Remember, balancing cannot alter the chemical formulas of the reactants or products, only their coefficients. By mastering this skill, one can accurately predict the quantities of substances consumed and produced in a chemical reaction, ensuring a clear understanding of the reaction's stoichiometry.