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 \(\mathrm{SO}_{4}^{2-}\), (c) \(\mathrm{Mg}^{2+}\) and \(\mathrm{PO}_{4}{ }^{3-}\).

Short answer

The chemical formulas for the ionic compounds formed by the given ions are: (a) CaBr₂, (b) K₂CO₃, (c) Al(CH₃COO)₃, (d) (NH₄)₂SO₄, and (e) Mg₃(PO₄)₂.

Step-by-step solution

(a) Ca^2+ and Br^(-1)

: In this case, we have a \( \mathrm{Ca}^{2+}\) ion with a +2 charge and a \( \mathrm{Br}^{-}\) ion with a -1 charge. In order to balance the charges, we need two \( \mathrm{Br}^{-}\) ions for each \( \mathrm{Ca}^{2+}\) ion. Thus, the resulting chemical formula for the ionic compound formed by these ions will be \( \mathrm{CaBr}_{2}\).

(b) K^+ and CO_3^(2-)

: For this compound, we have a \( \mathrm{K}^{+}\) ion with a +1 charge and a \( \mathrm{CO_{3}}^{2-}\) ion with a -2 charge. To balance the charges, we will need two \( \mathrm{K}^{+}\) ions for each \( \mathrm{CO_{3}}^{2-}\) ion. The chemical formula for this compound will be \( \mathrm{K}_{2}\mathrm{CO}_{3}\).

(c) Al^3+ and CH_3COO^-

: This time, we have an \( \mathrm{Al}^{3+}\) ion with a +3 charge and a \( \mathrm{CH}_{3} \mathrm{COO}^{-}\) ion with a -1 charge. To balance the charges, we will need three \( \mathrm{CH}_{3} \mathrm{COO}^{-}\) ions for each \( \mathrm{Al}^{3+}\) ion. The chemical formula for this compound will be \( \mathrm{Al(CH}_{3}\mathrm{COO})_{3}\).

(d) NH_4^+ and SO_4^(2-)

: For this compound, we have an \( \mathrm{NH}_{4}^{+}\) ion with a +1 charge and a \( \mathrm{SO}_{4}^{2-}\) ion with a -2 charge. We will need two \( \mathrm{NH}_{4}^{+}\) ions for each \( \mathrm{SO}_{4}^{2-}\) ion in order to balance the charges. The chemical formula for this compound will be \( \mathrm{(NH}_{4}\mathrm{)_{2}SO}_{4}\).

(e) Mg^2+ and PO_4^(3-)

: Lastly, we have a \( \mathrm{Mg}^{2+}\) ion with a +2 charge and a \( \mathrm{PO}_{4}^{3-}\) ion with a -3 charge. In this case, we will need three \( \mathrm{Mg}^{2+}\) ions and two \( \mathrm{PO}_{4}^{3-}\) ions to balance the charges. The chemical formula for this compound will be \( \mathrm{Mg}_{3}(\mathrm{PO}_{4})_{2}\).

Key concepts

Chemical Formula

Understanding the chemical formula of a substance is fundamental in the field of chemistry. It tells us the exact number and type of atoms that make up a molecule. In ionic compounds, the formula is determined by the ratio of cations (positively charged ions) to anions (negatively charged ions) required to achieve charge neutrality.

For example, calcium bromide, with its chemical formula \(\mathrm{CaBr}_2\), has one calcium ion (\(\mathrm{Ca}^{2+}\)) for every two bromide ions (\(\mathrm{Br}^{-}\)), ensuring that the total positive charge equals the total negative charge. Mastering the art of predicting these formulas is not just a matter of memorization but understanding the charge relationships between ions.

Ionic Bonding

Ionic bonding is the electrostatic force of attraction between oppositely charged ions. These ions are formed when atoms either lose or gain electrons to fulfill their need for a more stable electron configuration, typically resembling the nearest noble gas. Cations lose electrons and acquire a positive charge, while anions gain electrons, acquiring a negative charge.

For instance, in the compound \(\mathrm{K}_2\mathrm{CO}_3\), potassium (\(\mathrm{K}^+\)) donates an electron becoming a cation, while the carbonate ion (\(\mathrm{CO}_3^{2-}\)) accepts the electrons, thus becoming an anion. These oppositely charged ions are held together strongly in a lattice structure due to their electrostatic attraction.

Charge Balance in Ionic Compounds

Charge balance in ionic compounds is a principle that states the total positive charge must equal the total negative charge to create a neutral compound. This is why we often see ions combining in certain ratios.

• The formula \(\mathrm{Al(CH}_3\mathrm{COO})_3\) shows the aluminum cation \(\mathrm{Al}^{3+}\) balanced by three acetate anions \(\mathrm{CH}_3\mathrm{COO}^{-}\), where the charges in total sum to zero.
• In \(\mathrm{(NH}_4\mathrm{)_2SO}_4\), two ammonium cations each with a charge of +1 balance a sulfate anion with a charge of -2.

Through charge balance, students can predict the proper stoichiometric amounts of ions necessary to form a specific ionic compound.

Polyatomic Ions

Polyatomic ions are charged entities composed of two or more covalently bonded atoms that function as a single ion. They can have a positive or negative charge, and like monatomic ions, are essential in the formation of ionic compounds.

When predicting formulas of ionic compounds with polyatomic ions, such as \(\mathrm{Mg}_3(\mathrm{PO}_4)_2\), it is crucial to recognize that these ions stay together as a group. The magnesium ion \(\mathrm{Mg}^{2+}\) balances the charge of the phosphate ion \(\mathrm{PO}_4^{3-}\) with a 3:2 ratio. Understanding polyatomic ions helps students handle more complex compounds beyond those formed by individual metal and non-metal ions.