Question

Name each ionic compound containing a polyatomic ion. $$ \text { a. }\mathrm{Ba}(\mathrm{OH})_{2} \quad \text { b. } \mathrm{NH}_{4} \mathrm{I} \quad \text { c. } \mathrm{NaBrO}_{4} \quad \text { d. } \mathrm{Fe}(\mathrm{OH})_{3} $$

Short answer

a) Barium hydroxide, b) Ammonium iodide, c) Sodium perbromate, d) Iron(III) hydroxide

Step-by-step solution

Identifying the Ionic Compounds

The given compounds are ionic and contain polyatomic ions. Barium hydroxide, ammonium iodide, sodium perbromate, and iron(III) hydroxide respectively have the chemical formulas \(\mathrm{Ba}(\mathrm{OH})_{2}\), \(\mathrm{NH}_{4}\mathrm{I}\), \(\mathrm{NaBrO}_{4}\), and \(\mathrm{Fe}(\mathrm{OH})_{3}\). Each compound consists of a metal cation and a polyatomic anion, except ammonium iodide that contains the ammonium cation which is a polyatomic ion.

Naming the Ionic Compounds

For each compound, the name starts with the name of the metal cation or ammonium ion followed by the name of the polyatomic ion. Barium hydroxide is named by the metal barium followed by the polyatomic hydroxide ion. Ammonium iodide is named for the ammonium ion followed by iodide. Sodium perbromate derives its name from sodium and the polyatomic perbromate ion. Iron(III) hydroxide involves the cation iron with a Roman numeral indicating the charge, and hydroxide as the polyatomic ion.

Writing the Names

The names of the compounds are as follows: a) \(\mathrm{Ba}(\mathrm{OH})_{2}\) is barium hydroxide, b) \(\mathrm{NH}_{4}\mathrm{I}\) is ammonium iodide, c) \(\mathrm{NaBrO}_{4}\) is sodium perbromate, d) \(\mathrm{Fe}(\mathrm{OH})_{3}\) is iron(III) hydroxide. These names reflect the metal and the polyatomic ions that make up each compound.

Key concepts

Polyatomic Ions

Understanding polyatomic ions is crucial when learning to name ionic compounds. These ions are composed of two or more atoms covalently bonded together, and they function as a single charged entity. In ionic compounds, they can take on the role of either the cation or the anion, but they are most commonly found as anions. Let's look at the compounds from the exercise:

For instance, OH^{-1} is the hydroxide ion, which contains one oxygen and one hydrogen atom bonded together with a negative one charge. In the compound barium hydroxide Ba(OH)_2, one barium cation Ba^{2+} pairs with two hydroxide anions to balance the charge. Similarly, the ammonium compound contains the ammonium ion NH_{4}^{+}, a positively charged ion made up of nitrogen and hydrogen atoms.

It is essential to memorize common polyatomic ions and their charges to name ionic compounds correctly. Some examples include carbonate CO_{3}^{2-}, sulfate SO_{4}^{2-}, and nitrate NO_{3}^{-}. By recognizing these patterns, you enable yourself to break down and understand a wide variety of chemical compounds.

Chemical Nomenclature

Chemical nomenclature is the standardized process for naming chemical compounds. It allows scientists and students alike to communicate about chemical substances unambiguously. In dealing with ionic compounds containing polyatomic ions, the name of the cation is stated first, followed by the name of the anion.

For a monoatomic cation (a single atom with a positive charge), simply use the element's name like sodium or iron. With transition metals, you often need to include a Roman numeral in parentheses to indicate the metal's charge, such as iron(III) in iron(III) hydroxide. For a polyatomic cation like ammonium, use its common name, ammonium.

For anions, you typically use the root of the element's name with an '-ide', '-ate', or '-ite' suffix. The '-ide' suffix is used for simple anions like chloride or iodide, whereas '-ate' and '-ite' are often used for polyatomic anions with oxygen, such as nitrate NO_{3}^{-} or sulfite SO_{3}^{2-}, where '-ate' usually indicates a higher number of oxygen atoms than '-ite'. Remember, correct nomenclature is crucial for clear communication in chemistry.

Ionic Compound Formulas

The formula of an ionic compound communicates not only the elements within the compound but also their proportions based on charge balance. Since ionic compounds are neutral overall, the total positive charge from the cations must equal the total negative charge from the anions. The subscripts in the formulas represent the number of ions necessary to achieve this balance.

In the exercise, barium hydroxide, with the formula Ba(OH)_2, has one barium ion with a 2+ charge balancing with two OH^{-1} ions. Ammonium iodide NH_{4}I involves a single NH_{4}^{+} ion paired with one iodide ion I^{-}.

When writing formulas, it's often necessary to use parentheses to indicate multiple polyatomic ions. For example, iron(III) hydroxide is written as Fe(OH)_3 to depict one iron ion with a 3+ charge and three hydroxide ions.

Learning to write these formulas correctly requires practice and a good grasp of valencies and charge balance. This knowledge then enables you to visualize the composition and proportions of elements within these compounds, allowing for more in-depth understanding and manipulation of chemical substances.