Question

Name the following ionic compounds: (a) \(\mathrm{K}_{2} \mathrm{O}\), (b) \(\mathrm{NaClO}_{2},(\mathrm{c}) \mathrm{Sr}(\mathrm{CN})_{2},(\mathrm{~d}) \mathrm{Co}(\mathrm{OH})_{2},(\mathrm{e}) \mathrm{Fe}_{2}\left(\mathrm{CO}_{3}\right)_{3}\) (f) \(\mathrm{Cr}\left(\mathrm{NO}_{3}\right)_{3},(\mathrm{~g})\left(\mathrm{NH}_{4}\right)_{2} \mathrm{SO}_{3},(\mathrm{~h}) \mathrm{NaH}_{2} \mathrm{PO}_{4}\), (i) \(\mathrm{KMnO}_{4}\) (j) \(\mathrm{Ag}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\)

Short answer

(a) potassium oxide, (b) sodium chlorite, (c) strontium cyanide, (d) cobalt(II) hydroxide, (e) iron(III) carbonate, (f) chromium(III) nitrate, (g) ammonium sulfite, (h) sodium dihydrogen phosphate, (i) potassium permanganate, (j) silver dichromate.

Step-by-step solution

Ionic Compound a: K2O

In this compound, the cation is potassium (K) and the anion is oxygen (O). The chemical name for potassium is the same as its symbol, and the chemical name for oxygen's anion is oxide. Therefore, the name of this compound is potassium oxide.

Ionic Compound b: NaClO2

In this compound, the cation is sodium (Na) and the anion is chlorite (ClO2). The name of this compound is sodium chlorite.

Ionic Compound c: Sr(CN)2

In this compound, the cation is strontium (Sr) and the anion is cyanide (CN). The name of this compound is strontium cyanide.

Ionic Compound d: Co(OH)2

In this compound, the cation is cobalt (Co) and the anion is hydroxide (OH). Cobalt can have multiple oxidation states, so we need to determine the charge of the cobalt ion. The hydroxide ion has a charge of -1, and there are two of them, which contributes a total charge of -2. Since the ionic compound is neutral, the cobalt ion must have a charge of +2. The name of this compound is cobalt(II) hydroxide.

Ionic Compound e: Fe2(CO3)3

In this compound, the cation is iron (Fe) and the anion is carbonate (CO3). Iron has multiple oxidation states, so we need to determine its charge. The carbonate ion has a charge of -2, and there are three of them, which contributes a total charge of -6. There are two iron ions, so each must have a charge of +3. The name of this compound is iron(III) carbonate.

Ionic Compound f: Cr(NO3)3

In this compound, the cation is chromium (Cr) and the anion is nitrate (NO3). Chromium has multiple oxidation states, so we need to determine its charge. The nitrate ion has a charge of -1, and there are three of them, which contributes a total charge of -3. Since the ionic compound is neutral, the chromium ion must have a charge of +3. The name of this compound is chromium(III) nitrate.

Ionic Compound g: (NH4)2SO3

In this compound, the cation is ammonium (NH4) and the anion is sulfite (SO3). The name of this compound is ammonium sulfite.

Ionic Compound h: NaH2PO4

In this compound, the cation is sodium (Na) and the anion is dihydrogen phosphate (H2PO4). The name of this compound is sodium dihydrogen phosphate.

Ionic Compound i: KMnO4

In this compound, the cation is potassium (K) and the anion is permanganate (MnO4). The name of this compound is potassium permanganate.

Ionic Compound j: Ag2Cr2O7

In this compound, the cation is silver (Ag) and the anion is dichromate (Cr2O7). Silver has only one oxidation state, so we do not need to identify its charge. The name of this compound is silver dichromate.

Key concepts

Chemical Nomenclature

Chemical nomenclature is the standardized system for naming chemical substances. Each compound's name is composed of the names of the cation (positively charged ion) and anion (negatively charged ion). When naming ionic compounds, we first state the name of the cation followed by the name of the anion. For example, in the ionic compound sodium chloride (\textbf{NaCl}), 'sodium' is the name of the cation and 'chloride', derived from chlorine, refers to the anion.

There are specific rules to follow in chemical nomenclature, especially for elements that can have more than one possible charge. For these elements, typically transition metals, we use Roman numerals in parentheses after the name of the cation to indicate its oxidation state. For instance, in iron(III) chloride (\textbf{FeCl}\(_{\textbf{3}}\)), the (III) denotes that iron has a charge of +3.

To help students understand and grasp the concept of naming ionic compounds, it is crucial to emphasize the importance of knowing the names and charges of common ions, understanding how to determine the oxidation states of transition metals, and practicing the naming of compounds with both simple and polyatomic ions.

Ionic Compound Formulas

The formulas for ionic compounds reflect the ratio of ions present to ensure overall electrical neutrality. A basic rule in forming ionic compound formulas is that the total positive charge must balance the total negative charge. Ions come together in a ratio that neutralizes their charges, resulting in a formula with no overall charge.

Take potassium oxide (\textbf{K}\(_{2}\)\textbf{O}), for example. Potassium has a +1 charge, and oxygen, in its ionic form as oxide, has a -2 charge. Two potassium ions are required to balance one oxide ion, resulting in the formula \textbf{K}\(_{2}\)\textbf{O}. Whenever dealing with polyatomic ions like sulfate (\textbf{SO}\(_{4}^{2-}\)) or nitrate (\textbf{NO}\(_{3}^{-}\)), the whole polyatomic ion acts as a single unit in the compound. This is crucial when writing the chemical formula for compounds like ammonium sulfate, where you need two ammonium (\textbf{NH}\(_{4}^{+}\)) ions to balance one sulfate ion.

In educational content, demonstrating this balance with clear examples and engaging visuals, and providing practice in writing formulas from names and vice versa can greatly enhance comprehension.

Oxidation States

Oxidation states, also referred to as oxidation numbers, indicate the degree of oxidation or reduction of an atom in a chemical compound. These states are expressed as integers; for instance, a +3 oxidation state signifies a loss of three electrons from the neutral state. Identifying the correct oxidation state is crucial in naming compounds where the elements have more than one possible oxidation state.

Consider iron(III) carbonate (\textbf{Fe}\(_{2}\)\textbf{(CO}\(_{3})_{3}\)), the (III) indicates each iron atom has lost three electrons, giving it a +3 oxidation state. Since the carbonate ion has a -2 charge and there are three carbonate ions, the two iron ions must each have a +3 charge to balance the -6 charge from the carbonate ions. This explains why iron has the oxidation state of +3 in this compound.

To facilitate understanding, it's effective to explain the concept of oxidation states with simple examples, and then progressively introduce more complex ions and their corresponding states. Exercises that involve assigning oxidation states to different atoms in a compound, or determining the correct ionic charge based on oxidation states, are valuable for deepening student understanding.