Problem 98 - Chemistry: The Central Science

Question

Name each of the following oxides. Assuming that the compounds are ionic, what charge is associated with the metallic element in each case? (a) \(\mathrm{NiO}\), (b) \(\mathrm{MnO}_{2}\), (c) \(\mathrm{Cr}_{2} \mathrm{O}_{3}\), (d) \(\mathrm{MoO}_{3}\).

Step-by-Step Solution

Answer

(a) Nickel(II) oxide, where nickel has a charge of +2. (b) Manganese(IV) oxide, where manganese has a charge of +4. (c) Chromium(III) oxide, where each chromium ion has a charge of +3. (d) Molybdenum(VI) oxide, where molybdenum has a charge of +6.

Step 1: (a) Identifying the metal and naming NiO

The metallic element in this case is nickel (Ni). Since oxygen has a -2 oxidation state, the charge on the nickel ion must be +2 to balance the charges. Therefore, the name of \(\mathrm{NiO}\) is "Nickel(II) oxide".

Step 2: (b) Identifying the metal and naming MnO2

The metallic element in this case is manganese (Mn). Each oxygen ion has a -2 oxidation state, and as there are two oxygen ions, the total negative charge is -4. To balance this charge, manganese must have a +4 charge. Therefore, the name of \(\mathrm{MnO}_{2}\) is "Manganese(IV) oxide".

Step 3: (c) Identifying the metal and naming Cr2O3

The metallic element in this case is chromium (Cr). Each oxygen ion has a -2 oxidation state, and as there are three oxygen ions, the total negative charge is -6. As there are two chromium ions, each chromium ion must have a +3 charge to balance the total negative charge. Therefore, the name of \(\mathrm{Cr}_{2} \mathrm{O}_{3}\) is "Chromium(III) oxide".

Step 4: (d) Identifying the metal and naming MoO3

The metallic element in this case is molybdenum (Mo). Each oxygen ion has a -2 oxidation state, and as there are three oxygen ions, the total negative charge is -6. To balance this charge, molybdenum must have a +6 charge. Therefore, the name of \(\mathrm{MoO}_{3}\) is "Molybdenum(VI) oxide".

Key Concepts

Oxidation States

In chemistry, oxidation states, also known as oxidation numbers, are very important. They help us determine how elements combine and interact in compounds. An oxidation state is a hypothetical charge that an atom would have if all bonds to atoms of different elements were fully ionic. It helps us keep track of electrons in a reaction.

For example, in oxides, oxygen typically has an oxidation state of -2. When naming ionic compounds, it’s crucial to consider the oxidation states of each element involved. This is because the overall charge in a stable ionic compound must be zero.

• In NiO, nickel must have an oxidation state of +2 to balance out the -2 from oxygen.
• For MnO₂, manganese needs a +4 oxidation state since there are two oxygens, each with -2.
• When it comes to Cr₂O₃, each chromium must be +3 as there are three oxygens generating a charge of -6.
• In MoO₃, molybdenum balances three oxygens, leading it to have a +6 oxidation state.

Understanding these states helps in naming the compounds accurately, reflecting the specific charges of the metallic elements.

Metallic Elements

Metallic elements play a vital role in forming ionic compounds such as oxides. These are generally metals from the d-block of the periodic table, also known as transition metals. They can have various oxidation states, making it necessary to understand their specific charges in different compounds.

Transition metals like nickel, manganese, chromium, and molybdenum, as found in the given oxides, have the ability to form different ions based on the compound they are in. This versatile nature stems from their ability to use d-orbitals for bonding. These d-orbitals allow them to have multiple stable ionic forms, contributing to the rich chemistry of transition metals.

• Nickel in NiO adopts a +2 oxidation state as it forms Nickel(II) oxide.
• Manganese forms a +4 ion in MnO₂, hence it is Manganese(IV) oxide.
• Chromium takes a +3 state in Cr₂O₃, which is Chromium(III) oxide.
• Molybdenum in MoO₃ carries a +6 state, leading to Molybdenum(VI) oxide.

Recognizing the changes in these oxidation states is vital for correctly naming each compound.

Balancing Charges

Balancing charges is a fundamental aspect of forming stable ionic compounds. In these scenarios, the total positive charge must equal the total negative charge, ensuring the compound as a whole is electrically neutral.

When naming ionic compounds like the oxides discussed, it’s necessary to balance the charges of the metallic and non-metallic ions. Oxygen consistently presents a -2 charge in oxides. Hence, the metallic elements must adjust their charges accordingly:

• In NiO, nickel balances a single oxygen with its +2 charge.
• In MnO₂, two oxygen ions at -2 each sum to -4, requiring manganese to be +4.
• For Cr₂O₃, three oxygens lead to a -6 charge, balanced by two chromiums each at +3.
• In MoO₃, three oxygens total to a -6 charge, which molybdenum offsets with a +6 charge.

The principle of charge balancing is what ensures the stability and accurate naming of ionic compounds. Paying attention to this aspect simplifies the understanding of ionic compound formulas.