Question

Write the formulas for the following ionic compounds: (a) copper bromide (containing the \(\mathrm{Cu}^{+}\) ion), (b) manganese oxide (containing the \(\mathrm{Mn}^{3+}\) ion), (c) mercury iodide (containing the \(\mathrm{Hg}_{2}^{2+}\) ion), (d) magnesium phosphate (containing the \(\mathrm{PO}_{4}^{3-}\) ion).

Short answer

(a) \( \mathrm{CuBr} \), (b) \( \mathrm{Mn}_{2}\mathrm{O}_{3} \), (c) \( \mathrm{Hg}_{2}\mathrm{I}_{2} \), (d) \( \mathrm{Mg}_{3}\left(\mathrm{PO}_{4}\right)_{2} \).

Step-by-step solution

Identify the Ions

First, identify the cation and anion in each compound given in the exercise.(a) Copper bromide has - Cation: \( \mathrm{Cu}^{+} \) - Anion: \( \mathrm{Br}^{-} \)(b) Manganese oxide has - Cation: \( \mathrm{Mn}^{3+} \) - Anion: \( \mathrm{O}^{2-} \)(c) Mercury iodide has - Cation: \( \mathrm{Hg}_{2}^{2+} \) - Anion: \( \mathrm{I}^{-} \)(d) Magnesium phosphate has - Cation: \( \mathrm{Mg}^{2+} \) - Anion: \( \mathrm{PO}_{4}^{3-} \)

Determine the Formula for Copper Bromide

For copper bromide:- The charges for \( \mathrm{Cu}^{+} \) and \( \mathrm{Br}^{-} \) are equal and opposite, so one of each ion will balance the charges.- The formula is \( \mathrm{CuBr} \).

Determine the Formula for Manganese Oxide

For manganese oxide:- The \( \mathrm{Mn}^{3+} \) ion needs to be balanced with \( \mathrm{O}^{2-} \). - To balance the overall charge to zero, two \( \mathrm{Mn}^{3+} \) ions (total charge +6) will combine with three \( \mathrm{O}^{2-} \) ions (total charge -6).- The formula is \( \mathrm{Mn}_{2}\mathrm{O}_{3} \).

Determine the Formula for Mercury Iodide

For mercury iodide:- The \( \mathrm{Hg}_{2}^{2+} \) ion needs to be balanced with \( \mathrm{I}^{-} \).- Two \( \mathrm{I}^{-} \) ions are needed to balance the 2+ charge of \( \mathrm{Hg}_{2}^{2+} \).- The formula is \( \mathrm{Hg}_{2}\mathrm{I}_{2} \).

Determine the Formula for Magnesium Phosphate

For magnesium phosphate:- Three \( \mathrm{Mg}^{2+} \) ions (totalling +6 charge) are needed to balance with two \( \mathrm{PO}_{4}^{3-} \) ions (totalling -6 charge).- The formula is \( \mathrm{Mg}_{3}\left(\mathrm{PO}_{4}\right)_{2} \).

Key concepts

Chemical Formulas

Ionic compounds are formed from the attraction between positively charged cations and negatively charged anions. To write a chemical formula, follow these key steps:
Identify the ions involved and their charges. Ensure the total positive and negative charges balance each other to achieve a neutral compound. For example, in copper bromide, with the cation \(\mathrm{Cu}^{+}\) and anion \(\mathrm{Br}^{-}\), the charges are equal and opposite.

• The steps for balancing involve ensuring the total charges are equal.
• The subscript numbers in the formula represent the ratio of ions needed.

This process results in the simplest ratio of ions, expressed in the chemical formula, like \(\mathrm{CuBr}\) for copper bromide.

Cation and Anion Identification

Identifying cations and anions is crucial for writing chemical formulas. Cations are positively charged ions, usually metals, while anions are negatively charged ions, often non-metals or polyatomic ions.
In the exercise, you see examples such as manganese (\(\mathrm{Mn}^{3+}\)) as the cation and oxide (\(\mathrm{O}^{2-}\)) as the anion in manganese oxide. This identification forms the foundation for predicting and balancing ionic compounds.

• Cations are generally written first in formulas.
• Anions follow the cations in the written formula.

Understanding the types of ions present helps in predicting how they will combine to form a stable compound.

Charge Balancing

Balancing the charges of cations and anions results in a neutral ionic compound. It's all about finding the right ratio of ions to balance their total charges. In the manganese oxide example, \(\mathrm{Mn}^{3+}\) and \(\mathrm{O}^{2-}\) ions must balance.
Two manganese ions (\(2 \times +3 = +6\)) need three oxide ions (\(3 \times -2 = -6\)) to achieve neutrality:

• Add subscripts to indicate the number of each ion.
• Ensure the total charges cancel each other out.

This leads to the formula \(\mathrm{Mn}_{2}\mathrm{O}_{3}\). Such methods ensure compounds are stable and correctly formed.

Transition Metals

Transition metals, such as copper, manganese, and mercury, often exhibit variable oxidation states. This means they can form different types of ions. For instance, copper can be \(\mathrm{Cu}^{+}\) or \(\mathrm{Cu}^{2+}\), affecting how compounds like copper bromide are balanced.
The exercise's reference to copper bromide (\(\mathrm{Cu}^{+}\)) illustrates this concept. Transition metals' ability to have different charges requires special attention when writing formulas:

• Always specify the charge when given a compound name.
• Balance compounds according to the specified oxidation state.

These metals' unique properties add variety but can also complicate formula writing. Understanding their charge states is essential to mastering their chemistry.