Question

Classify each of the following as a monoatomic cation, monoatomic anion, polyatomic cation, or polyatomic anion: (a) ammonium ion, \(\mathrm{NH}_{4}^{+}\) (b) strontium ion, \(\mathrm{Sr}^{2+}\) (c) sulfide ion, \(\mathrm{S}^{2}\) (d) sulfate ion, \(\mathrm{SO}_{4}{ }^{2-}\)

Short answer

(a) Polyatomic cation, (b) Monoatomic cation, (c) Monoatomic anion, (d) Polyatomic anion.

Step-by-step solution

Identify the characteristics of each ion

To classify ions, determine the number of atoms involved and the type of charge. **Monoatomic ions** consist of a single atom, while **polyatomic ions** contain multiple atoms. **Cations** have a positive charge, and **anions** have a negative charge.

Analyze the ammonium ion, \(\mathrm{NH}_{4}^{+}\)

The ammonium ion has the chemical formula \(\mathrm{NH}_{4}^{+}\), which consists of 5 atoms (1 nitrogen and 4 hydrogens) and carries a positive charge. It is a polyatomic cation.

Analyze the strontium ion, \(\mathrm{Sr}^{2+}\)

The strontium ion is represented by \(\mathrm{Sr}^{2+}\). It consists of a single atom of strontium and has a positive charge. Therefore, it is a monoatomic cation.

Analyze the sulfide ion, \(\mathrm{S}^{2-}\)

The sulfide ion is \(\mathrm{S}^{2-}\). It comprises only one sulfur atom and carries a negative charge, making it a monoatomic anion.

Analyze the sulfate ion, \(\mathrm{SO}_{4}^{2-}\)

The sulfate ion \(\mathrm{SO}_{4}^{2-}\) consists of 5 atoms (1 sulfur and 4 oxygens) and has a negative charge. It is classified as a polyatomic anion.

Key concepts

Monoatomic and Polyatomic Ions

Ions are atoms or molecules that have gained or lost electrons, giving them a charge. They can be divided into two main categories based on the number of atoms involved: monoatomic ions and polyatomic ions. Understanding these types helps in identifying and classifying different ions.

• Monoatomic Ions: These ions consist of a single atom. For example, the strontium ion (\(\mathrm{Sr}^{2+}\)) and the sulfide ion (\(\mathrm{S}^{2-}\)) are both monoatomic ions because they each come from individual strontium and sulfur atoms, respectively.
• Polyatomic Ions: These ions consist of two or more atoms bonded together, behaving as a single charged entity. An example is the ammonium ion (\(\mathrm{NH}_{4}^{+}\)), which is composed of one nitrogen atom and four hydrogen atoms. Another example is the sulfate ion (\(\mathrm{SO}_{4}^{2-}\)), consisting of one sulfur atom and four oxygen atoms.

Cations and Anions

Ions are further categorized based on their charges as either cations or anions. The behavior of these charged particles is fundamental in chemical reactions and bonding.

• Cations: These are positively charged ions. They form when an atom loses electrons. In our examples, the ammonium ion (\(\mathrm{NH}_{4}^{+}\)) and strontium ion (\(\mathrm{Sr}^{2+}\)) are cations. The positive charge indicates these ions have fewer electrons compared to their protons.
• Anions: These are negatively charged ions, formed when an atom gains electrons. The sulfide ion (\(\mathrm{S}^{2-}\)) and sulfate ion (\(\mathrm{SO}_{4}^{2-}\)), both carry a negative charge. Here, the extra electrons result in a negative charge, outnumbering the protons.

Chemical Formulas

Chemical formulas provide vital information about the composition and charge of ions. They serve as a shorthand notation of the chemical components.

• A chemical formula like \(\mathrm{NH}_{4}^{+}\) (ammonium) reveals the presence of one nitrogen and four hydrogen atoms, along with its positive charge.
• The formula \(\mathrm{Sr}^{2+}\) (strontium) indicates a single strontium atom with a double positive charge, while \(\mathrm{S}^{2-}\) (sulfide) shows a sulfur atom with a double negative charge.
• In the case of \(\mathrm{SO}_{4}^{2-}\) (sulfate), the formula tells us there's a combination of one sulfur and four oxygen atoms, with the overall charge being negative.

Understanding chemical formulas is crucial as they are used to predict and balance chemical reactions in various applications, underpinning much of chemical science.