Question

Classify each of the following as a monoatomic cation, monoatomic anion, polyatomic cation, or polyatomic anion: (a) hydronium ion, \(\mathrm{H}_{3} \mathrm{O}^{+}\) (b) aluminum ion, \(\mathrm{Al}^{3+}\) (c) chloride ion, \(\mathrm{Cl}^{-}\) (d) chlorate ion, \(\mathrm{ClO}_{3}^{-}\)

Short answer

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

Step-by-step solution

Classify the Hydronium Ion

The hydronium ion, \(\mathrm{H}_{3} \mathrm{O}^{+}\), consists of multiple atoms. The presence of a positive charge identifies it as a cation. Therefore, it is classified as a polyatomic cation.

Classify the Aluminum Ion

The aluminum ion, \(\mathrm{Al}^{3+}\), contains only one type of atom and has a positive charge. Monoatomic ions consist of a single atom, and given its charge, this is a monoatomic cation.

Classify the Chloride Ion

The chloride ion, \(\mathrm{Cl}^{-}\), contains only one chlorine atom and has a negative charge. Monoatomic ions have a single atom, and because of its negative charge, it is categorized as a monoatomic anion.

Classify the Chlorate Ion

The chlorate ion, \(\mathrm{ClO}_{3}^{-}\), consists of multiple atoms, including chlorine and oxygen, and carries a negative charge. It is classified as a polyatomic anion because it is composed of multiple atoms and has a negative charge.

Key concepts

Monoatomic Ions

Monoatomic ions are ions that consist of a single atom with a positive or negative charge.
They gain their charges by losing or gaining electrons, making them either cations or anions.
These ions are simple and form due to the transfer of electrons.

• Cations: These are positively charged monoatomic ions. They form when an atom loses one or more electrons, resulting in more protons than electrons in the atom. An example of this is the aluminum ion, \(\mathrm{Al}^{3+}\), which has three fewer electrons than protons.
• Anions: These are negatively charged monoatomic ions. They form when an atom gains additional electrons, resulting in more electrons than protons. Chloride ion, \(\mathrm{Cl}^{-}\), is an example where chlorine has gained an extra electron.

Polyatomic Ions

Unlike monoatomic ions, polyatomic ions are composed of two or more atoms bonded together, and they act as a single charged entity.
They often consist of multiple types of atoms and carry a positive or negative charge.

• Types of Polyatomic Ions: These ions can be cations like the hydronium ion, \(\mathrm{H}_{3} \mathrm{O}^{+}\), which carries a positive charge and consists of three hydrogen atoms and one oxygen atom. Another example is the polyatomic anion, chlorate ion, \(\mathrm{ClO}_{3}^{-}\), consisting of chlorine and oxygen atoms.
• Stability: Polyatomic ions are stable due to the strong covalent bonds within the ions. However, the entire group of atoms functions together as a unit, gaining or losing electrons to maintain stability.

Understanding polyatomic ions is crucial for studying reactions in chemistry, especially since they often make up a significant part of compounds.

Cations and Anions

Ions are broadly classified into cations and anions, based on their charge.
Identifying ions as cations or anions helps in understanding chemical reactions and forming compounds.

• Cations: These are ions with a positive charge. They have more protons than electrons. This positive charge often helps them attract and bond with anions in ionic compounds. Cations can be monoatomic like \(\mathrm{Al}^{3+}\) or polyatomic like \(\mathrm{H}_{3} \mathrm{O}^{+}\).
• Anions: Ions with a negative charge, having more electrons than protons. Anions readily bond with cations to form ionic compounds. Examples include monoatomic anions like \(\mathrm{Cl}^{-}\) and polyatomic anions such as \(\mathrm{ClO}_{3}^{-}\).

Both cations and anions are essential for the formation of a wide range of ionic compounds seen in nature and chemistry. Knowing these basics helps learners comprehend more advanced topics in chemistry, such as reactions and solubility.