Ion Definition

What is an Ion?

An ion can be defined as a chemical species which holds a positive or negative charge of some magnitude.

The term ‘ion’ can be used to refer to atoms or molecules that have non-zero net charges associated with them. Therefore, all ions have either a greater number of protons than electrons in their overall atomic or molecular structure, or they have a greater number of electrons than protons in their atomic/molecular structures. The ions that contain a greater number of protons than electrons are known to hold a net positive charge. These ions are commonly referred to as cations. On the other hand, the ions that contain a greater number of electrons than protons are known to hold a net negative charge. These ions are commonly known as anions.

It is important to note that the electrostatic forces of attraction that can arise between positively charged cations and negatively charged anions are the driving force behind the formation of ionic bonds. When two oppositely charged ions form an ionic bond with each other, the resulting type of compound is commonly referred to as an ionic compound.

If an ion is made up of only one type of atom (each holding some net charge, positive or negative), it can be referred to as an atomic ion or a monoatomic ion. On the other hand, if an ion is made up of two or more atoms, it can be referred to as a polyatomic ion or a molecular ion.

Table of Contents

• • How are Ions Created?
  • Representing the Charge on an Ion
  • Solved examples on Ion Notation
  • Examples of Ions

How are Ions Created?

Several methods exist for the preparation of ions. For example, spontaneous collisions between the molecules of a liquid or gaseous fluid can result in one of the electrons being knocked off an atom/molecule. This results in the formation of a positively charged ion and a free electron. This type of ionization is commonly referred to as physical ionization. The free-electron may even go on to attach itself to another atom or molecule, resulting in the formation of a new negatively charged anion.

Another important process through which ions can be created is through chemical interactions. For example, when an ionic compound such as salt is dissolved in a suitable solvent (such as water), the atoms that constitute the salt undergo dissociation and form free ions. When common salt, also known as sodium chloride, is dissolved in water, it undergoes dissociation to yield sodium cations and chloride anions. It can be noted that the sodium cations are denoted by the symbol Na⁺ and the chloride anions are denoted by the symbol Cl^–.

Other notable processes through which ions can be formed include the passage of direct currents through certain conducting solutions, which results in the formation of ions in the solution. It can also be noted that the dissolving of anodes via the process of ionization also yields large amounts of free ions.

Representing the Charge on an Ion

In order to represent an ion and the charge held by it, the chemical formula of the ion must be written first. Then, the symbol of the type of charge held by the ion (‘+’ for positive charges and ‘-’ for negative charges) must be written in superscript, followed by the magnitude of the charge held by the ion (also in superscript). It is important to note that the sign is written after the magnitude of the charge for anions that hold charges whose magnitudes are greater than -1. It can also be noted that the magnitude of the charge is omitted if its value is equal to one. Therefore, the sodium cation, whose charge is +1, can be represented simply as Na⁺.

Solved Examples on Ion Notation

The Carbonate Anion

The chemical formula of the carbonate anion is CO₃. The magnitude of the negative charge held by it is 2. Therefore, the effective charge held by the ion is -2. The carbonate ion can, therefore, be represented by the symbol CO₃^2-

The Lithium Cation

The symbol of the lithium atom is Li. Since the lithium cation holds a net charge of +1, the monoatomic ion can be represented as Li⁺.

The Oxide Anion

The symbol of oxygen is O. The charge held by the oxide anion is equal to -2. Therefore, the oxide anion can be denoted by the symbol O^2-.

The Zinc Cation

The symbol of zinc is Zn. The positive charge held by the zinc cation has a magnitude of 2. Therefore, the zinc cation can be denoted by the symbol Zn²⁺.

Examples of Ions

Examples of Simple Cations (Monoatomic Cations)

Five common examples of monoatomic cations are provided below.

• • • • The aluminium cation is denoted by the chemical formula Al³⁺.
      • The calcium cation is denoted by the chemical formula Ca²⁺.
      • The Iron(II) cation is denoted by the chemical formula Fe²⁺. It can also be noted that iron also has the ability to form the Fe³⁺ cation.
      • The manganese(II) cation is denoted by the chemical formula Mn²⁺. It is important to note that manganese also has the ability to form the Mn³⁺ and Mn⁴⁺ cations.
      • The hydrogen cation commonly known as a proton, is denoted by the chemical formula H⁺.

Examples of Polyatomic Cations (Molecular cations)

Three common examples of polyatomic cations are provided below.

• • • • The hydronium cation is denoted by the chemical formula H₃O⁺.
      • The ammonium cation is denoted by the chemical formula NH₄⁺.
      • The mercurous cation is denoted by the chemical formula Hg₂²⁺.

Examples of Simple Anions (Monoatomic Anions)

Five common examples of monoatomic anions are provided below.

• • • • The fluoride anion is denoted by the chemical formula F^–.
      • The chloride anion is denoted by the chemical formula Cl^–.
      • The bromide anion is denoted by the chemical formula Br^–.
      • The iodide anion is denoted by the chemical formula I^–.
      • The sulfide anion is denoted by the chemical formula S^2-.

Examples of Polyatomic Anions (Molecular Anions)

Five common examples of polyatomic anions are provided below.

• • • • The nitrate anion is denoted by the chemical formula NO₃^–.
      • The nitrite anion is denoted by the chemical formula NO₂^–.
      • The hydroxide anion is denoted by the chemical formula OH^–.
      • The carbonate anion is denoted by the chemical formula CO₃^2-.
      • The sulfate anion is denoted by the chemical formula SO₄^2-.

Examples of Anions Formed by Organic Compounds

Some anions are formed via the deprotonation of certain organic acids. Other anions are known to be derived from certain organic compounds. Common examples of anions that are derived from organic compounds are listed below.

• • • • The formate anion is denoted by the chemical formula HCOO^–.
      • The acetate anion is denoted by the chemical formula CH₃COO^–.
      • The cyanide anion is denoted by the chemical formula CN^–.

To learn more about the definition of ions and other related concepts such as the periodic trends in the ionization enthalpies of the elements, register with BYJU’S and download the mobile application on your smartphone.