What is an Ion?

Ever wondered why salt dissolves in water or how your nerves transmit signals throughout your body? The reason is ions—small charged particles created when atoms gain or lose electrons. From assisting in the construction of stable compounds to energising your heartbeat, ions are everywhere. This is why in this lesson, we’ll explore everything about these tiny yet powerful particles. So let’s begin! 

1.0Definition of Ion

The definition of Ions can be stated as: “An atom or a group of atoms that carries an electric charge due to loss or gain of single or multiple electrons is known as an ion”. 

The sign of the charge of an atom also depends on whether an atom loses or gains electrons. That is, if an atom loses electron/s, it becomes a positively charged ion, while if an atom gains electron/s, it forms a negatively charged ion.

To represent the charge held by an ion, first write the chemical formula of the atom, then represent the symbol of the type of charge(+ or –) in superscript in the formula. Some of the most common examples of ions are sodium ion (Na⁺), Calcium ion (Ca⁺), Chloride ion (Cl^–), and Oxide ion (O^2–).

2.0Types of Ions

Ions are divided into two major groups, formed via the transfer of electrons, and carry opposite charges, ultimately making them behave chemically differently. These two types are: 

• Cations (Positive Ions): Cations are the positively charged atoms, formed by losing an electron. The loss of an electron results in a smaller number of electrons than protons, ultimately giving them their characteristic sign of charge. Cations are typically formed by metallic elements. Also, the size of cations is less than that of their parent atom due to a decreased number of electrons in their shells. Examples of cations are sodium ion (Na⁺), Calcium ion (Ca⁺), etc. 
• Anions (Negative Ions): Anions are negatively charged atoms, formed by gaining electron/s. As a result of this, the number of electrons in the atom becomes more than the number of protons, ultimately forming a negative charge. Anions are typically formed by non-metallic elements. As a result of increased repulsion between electrons, the anions are larger than their parent atom. Examples of anions include the Chloride ion (Cl^–) and the Oxide ion (O^2–). 

3.0Formation of Ions

The formation of ions for both types of ions (Cations and Anions) is a result of ionisation. It is the process by which an atom gains or loses an electron to achieve a more stable electronic configuration, typically resembling the nearest noble gas configuration. 

The need to achieve stability is driven by the octet rule, which states that atoms are stable if they contain eight electrons in their outermost shell (with the exceptions of hydrogen and helium, which only need two). Things outside the atom, like heat, electricity, or chemical reactions, can also give the energy necessary to release or pull electrons, leading to ionisation.

During Cation formation, atoms, especially metals, tend to lose electrons from their outermost shell. For example: 

• Sodium (Na) → loses 1 electron → Na⁺ (Cation)
• Magnesium (Mg) → loses 2 electrons → Mg²⁺ (Cation) 

On the other hand, during anion formation, non-metallic atoms gain electrons to fill their valence shell to satisfy the octet rule. Such as: 

• Chlorine (Cl) → gains 1 electron → Cl⁻ (Anion)
• Oxygen (O) → gains 2 electrons → O²⁻ (Anion)

4.0Ionic Compounds

Ionic compounds result when cations and anions come together via a strong electrostatic force referred to as an ionic bond. Ionic compounds are usually products of reactions between metals (that lose electrons to become cations) and non-metals (that gain electrons to become anions). Ionic compounds are also classified into two categories, which are: 

Monoatomic Ionic Compounds: These are ions formed from a single atom, for example: 

• Sodium chloride (NaCl) → Na⁺ (monoatomic cation) + Cl⁻ (monoatomic anion)
• Magnesium oxide (MgO) → Mg²⁺ + O^2–

Polyatomic Ionic Compounds: Polyatomic ionic compounds are formed from groups of atoms covalently bonded together that carry an overall charge. Such as: 

• Ammonium nitrate (NH₄NO₃) → NH₄⁺ (Polyatomic cation) + NO₃⁻ (Polyatomic anion)
• Calcium carbonate (CaCO₃) → Ca²⁺ (Polyatomic cation) + CO₃²⁻ (Polyatomic anion)

5.0Properties of Ions: 

Based on the types of ions (cations and anions), the properties of ions can vary widely. However, both of these types follow some common physical and chemical properties, which are: 

1. Electrical Conductivity: Ions are excellent conductors of electricity, however, only in their molten state or when dissolved in water. This is because, in molten states, ions are free to move and hence, carry electricity without any hindrance. This is the reason why saltwater conducts electricity, while solid salt doesn’t. 
2. Reactivity: In comparison to their neutral atoms, ions are highly reactive. Cations and anions efficiently combine with oppositely charged ions to form stable compounds. This high reactivity is the reason behind the formation of ionic compounds, acid-base reactions, and many more. 
3. Solubility: Most ionic compounds easily dissolve in polar solvents such as water. This is due to water molecules being able to surround and dissociate the ions (hydration), allowing them to move freely in solution.
4. Melting and Boiling Points: The melting and boiling points of ionic compounds are usually very high due to the strong electrostatic forces working between oppositely charged ions. 
5. Crystalline Structure: In their solid form, ions organise into regular, repeating crystal lattices. This ordered pattern provides many ionic compounds with a rigid, brittle consistency, which causes them to appear as crystalline solids such as table salt (NaCl).
6. Biological Importance: In human physiology and medicine, ions such as Na⁺, K⁺, and Ca⁺ are essential for nerve signal transmission, muscle contraction, and heartbeat regulation. While Cl^– helps maintain fluid and pH balance in the body.