Atoms are most stable when they possess a full outer shell of electrons, a configuration characteristic of the noble gases (Group VIII). To achieve this state, atoms often undergo electron transfer, leading to the formation of charged particles called ions. The resulting attraction between these ions constitutes an ionic bond. The IGCSE 0654 Extended syllabus requires a detailed explanation of ion formation, the nature of the ionic bond, and the macroscopic properties of ionic compounds.

An ion is an atom that has gained or lost electrons, resulting in a net electrical charge.

Formation of Cations (Positive Ions)

• Source: Generally formed from metal atoms (Groups I, II, and III).
• Mechanism: Metal atoms possess few electrons in their outer shell. It is energetically favorable to lose these electrons.
• Result: Because the atom now has more protons than electrons, it carries a positive charge.
• Example: Sodium ($Na$) loses one electron to become the sodium ion ($Na^{+}$).

Formation of Anions (Negative Ions)

• Source: Generally formed from non-metal atoms (Groups V, VI, and VII).
• Mechanism: Non-metal atoms have shells that are nearly full. It is energetically favorable to gain electrons to complete the shell.
• Result: The atom has more electrons than protons, carrying a negative charge.
• Example: Chlorine ($Cl$) gains one electron to become the chloride ion ($Cl^{-}$).

An ionic bond is defined as the strong electrostatic attraction between oppositely charged ions. This attraction is multi-directional and leads to the formation of a giant lattice—a regular, repeating three-dimensional arrangement of positive and negative ions.

The macroscopic properties of ionic substances, such as sodium chloride (NaCl), are direct consequences of the strength and structure of the ionic lattice.

High Melting and Boiling Points

Ionic bonds are exceptionally strong. A significant amount of thermal energy is required to overcome these electrostatic forces throughout the entire giant lattice. Consequently, ionic compounds are solids at room temperature and have very high melting and boiling points.

Electrical Conductivity

Electrical conductivity requires the presence of mobile charged particles.

• Solid State: In a solid lattice, the ions are locked in fixed positions and cannot move. Therefore, solid ionic compounds are insulators.
• Molten or Aqueous State: When melted (molten) or dissolved in water (aqueous), the lattice structure breaks down. The ions become mobile and can carry an electric current.

Solubility

Most ionic compounds are soluble in water. Water molecules are polar and can surround the individual ions, "pulling" them out of the lattice and into the solution.

Ionic bonding is the fundamental mechanism by which metals and non-metals combine. By transferring electrons to achieve stable electronic configurations, atoms create a powerful electrostatic network that results in some of the most stable and physically robust substances in the chemical world.