Ionic bonding is the fundamental force that combines metals and non-metals. While the Core syllabus identifies the basic attraction between ions, the IGCSE 0654 Extended syllabus requires a deeper analysis of the giant lattice structure and a rigorous explanation of how this 3D network dictates the physical properties of ionic compounds. By visualizing the regular arrangement of ions, we can explain why salts have high melting points and why their electrical conductivity is state-dependent.

An ionic bond is formed when a metal atom transfers one or more electrons to a non-metal atom.

• Metallic Ions (Cations): Lose electrons to achieve a full outer shell, becoming positively charged.
• Non-metallic Ions (Anions): Gain electrons to achieve a full outer shell, becoming negatively charged.
• Visualizing Transfer: Dot-and-cross diagrams are used to illustrate this transfer. The metal is shown losing its valence dots, while the non-metal is shown gaining those same dots (marked as crosses) to complete its valence shell.

Ionic compounds do not exist as discrete molecules. Instead, they form a giant lattice structure.

Characteristics of the Lattice

• Regular Arrangement: The ions are organized in a highly ordered, repeating three-dimensional pattern.
• Alternating Charges: Every positive ion is surrounded by negative ions, and every negative ion is surrounded by positive ions. This maximizes the attractive forces and minimizes the repulsive forces between like charges.
• Omni-directional Attraction: The electrostatic attraction acts in all directions throughout the entire giant structure. There are no "weak spots" in the network.

The macroscopic properties of substances like sodium chloride ($NaCl$) are a direct consequence of the lattice's stability.

(a) High Melting and Boiling Points

The electrostatic attraction between oppositely charged ions is incredibly strong. Because the lattice is "giant," melting a salt requires breaking millions of these strong bonds simultaneously across the 3D structure.

• Energetic Requirement: This process demands a vast amount of thermal energy. Consequently, all ionic compounds have very high melting and boiling points and are solids at room temperature.

(b) Electrical Conductivity

For a substance to conduct electricity, it must contain mobile charged particles.

• Solid State: In a solid lattice, the ions are held in fixed positions by strong electrostatic forces. They can vibrate but cannot move from place to place. Therefore, solid ionic compounds do not conduct electricity.
• Molten or Aqueous State: When the compound is melted (molten) or dissolved in water (aqueous), the lattice structure is broken down. The ions are freed from their fixed positions and are mobile. These mobile ions can move toward electrodes, allowing the substance to conduct an electric current.

The giant ionic lattice is one of nature's most stable structural motifs. By arranging alternating charges in a precise, repeating 3D network, ionic compounds achieve a level of structural integrity that results in high-temperature stability. Understanding the transition from a rigid insulator to a mobile conductor is key to mastering the behavior of salts in both laboratory and industrial settings.