Ionic Bonds and Lattice Structures

Ionic compounds (like table salt) are built from charged particles called ions. They form strong attractions and pack into large, regular crystals called giant ionic lattices.

Ions: how they form

• Cations are positive ions. They form when atoms lose electrons (usually metals, e.g. Group I and II).
• Anions are negative ions. They form when atoms gain electrons (usually non-metals, e.g. Group VII and VI).
• Atoms form ions to get a full outer shell of electrons (similar to noble gases).

What is an ionic bond?

An ionic bond is a strong electrostatic attraction between oppositely charged ions. Think of many tiny magnets pulling each other in all directions.

Forming ionic bonds (examples)

Sodium chloride (NaCl): sodium loses one electron; chlorine gains one electron.

$$\text{Na} \rightarrow \text{Na}^+ + e^- \qquad \text{Cl} + e^- \rightarrow \text{Cl}^-$$

After transfer, $\text{Na}^+$ and $\text{Cl}^-$ attract strongly.

Magnesium oxide (MgO): magnesium loses two electrons; oxygen gains two electrons.

$$\text{Mg} \rightarrow \text{Mg}^{2+} + 2e^- \qquad \text{O} + 2e^- \rightarrow \text{O}^{2-}$$

In dot-and-cross diagrams, show electrons from the metal as one symbol and from the non-metal as another. After transfer, place each ion in brackets with its charge, for example $[\,\text{Na}^+\,]$ and $[\,\text{Cl}^-\,]$.

Giant ionic lattice

Ionic compounds form a giant lattice: a regular 3D arrangement of alternating positive and negative ions. Each ion is surrounded by oppositely charged ions. Imagine a 3D checkerboard or a stack of spheres where plus and minus repeat throughout the crystal.

Properties explained by structure

• High melting and boiling points: Many strong attractions act in all directions. A lot of energy is needed to separate the ions.
• Electrical conductivity: Ionic compounds do not conduct when solid because ions are fixed in place. When molten or dissolved in water (aqueous), ions are free to move and carry charge, so they conduct well.

Common misconceptions

• Ionic bonding is not sharing electrons. It is the attraction after electrons are transferred.
• Ionic compounds are not molecules. Use formula units like NaCl or MgO.
• A solid ionic crystal does not conduct electricity because its ions cannot move.