4.1.2 Ionic Compounds

Ionic Lattices

The ions form a lattice structure which is an evenly distributed crystalline structure
Ions in a lattice are arranged in a regular repeating pattern so that positive charges cancel out negative charges
Therefore the final lattice is overall electrically neutral

Ionic solids are arranged in lattice structures

Different types of structure and bonding have different effects on the physical properties of substances such as their melting and boiling points, electrical conductivity and solubility

Ionic compounds are strong
- The strong electrostatic forces in ionic compounds keep the ions held strongly together
They are brittle as ionic crystals can split apart
Ionic compounds have high melting and boiling points
- The strong electrostatic forces between the ions in the lattice act in all directions and keep them strongly together
- Melting and boiling points increase with the charge density of the ions due to the greater electrostatic attraction of charges
- Mg²⁺O^2– has a higher melting point than Na⁺Cl^–
Ionic compounds are soluble in water as they can form ion-dipole bonds
Ionic compounds only conduct electricity when molten or in solution
- When molten or in solution, the ions can freely move around and conduct electricity
- As a solid, the ions are in a fixed position and unable to move around

Table comparing the characteristics of giant ionic lattices with other structure types

	Giant ionic	Giant metallic	Simple covalent	Giant covalent
Melting / boiling point	High	Moderately high to high	Low	Very high
Electrical conductivity	Only when molten or in solution	When solid or liquid	Do not conduct electricity	Do not conduct electricity (except graphite)
Solubility	Soluble	Insoluble but some may react	Usually insoluble unless they are polar	Insoluble
Hardness	Hard, brittle	Hard, malleable	Soft	Very hard (diamond and silica) or soft (graphite)
Physical state at room temperature	Solid	Solid	Solid, liquid or gas	Solid
Forces	Electrostatic attraction between ions	Delocalised electrons attracting positive ions	Weak intermolecular forces and covalent bonds within a molecule	Electrons in covalent bonds between atoms
Particles	Ions	Positive ions in a sea of electrons	Small molecules	Atoms
Examples	NaCl	Copper	Br₂	Graphite, silicon(IV) oxide

The table below shows the physical properties of substances X, Y and Z.

Substance	Melting point (^oC)	Electrical conductivity when molten	Solubility in water
X	839	Good	Soluble
Y	95	Very poor	Almost insoluble
Z	1389	Good	Insoluble

Which one of the following statements about X, Y and Z is completely true?

Statement 1: X has a giant ionic structure, Y has a giant molecular structure, Z is a metal

Statement 2: X is a metal, Y has a simple molecular structure, Z has a giant molecular structure

Statement 3: X is a metal, Y has a simple molecular structure, Z has a giant ionic structure

Statement 4: X has a giant ionic structure, Y has a simple molecular structure, Z is a metal

Answer:

Compound X has a relatively high melting point, is soluble in water and conducts electricity when molten
- This suggests that X has a giant ionic structure
Compound Y has a low melting point which suggests that little energy is needed to break the lattice
- This suggests that Y is a simple molecular structure
- This is further supported by its low electrical conductivity and it being almost insoluble in water
Compound Z has a very high melting point, which is characteristic of either metallic, giant ionic lattices or giant covalent / molecular lattices
- However since it is insoluble in water, compound Z must be a metal
Therefore, the correct answer is Statement 4