Brittleness is a property of a substance that fractures when subjected to force. When stress is applied to a substance, it has at least some elastic limit that keeps it from breaking. Ionic substances have very little of that which makes them poor resistors to stress, a very little elastic limit, and a very low tensile strength.

An ionic compound forms by the arrangement of oppositely charged ions in a rigid, closely packed structure called a unit cell. It repeats itself to produce a crystal. These ionic substances are mostly brittle because when they are beaten, hammered, or pressed, layers slightly move but the same charged ions repel each other on coming face to face. This breaks a giant lattice into smaller crystals.

Why are ionic substances brittle?

Ionic lattice, having a specific arrangement of sodium and chlorine crystals to make a unit cell

In ionic substances, ions of different elements are bonded (held together) in symmetrical arrangement due to the attractive forces between oppositely charged ions. They are formed by the loss of electron(s) by one type of atom and then gain by the electronegative atom. It is a non-directional bond and can be represented by a hard-sphere of opposite charges in different arrangements.

A crystal of ionic solid is considered to be made of repeating units also known as representative units as they repeat in a regular manner. These spheres describe cations and anions in the unit cells that are stacked up to gather to form a crystal lattice. The structure of ionic compounds is different when compared with metallic lattices.

Comparison of metallic and ionic lattices

In metallic compounds, metal atoms are layered above and below a sea of electrons. When they are pressed or hammered to slide over each other, for which they are malleable. In case when ionic compounds are pressurized, they show very little mobility of layers as the same charges start to confront each other and start having electrostatic repulsion. They break into smaller crystal parts which becomes the reason for their brittleness.

Cause of brittleness in ionic substances

  • Electrostatic repulsions between similar ions are the main cause of brittleness when lattices are subjected to a slide like metallic lattices.
  • Destruction of unit cellular structure, necessary to keep the malleable character.
  • Ionic substances do not have enough sea of electrons, like similar atoms combined by bonds e.g. metallic bonds.
  • Very strong, and highly polar bonds i.e. ionic bonds making them up i.e. electronegativity difference above 1.7