Ionic solids are, by virtue of their very nature, inevitably both hard and brittle. The way in which ions are attracted to one another makes this development an inevitability, resulting in some rather common structures that we we see every day but may never think of in scientific terms. Sodium chloride - plain old table salt - is a good example.
First, though, what is an ionic solid? Otherwise known as ionic compounds, these solids are groups of oppositely-charged ions that are being held together through the forces of attraction. They become densely packed together, forming salts like NaCl, and appear under close examination as latticed structures. Examples of ionic solids beyond NaCl include:
- Caesium Chloride (CsCl), used in the treatment of cancer;
- Zinc Sulphide (ZnS), used as a phosphor in glow-in-the-dark products; and
- Calcium Fluoride (CaF2), used in aluminum metallurgy.
Ionic solids are formed when a mass of cations (positively charged metal ions) and anions (negatively charged non-metal ions) are brought together in the same place. These normally microscopic ions then join together via electrostatic attraction and begin to form the crystallized salts that we use every day. Their strong bonds keep the new crystals from falling apart.
Why, then, are ionic solids so hard? The answer is quite simple, and already hinted at: the bond between the cations and anions is resistant to separation. Take two magnets of opposite charges and place them together. They'll stick in an instant, right? What's more, they'll be tough to pry apart.
Ionic solids work in the same manner. The opposing charges compel the two or more cations and anions to unstoppably link together, the masses of which create the tough, brittle crystals typically associated with ionic solids. Ionic solids are also known as Closely Packed Structures, which means that each molecule maximizes the number of immediate neighbors it has, creating a rather tough grain of salt. It's quite difficult to beak ionic solids apart as a result, as they have no sense for flexibility. An ionic solid is either together or apart, nothing in-between. This force of attraction also explains why ionic solids have such a high boiling point - NaCl, for example, has a boiling point of 1413 C - as you need a lot of heat energy in order to tear those cations and anions apart.
It's interesting to note, in closing, that ionic solids, thanks to their nature as strong attractors, are good conductors of electricity. It's this property of salts that makes water a good conductor of electricity, in turn, as normally water is not so good on its own. The presence of ionic solids in water makes it dangerous to have anything electrical near, say, a bathtub or a shower, as the solids in the water will help pull the electricity quickly through the liquid and zap anyone similarly exposed.