“The crystalline solids created by carefully packed ions of opposite charge,” according to the definition of ionic compounds.
When metals combine with nonmetals, ionic compounds are typically generated.”
To establish stability in ionic compounds, a metal loses electrons and a nonmetal receives electrons to complete its valence shell (i.e. has the configuration of the nearest noble gas).
Ionic bonds, which are electrostatic forces of attraction between a positive and a negative charge, hold the respective metal and nonmetal ions together (i.e. cation and anion).
This electrostatic force is the primary cause of ionic compound hardness.
The oppositely charged ions in an ionic combination are packed firmly together and create an extremely hard and rigid crystalline structure.
Final Answer:
Ionic compounds are typically hard because the ions are kept together by a strong force of attraction, since positive and negative ions are strongly attracted to one another and difficult to separate.