This type of bond usually forms between a metal and a nonmetal. Metals have a weak hold on their outer electrons. Nonmetals have a very strong pull for new electrons.
When they meet, the nonmetal simply takes the outer electrons from the metal. The metal becomes a positive cation because it lost negative charge. The nonmetal becomes a negative anion because it gained negative charge.
These opposite charges now attract each other like powerful magnets. Students often picture just two ions paired up together in an isolated group. In reality, ionic bonds never form single isolated pairs.
The newly formed ions gather into a giant, repeating three-dimensional grid called a crystal lattice. Every positive ion is surrounded by negative ions on all sides. Every negative ion is surrounded by positive ions.
This massive web of attraction makes ionic compounds very hard and brittle. It also gives these compounds extremely high melting points. The strong electrical bonds require massive amounts of heat energy to break apart.
Ionic solids cannot conduct electricity because the ions are locked firmly in place. If you melt them or dissolve them in water, the grid falls apart. The freed ions can then move around and easily carry an electric current.