The internal structure of a crystalline solid follows strict geometric rules. The smallest repeating piece of this pattern is called a unit cell. You can think of it like a single brick in a large building.
Stacking these identical cells in all directions builds the entire crystal. This perfect internal order gives the solid a specific shape with flat faces. It also dictates how the material handles heat.
A crystalline solid melts at one exact temperature. When you reach that specific melting point, all the identical bonds break at exactly the same time. A common misconception is that all crystals must look like shiny gems.
In reality, many everyday metal tools and rocks are made of tiny crystals packed tightly together. Chemists sort these solids into four main groups based on their bonds. Ionic solids stick together using opposite electrical charges.
Molecular solids rely on weaker forces between whole molecules. Covalent network solids share electrons in a massive continuous web. Metallic solids let a sea of electrons flow freely around metal atoms.
Materials that lack this internal order are called amorphous solids. Glass and plastic are common examples of amorphous materials. They just soften gradually when heated instead of melting suddenly.