Even though a solid looks completely still, its particles are actually moving. Atoms and molecules in a solid constantly vibrate back and forth in place. The type of bond holding them together decides the traits of the solid.
Ionic bonds make hard and brittle solids that melt at very high temperatures. Metallic bonds allow electrons to move freely, making metals good conductors of heat and electricity. Covalent network bonds create extremely hard materials like diamond and quartz.
Intermolecular forces form softer molecular solids like ice or candle wax. Chemists group solids into two main types based on their internal structure. Crystalline solids have particles arranged in a perfect three-dimensional repeating pattern.
This neat order gives them flat outer faces and sharp melting points. When you heat a crystal, all the bonds break at the exact same temperature. Amorphous solids have particles jumbled together without any regular repeating pattern.
Common everyday materials like glass and rubber are examples of amorphous solids. When you heat them, they just get softer and softer over a wide temperature range. Students often think that any hard material must be a true crystal. However, true crystals require that perfect and repeating atomic pattern inside.