Why is diamond harder than gold?

stronger bonds between atoms. Why? because diamond forms under intense pressure that forces atoms closer together than they want and once there, they are held together tightly (many neighbors hold a given atom in place; a lot of energy is needed to move an atom against all those "strings" holding it in place. You could think of it as the energy of pressure gets stored in the atomic structure.

gold forms a weak chemical structure. easier to move an individual atom in the structure so the solid is more malleable and softer. individual bonds are not strong. this is basically true of everything: how hard and malleable a substance is depends on how easily atoms and molecules can get moved around. loose structures with weak covalent bonds tend tobe the easiest to disrupt.