Answer: orderly arrangement of particles
Explanation:
In a crystaline solid, the particles that compose the solid are arranged or packed in an orderly manner to form a three dimensional crystal lattice with a defined structure. Sodium chloride is a crystaline solid.
In an amorphous solid, the particles that compose the solid aren't arranged in an orderly manner hence the solid tends to be brittle, e.g glass
Because the speed of light is determined by the medium that is it in. If light is in a vacuum it goes 300 million meters per second but it is slower in air, and even slower in water because water is more dense than the previous two.
