Atomic structure refers to the way the atoms are arranged to form a molecule. Crystalline structure refers to the manner in which the molecules are arranged to form a crystal. For example: H2O has a structure as H-O-H where the 2 H's for a "V" shape with an angle of about 100 degrees of separation. When water freezes, the H2O molecules re-arrange themselves to form crystals. The bonds are not strong which is why you can shatter ice easily. Si2O3 (silicon oxide) can also form crystal to create glass. Plastics (carbon-hydrogen chains ) on the other hand cannot form crystals, this is why plastic breaks and does not shatter.