Ionic Crystals
These were described earlier. Example: NaCl. The ions in NaCl are water soluble, giving a conductive solution. Solid NaCl is a poor conductor.
Molecular Crystals
I2 is a purple solid that is so weakly bound that gentle heating causes I2 gas to form without first forming a liquid in a process called sublimation.
Covalent Network Solids
These are among the strongest solids known. SiO2 and C (diamond) are well-known examples.
Metallic Crystals
We have previously described the "sea of electrons" in which metal atoms swim, allowing metals to conduct heat and electricity. Metals are often deformable when pure. Making polycrystalline alloys "puts grit in the gears" and provides greater hardness.