Weak Forces Decide Which Face Grows

Hydrogen bonds and van der Waals forces are much weaker than covalent, ionic, or metallic bonds, and they act over short ranges. They are also largely non-directional. Yet at a growing crystal surface, they determine how easily a new particle can attach to a particular face. A face where these weak interactions are more favourable grows faster, and the relative growth rates of different faces determine the crystal's final habit. A change in the interaction pattern can also favour a different lattice arrangement, producing a different polymorph.