# Why does crystalline solids have well defined 3d geometry

Crystalline solids have well defined geometry because the intermolecular forces are uniform throughout the solid unlike amorphous solid where it varies from place to place.

But still the question hold why does some solids have this property like why $\ce{NaCl}$ forms a crystalline solid but not glass or something else.

I thought it is because crystalline solids are ionic but diamond is an counterexample among many.

Question :

Why does some solids have crystalline structure while others don't ?

Keep two concepts separate: the solid crystalline state is not bound to occur on ionic compounds only.

In a simple view, crystalline describes a regular arrangement of matter, with predictable order both in short, as well in long range. For the crystalline state, it is unimportant if the constituents of the crystal are atoms (like metal atoms), charged particles (like your example of NaCl), or neutral molecules (like benzoic acid) or even a mixture of neutral and charged molecules (many protein crystals contain water).

Some materials are easier packed in such a regular pattern, than others. In the instance of metals, their (on first approximation) sphere like atomic geometry and similar radius eases such a settlement. Inorganic salts tend to be a bit more complicated, as often smaller ions are localized in gaps built by larger ions (octahedral / tetrahedral gaps).

Organic molecules may equally build crystals, yet their shape and conformational flexibility around single bonds (for example in alkyl chains) and multiple intermolecular interactions may keep them over a long time in a glassy state (like supercooled benzoic acid, or some brands of honey). More than for the aforementioned examples: formation of a nice crystal may be thermodynamically favourable, but it is not enough "just to cool down" below the melting temperature to reach this energetic minimum. There must be seeds of crystallization, and typical conditions of their formation (temperature, concentration, etc.) are not necessarily the same where nicely shaped crystals growth further.

It is not surprising crystals attract attention, for example in material science and crystallography, and that there are journals with focus on the relationship between molecular properties, and macroscopic / crystallographic ones; like CrystEngComm or Crystal Growth & Design.

• "some brands of honey", is honey a solid ?
– user31607
Apr 4 '17 at 7:55
• @A---B While "honey" is a mixture of several compounds, it is save to assume it largely consists of sugars, with fructose and glucose as the most important contribution among them. These, if pure, are solids. The ease honey crystallizes out correlates with the ratio of fructose : glucose, because yes, if not solid, honey is a supercooled liquid (cf. en.wikipedia.org/wiki/Honey#Phase_transitions, for example). It is one of the examples, outside the chem lab, that illustrate how thermodynamics is about energies, and kinetics about the rates reactions and phase transitions occur. Apr 4 '17 at 8:23