# Crystals Grouped by Chemical Properties

Gems and minerals are recognized by their crystal structural symmetry, chemical, and physical properties. For crystals, they are divided into 2 categories i.e. the lattices can be primitive (only one lattice point per unit cell) or non-primitive (more than one lattice point per unit cell). In the previous article, we learned about 7 crystal structures or system which are the following:-

1. Cubic
2. Hexagonal
3. Tetragonal
4. Trigonal
5. Orthorhombic
6. Monoclinic
7. Triclinic

It should be kept in mind that there is no specific way to distribute crystals or divide them. Though here, we are describing 4 crystals type properties wise on the basis of chemical binding of the constituent atoms. They help us understand minerals and gemstone identification.

### Group of Crystals by their Properties

According to the physical properties and chemical properties, crystals are divided into 4 main categories:;-

1. Covalent Crystals
2. Metallic Crystals
3. Ionic Crystals
4. Molecular Crystals

Covalent Crystals:

• In this type of crystals, all atoms consist of true covalent bonds

between each of them. It can be called like one big molecule.

• Many covalent crystals are melted at high temperature or they have a very high melting point of. Examples are diamonds and zinc sulfide crystals.
• Very stable crystals

Metallic Crystals:

• Metallic crystals usually have metal elements or formed by metal elements.
• Their main point is that they conduct electricity.
• They are malleable
• Their surface usually shines.
• They are ductile
• Outer electrons of metal atoms float around the lattice because these metal atoms individually sit on lattice sites.
• High melting point
• Tend to be very dense

Ionic Crystals:

• Are usually hard substance
• Posses ionic bond
• Relative high melting point and boiling point
• oppositely charged ions are joined together by strong electrostatic forces
• Conduct electricity in form of a solution and molten state both
• Usually are brittle
• Can not be spread on a sheet or drawn on a sheet
• They are not ductile
• Table salt is an example

Molecular Crystals:

• Molecules are joined together by weak Van der wall force or hydrogen bonding
• Noncovalent interaction let the molecular crystal held together
• An example is rock candy, sulfur, table sugar. like,$H_2, O_2, CO_2, I_2, S_8$ sugar etc.
• Low melting and boiling point
• They evaporate soon and thus are volatile i.e $NH_3(s)$ $(kJ mol^{-1})$: 23.55
• Hardness is soft

One material can have many crystal structures or lattice. It is not a hard and fast rule that we will find all crystals same we have described above.