# Types of Crystals

Types of Crystals

Although there is no unique way to classify all the crystalline solids found in nature, even then these are divided into four important types on the basis of chemical binding of the constituent atoms.

1. Ionic Crystals: These are formed by a combination of highly electropositive ions (cations) and highly electronegative ions (anions). Thus strong electrostatic force of attraction acts within the ionic crystals. Therefore, a large amount of energy is required to separate ions from one

another. The type of the crystal lattice depends upon (i). The size of the ion (ii). The necessity for the preservation of electrical neutrality. Therefore alternate cations and anions in equivalent amount are arranged in the ionic crystal e.g. NaCl, KF, CsCl, $BaSO_4$ etc.

2. Covalent Crystals: These are formed by sharing of valence electrons between two atoms resulting in the formation of a covalent bond. The covalent bonds extend in two or three dimensions forming a giant interlocking structure called network. Diamond, silicon, quartz and graphite are good examples of this type.

3. Molecular Crystals: In these crystals, molecules occupy the lattice points of the unit cells, except in solidified noble gases in which the units are atoms, where the binding is due to van der Waals’ forces and dipole-dipole forces. Since van der Waals’ forces are non-directional, hence structure of the crystal is determined by geometric consideration only. Solids like $H_2, O_2, CO_2, I_2, S_8$, sugar etc. are well known examples of such crystals in which van der Waals forces are acting. Ice is the common example in which dipole-dipole forces of attraction (hydrogen bonding) are active. Many organic and inorganic crystals involve hydrogen bonds. Although these are comparatively weaker but they play a very important role in determining the structure of substances e. g. polynucleoides, proteins etc.

4. Metallic Crystals: These are formed by a combination of atoms of electropositive elements. These atoms are bound by metallic bonds. It may be defined as:

The force that binds a metal ion to a number of electrons within its sphere of influence is known as metallic bond.

OR

A bond which is formed between electropositive elements.

OR

The attractive force which holds the atoms of two or more metals together in a metal crystal or in an alloy. We know that the force of attraction between metal ions and valency electrons is very strong. This force of attraction is responsible for a compact solid structure of metal.

The important characteristics of the various types of crystals  are given in the following table:

Some important characteristics of various thype of crystals

1.       Ionic crystals

Units that occupy lattice points: cations and anions

Binding forces: Electrostatic attraction between ions

Hardness: Hard

Brittleness: Brittle

Melting point: High

Electrical Conduction: semiconductor, due to crystal imperfections, conductor in fused state

Solubility in Polar solvents: soluble

Heat of vaporization: NaCl(s)

$(kJ mol^{-1})$: 170.75

Heat of Fusion: NaCl

$(kJ mol^{-1}$ ): 28.45

Example: $NaCl, KNO_3, CsCl, Na_2SO_4, ZnS$

2.       Ionic crystals

Units that occupy lattice points: atoms

Binding forces: shared electrons

Hardness: very hard graphite is soft

Brittleness: intermediate

Melting point: very high

Electrical Conduction: non-conductor, graphite is good conductor

Solubility in Polar solvents: insoluble

Heat of vaporization: graphite

$(kJ mol^{-1})$: 718.43

Heat of Fusion:  -

$(kJ mol^{-1}$ ):  -

Example: Diamond, graphite, Quartz$(SiO_2)$, SiC

3.       Molecular Crystals

Units that occupy lattice points: molecules

Binding forces: van der Waals or Dipole-dipole

Hardness: soft

Brittleness: low

Melting point: low

Solubility in Polar solvents: soluble as well as insoluble

Heat of vaporization: $NH_3(s)$

$(kJ mol^{-1})$: 23.55

Heat of Fusion: $NH_3(s)$

$(kJ mol^{-1}$: 5.65

Examples: $H_2O(s), CO_2(s)$, sulphur, sugar, iodine noble gases

4.       Metallic crystals

Units that occupy lattice points: positive ions in a “sea or pond” of electrons

Binding forces: Electrostatic attraction between positively charged ions and negatively charged electrons.

Hardness: Hard or soft

Brittleness: low

Melting point: varying from moderate to high

Electrical Conduction: Good conductor

Solubility in Polar solvents: insoluble

Heat of vaporization: Cu(s)

$(kJ mol^{-1})$: 304.59

Heat of Fusion: $Cu(s)$

$(kJ mol^{-1}$: 13.016

Example: Na, Cu, Ag, Fe, Pt, alloys

Isomorphism:

The occurrence of a given substance in more than one solid crystalline form having different physical properties is known as polymorphism. This property when occurs in elements is known as allotropy.

Sometimes we come across examples of chemically different solids which crystallise in the same crystalline shape. Such substances are said to be Isomorphism (same shape), their chemical constitutions are very similar and in some cases crystals of one substance may continue to grow when placed in a saturated solution of the other e. g. potash alum and chrome alum crystals have the same shape and can be grown in each other’s solutions.

Mitscherlich deduced that isomorphous substances have similar chemical formula e.g. phosphates and arsenates are said to be isomorphous with one another viz. $Na_2HPO_4,.12H_2O \text{and} Na_3AsO_4.12H_2O$. However, the law is not without exceptions.

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