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Characteristics of Crystalline Solids

◆ Crystalline solids are usually very stable―the Gibbs free energy of these solids is very low. Their lattice energy is very high (energy given off to form the lattice). Therefore, a large amount of energy is needed to break the lattice structure of the solids (latent heat of fusion). ◆ Some atoms that form the crystalline solid molecules are capable of crystallizing in such a way that they may give rise to more than one crystalline structure. This occurs due to different spatial arrangements of the same atoms and is called polymorphism or allotropy of crystalline solids. ◆ These solids show the property of anisotropy (the property of a material to exhibit different mechanical and physical characteristics like absorbance, refractive index, tensile strength, etc., along different axes.) ◆ These solids have a specific temperature at which they melt and are said to have a sharp melting point. This is because the atoms are arranged in such a manner that they are at equal distances from their neighboring atoms, and the forces of attraction between all the atoms is exactly the same. ◆ These solids have an ordered structure which tends to repeat itself at equal distances. Thus, the arrangement of atoms inside the crystalline solids can be said to be periodic. These repeating units have a definite structure and are called unit cells. They can be said to be the smallest unit of a crystalline solid. ◆ Crystals are usually microscopic structures, and the ones that are visible to us are usually due to the fusion of these crystallites, to form a polycrystalline structure. Although polycrystals are made of many crystals, they cannot be considered as true crystals because the arrangement of the atoms is not in a periodic manner. ◆ Crystalline solids have a distinctive internal structure that leads to distinct flat surfaces or faces; the angles at which these faces intersect is an intrinsic property of the solid. Depending on this, they can be classified as euhedral (have well-formed faces), subhedral (have somewhat flat surfaces) and anhedral (do not have well-formed faces). ◆ When a crystalline solid is cleaved, it breaks along fixed planes and gives rise to new faces. These faces intersect each other at angles that are same as that in the original solid.

Types of Crystalline Solids

These have metal atoms with a specific spatial arrangement that form the unit cells. Metals tend to lose their outermost valence electrons easily, giving rise to delocalized electrons and positively charged ions or cations. These delocalized electrons are capable of moving throughout the metal. The intermolecular forces that hold these atoms together are metallic bonds. Properties ● They maintain a proper rigid structure; however, their hardness may vary. ● Melting point of metals is usually very high with a few exceptions. ● They are lustrous, malleable, and ductile. ● They are good conductors of heat and electricity. Examples: Tin, aluminum, iron, gold, copper, etc.

In this, each unit cell is composed of a cation that is surrounded by a particular number of anions or, an anion that is surrounded by a particular number of cations. The intermolecular forces that holds these atoms together are electrostatic forces of attraction. Properties ● They maintain a proper rigid structure that is usually hard. ● Melting point is usually very moderate to very high. ● In a solid state, they are bad conductors of heat and electricity. ● They are soluble in polar solvents like water. Example: NaCl, MgO, NaNO₃, etc.

These can be further classified as: Polar: Here, the molecules that make each unit cell are polar in nature, i.e., these molecules have a partial positive and a partial negative charge. The intermolecular attractive forces that exist here are dipole-dipole forces of attraction. Properties ● They have little hardness. ● They are bad conductors of heat and electricity. ● Their melting point is low (exist as liquids or gases at room temperature), but is greater than that of non-polar molecular crystalline solids. Non-polar: These can be non-polar molecules or atoms that make each unit cell. The attractive intermolecular forces between them include van der Waals forces and London dispersion forces. These forces are very weak and give rise to the following physical characteristics. Properties ● They are soft in nature. ● They are bad conductors of heat and electricity. Hydrogen-bonded: In these solids, the molecules that form the unit cells are polar molecules, which contain either hydrogen, oxygen, fluoride, or nitrogen. Hydrogen bonds are the attractive forces that hold the molecules together. Properties ● They have a moderate to low melting point. ● They are soluble in solvents that possess hydrogen bonds.

Here, the atoms that make every unit cell are connected to the adjacent cell with the help of covalent bonds. The atoms form a large network of covalent bonds extending throughout the molecule. These give rise to a giant molecule. Properties ● They are hard and brittle due the covalent bonds they possess. ● They have a high melting point. ● Some are good insulators, while others like graphite are good conductors of electricity. Some like aluminum nitride are semiconductors, and some like diamond are bad conductors of electricity. Examples: Diamond (C), silicon carbide (SiC), graphite (C), silicon dioxide (SiO₂), etc.