Crystalline solids are one of the types of solid-state substances. Solids are those substances that have a melting point above room temperature at atmospheric pressure. Depending on the arrangement of their constituents, solids are of two types, namely Amorphous solid and Crystalline solids. Solids that have a regular and three-dimensional arrangement of constituent particles such as (atoms, molecules, or ions) are known as crystalline solids. A few examples of crystalline solids include sodium chloride, quartz, diamond, etc.
Properties of Crystalline Solids
The properties of crystalline solids are:
They have definite shapes and symmetries.
These are hard and rigid.
They have a high melting point.
They constitute a crystalline system.
Classification of Crystalline Solids
These solids are classified into four types on the basis of the nature of bonding present in their constituent particles.
Molecular Solids
Molecules are the constituent particles in these solids. These molecules are held together by weak Van der Waal’s forces of attraction. Due to the presence of weak forces, these solids are soft in nature. Molecular solids are bad conductors of electricity as there are no free electrons to conduct electricity. Their melting and boiling points are also low, so they vaporize easily. For example, ice, solid CO2 (dry ice), etc.
Molecular solids are again divided into 3 classes as follows.
Polar Molecular Solids
These molecular solids have a polar covalent bond between their molecules. The polarity in their bond is developed due to the difference in the electronegativity of the atoms which participate in bonding. Thus, partial charges are developed on atoms that form a dipole-dipole interaction force and this force holds the solid together—for example, SO2.
Non-Polar Molecular Solids
In these solids, atoms/elements form the molecule, which is further joined by a nonpolar bond to form this kind of molecular solid. These solids have weak Van Der Waals forces, so they are soft. No polarity is found in the bonds amongst these solids as the same atoms or molecules are joined like Cl2 (one chlorine atom is bonded to another by a single nonpolar bond).
Hydrogen-Bonded Molecular Solids
When Hydrogen makes a bond with fluorine, oxygen, or nitrogen, it is called a hydrogen bond. These are polar covalent bonds and are comparatively strong bonds. The polarity in hydrogen bonds is developed due to the electronegativity difference between hydrogen and the other element, which could be N/O/F. The solids in which these bonds are present are hydrogen-bonded molecular solids—for example, hydrogen fluoride (HF), water (H2O), etc.
Ionic Solids
These are the solids that are formed by ions. These ions are joined by the strong electrostatic forces of attraction within the solid. Ions are charged particles that are of two types- cations (positively charged) and anions (negatively charged). These ions are orderly arranged in the ionic solid.
The force of attraction between cations and anions are called an electrostatic force of attraction. These strong forces contribute to the hardness, brittleness, and high melting points of these solids. These solids conduct electricity only in a molten state/aqueous state. The reason is that only in these states the ions are free to move, unlike solid-state where they are fixed. Examples of such solids are sodium chloride (NaCl), lithium fluoride (LiF), etc.
Covalent Solids
These are also known as network solids as they are formed by an intense network of covalent bonds present in their adjacent atoms forming the solid. The constituent atoms/elements are neutral atoms and can be the same as in diamond (all atoms are of carbon joined together by covalent bonds) or can be different like in silicon carbide (SiC), also known as carborundum.
Diamond is the hardest substance in the world and it is a covalent solid. It is used in the glass cutting industry due to its hardness. Covalent solids are also bad conductors of electricity due to the absence of free electrons as all the electrons of constituent atoms are shared to form covalent bonds.
Metallic Solids
These solids have fixed positive ions surrounded by free electrons in their structure. Due to these free electrons, metallic solids are good conductors of electricity and heat. In the case of metallic solids, there are positive ions present in the pool of electrons. The melting and boiling points of metallic solids could range from moderate to high. These solids can be hard or soft (like sodium and potassium).
Metals like Copper, Nickel, Manganese are some examples.
Summary
Ionic crystals are made up of positive and negative ions.
Metal cations are surrounded by a "sea" of mobile valence electrons in metallic crystals.
Atoms that are covalently bound to one another make up the covalent crystals.
Weak intermolecular forces hold molecular crystals together.
FAQs on Classification of Crystalline Solids
1. Why graphite, a good conductor of electricity, is a covalent solid?
Graphite is a covalent solid with all the constituents as carbon is joined by the covalent bonds. Covalent solids are generally bad conductors of electricity as there are no free electrons to move within the area of the crystal. But, graphite is an exception in which carbon elements are joined in a hexagonal form and these hexagons are present in layers. Due to this hexagon layered arrangement, one electron out of four carbon atoms remains free to move out from each and every layer for the purpose of conducting electricity.
2. Arrange the following solids in increasing order of their hardness.
NaCl, diamond, ice, iron
Ice < NaCl < Iron < diamond
The hardness of a substance depends on the strength of bonds present in them. Ice has the weakest force due to the presence of Van Der Waal’s forces. Diamond is the hardest amongst them all because it is made up of carbon atoms, which makes it a rigid three-dimensional structure.
Iron is far denser than a diamond as each single steel molecule weighs much more than a single carbon atom. NaCl has ionic bonding.
3. Give the properties of crystalline solid?
Crystalline solids have well-defined edges and faces, diffract X-rays, and appear to have sharp melting points. Amorphous materials, on the other hand, have uneven or curved surfaces, lack well-resolved x-ray diffraction patterns, and melt across a wide temperature range.
