What are Solids? 

Solids are compounds with a definite structure and volume, hardness, elevated density but low elasticity. The components (atoms, molecules, and ions) in solids are packed tightly, and the strong intermolecular interactions hold them together.

Solids play an important role in our day to life. Food, utensils, and all such components which we use daily are examples of solid.

Solid Types 

Solids can be broadly classified into two forms crystalline solids and amorphous solids. This classification is fundamentally based on the arrangement of the components in the given solid.

Crystalline Solids  

A crystalline solid typically contains several little crystals, all showing a specific geometrical pattern. The atom, molecule, or ion components of a crystal are highly organized. It also shows a long line of order which indicates that the organization sequence of particles in crystalline solids regularly repeats itself throughout the crystal after regular intervals. Crystalline solids are also called true solids with particular melting points. Crystalline solids are anisotropic as they exhibit different values for certain physical properties, such as refractive index, when measured from different directions for the same crystalline form. The anisotropic nature is mainly due to the pattern of arrangement of different solids components in different directions. Crystalline solids, when cut, will generally give rise to two pieces, with both of the pieces being smooth and having a regular surface.

Examples of crystalline solids include quartz and sodium chloride.

Amorphous Solids 

They do not have a specific arrangement of their constituent particles. They don’t have sharp melting points; instead, they have a temperature range and melt slowly, allowing them to be molded into different shapes. Therefore, amorphous solids are also called supercooled liquids or pseudo solids as they are sometimes found in liquid state, but the flow speed is quite slow. One application of this property can be observed in the preparation of glass used in windows. It can be noticed that one end of the window glass is thicker than the other, mostly because the flow of glass is slow, leaving one end to be thicker than the other. Amorphous solids have a short range of arrangement of their constituent particles making the arrangement irregular. This property of amorphous solids leads to its isotropic nature, which means it has the same value for its physical properties when measured from any direction. Amorphous solids, when cut, will give two uneven surfaces.

Examples of amorphous solids include glass and plastics.

Crystalline Solids Classification 

Crystalline solids can further be separated into molecular, ionic, metallic, and covalent solids depending upon the intermolecular forces acting between the solid constituents.

Molecular Solids

Molecules solids are made with molecules as their main component. Molecular solids can be further divided into polar, non-polar, and hydrogen-bonded molecular solids.

Polar molecular solids

The molecules are kept together by significantly powerful dipole and dipole associations among them. These types of solids are flexible, and they do not conduct electricity. They possess high melting points compared to non-polar molecular solids, but at normal temperature and pressure, the majority are found in gases and liquid states. Examples of polar molecules include solid ammonia.

Non-polar molecular solids

These solids are also flexible in appearance and are also non-conductors of electricity like the polar molecular solids. The components are held together not by strong but instead weak forces such as London dispersion forces. These solids show a high melting point and are in liquid or gaseous form in normal conditions. Argon is an example of non-polar solids

Hydrogen bonded molecular solids

These solids have polar covalent bonds between atoms such as H and F. These solids show non-conductivity of electricity and are usually found in a soft state in normal conditions.

Ionic Solids 

These solids have ions as their comprising particle and are created by the 3-D assembly of cations and anions through powerful electrostatic forces (columbic). Ionic solids are rigid but brittle. These solids have high boiling and melting points. These solids act as perfect insulators in the solid-state as they don’t have freely moving ions. But when transformed into the liquid state, they can conduct electricity as they now have freely moving ions that conduct electricity.

Metallic solids 

Metallic solids are the positive ions held together through mobile free electrons that are evenly spread throughout the metal crystal. These solids are high conductors of electricity and thermal energy due to these free electrons, which are contributed by the metal atom involved in the crystal structure.

Covalent Solids 

These are also known as giant molecules. Covalent solids are very strong, and the atoms are placed tightly to their respective positions. Non–metal crystalline solids are formed by the covalent bonding between the neighboring atoms present in the crystal. These solids act as good insulators and have high melting points. Covalent solids are hard as well as brittle in nature.

Crystal Lattice 

A normal but recurring sequence of constituent elements is the major feature of crystalline solids. The arrangements are called crystal lattices if the three-dimensional layouts of the constituent elements are diagrammatically expressed. A standard three-dimensional array of points is then referred to as a crystal lattice. Fourteen possible three-dimensional lattices are postulated, and these are together called Bravais Lattices.

Unit Cells

A unit cell is the tiniest fragment of the crystal lattice that is recurring in various directions forming a whole lattice. Unit cells are classified into primitive unit cells and centered unit cells.

Primitive Unit Cells 

In these cells, the particle components are present at the corners of the unit cell.

Centered Unit Cells

This cell type has particle components located at positions other than the corner in a unit cell. It can be divided into three forms, namely body-centered unit cells, face-centered unit cells, and end-centered unit cells. Body-centered unit cells have a particle component at their body center and others present at the corners. Face-centered unit cells are those in which one of the particle constituents is found at the center of every face, along with those present at the corners of the lattice. In end-centered unit cells, one of the particle components is located in the middle of two opposed faces, and the others are located at the corners.

Common Mistakes 

Do not confuse between anisotropic and isotropic properties. Both say about the physical properties, but for anisotropic, the properties change, whereas, for isotropic, the properties remain the same.

Practice Problems 

Classify the following as amorphous, metallic, ionic, or molecular solids

NaCl, Cu, CO₂, graphite, H₂O.

Solution:

Ionic solid: NaCl

Molecular solid: H₂O, CO₂

Ionic solid: Cu

Metallic solid: graphite

Context and Applications  

This topic is significant in the professional exams for undergraduate and graduate courses, especially for Bachelors and Masters in Chemistry.