Solid State

Solid-state is nothing but one of the states of matter. In our early days of schooling, we have learnt that matter exists in three states – solid, liquid and gas. However, as we progress from lower to higher classes the concepts get more advanced and there are more things to learn. In this lesson, we will be dealing with the concept of solid-state in a broader aspect and understand all the underlying terms including the properties as well as the types of solids.

Solids generally exhibit certain characteristics that set them apart from liquids and gases. For example, they have the ability to resist any force that is applied to its surface. However, the solid-state of compounds largely depends on the properties of atoms such as their arrangement and the forces acting between them.

Table of Content

• Physical and Chemical Properties of Solids
• Solid State Chemistry
• Types of Solid States
• Crystalline Solid State
• Amorphous Solid State

Physical and Chemical Properties of Solids

• Solids are incompressible, meaning the constituent particle is arranged close to each other and because of that, there is negligible space between the constituent particle.
• Solids are rigid. This is due to lack of space between the constituent particles which make it rigid or fixed.
• Solids have definite mass, volume and shape due to which it has a compact arrangement of constituent particles.
• The intermolecular distance between molecules is short. Due to this, the force between the constituent particles (atoms, molecules or ions) is very strong.
• The constituents particles can only oscillate about their mean positions.

Also Read: Defects in Solids

Solid State Chemistry

Solid-state in Chemistry is the study of the structure, properties and the synthesis of solid materials. It is also sometimes called as materials chemistry. More significantly, in solid-state chemistry, we study the concept of a compound in a more deeper level. It basically helps us understand the compound from a molecular level to the crystal structure level.

In solid-state chemistry, nonstoichiometric compounds are categorized as exclusive solid-state compounds due to their structure and thermodynamic characteristics.

Types of Solid States

Based on the arrangement of constituent particles, solids are classified into two-state types:

1. Crystalline Solids
2. Amorphous Solids

Crystalline Solid State

Crystalline solids are those that have a typical geometry.  In such type of solids, there are definite arrangements of particles (atoms, molecules or ions ) throughout the 3-dimensional network of a crystal in a long-range order. Examples include Sodium Chloride, Quartz, Diamond, etc.

Properties of Crystalline Solids

• Crystalline solids have a sharp melting point and start melting when it reaches a particular temperature.
• The shape of crystalline solids are definite and having typical arrangements of particles.
• They show cleavage property i.e. when they are cut with the edge of a sharp tool they split into two pieces and the newly generated surfaces are smooth and plain.
• They have definite heat of fusion (amount of energy needed to melt a given mass of solid at its melting point).
• Crystalline solids are anisotropic which means their physical properties like electrical resistance or refractive index show different values when they are measured along with different directions in the same crystal.
• Crystalline solids are true solids.

Types of Crystalline Solids

On the basis of the nature of intermolecular forces or chemical bonding, crystalline solids are further classified into four categories. They are,

• Molecular Solids
• Ionic solids
• Metallic solids
• Covalent solids

Molecular solids

In molecular solids the constituent particles are molecules. They are further divided into three categories:

1. Non-Polar Molecular Solids

These solids are formed from molecules or atoms that share a non-polar covalent bond. The atoms or molecules are held by weak dispersion force or by London forces.

• The physical nature of non-polar solids are soft.
• They don’t conduct electricity so they are insulators.
• They have a very low melting point.
• Examples: H₂, Cl₂, I₂ etc.

2. Polar Molecular Solids

• These solids are held together by polar covalent bonds and the atoms/molecules are bonded by relatively stronger dipole-dipole interactions.
• The physical nature is soft and most of these are gases or liquids at room temperature.
• They do not conduct electricity and they have a higher melting point than the non -polar molecular solids.
• Examples: HCl, SO₂, NH₃ etc.

3. Hydrogen-Bonded Molecular Solids

The solids contain polar covalent bonds with Hydrogen, Fluorine, Oxygen and Nitrogen atoms. In these solids, molecules are held together via strong hydrogen bonding.

• The physical nature of such solids are hard.
• They do not conduct electricity.
• The physical state of these solids are volatile liquids or soft solids under room temperature.
• They have a low melting point.
• Examples: H₂O (Ice ).

Ionic Solids

In ionic solids the constituent particles are ions. These are formed by the arrangement of cations and anions by strong Coulombic forces.

• These are hard and brittle in nature.
• Ionic solids act as an insulator in a solid-state but are conductors in a molten and aqueous state.
• They have a high melting point.
• Example: NaCl, MgO, ZnS, CaF₂ etc.

Metallic Solids

• Positive metal ions in a sea of delocalized electrons. These electrons are evenly spread out throughout the crystal.
• Due to the presence of free and mobile electrons, they are responsible for high electrical and thermal conductivity.
• They are conductors in both solid and molten state.
• The physical nature of these solids are hard but they are malleable and ductile.
• They have high melting point than ionic solids.
• Examples: Fe, Cu, Ag, Mg, etc.

Covalent or Network Solids

A wide range of crystalline solids of non-metal form covalent bond between adjacent atoms throughout the crystal and form a giant molecule or large molecules.

• These solids are hard like diamond and soft like graphite which are isotopes of carbon.
• They are insulators as in the case of a diamond but in case of graphite due to free electrons, they conduct electricity and act as a conductor.

Amorphous Solid State

Amorphous solid-state comprises of those solids which have the property of rigidity and incompressibility but to a certain extent. They do not have a definite geometrical form or long range of order. Examples include glass, rubber, plastic, etc.

Properties of Amorphous Solids

• Amorphous solids are gradually softened over a range of temperature and they can be moulded into different shapes on heating.
• Amorphous solids are pseudo solids or super cooled liquids which means they have a tendency to flow very slowly. If you observe that the glass pans which is fixed to windows of old buildings they are found to be slightly thicker from the bottom than at the top.
• Amorphous solids have irregular shape i.e. their constituent particles do not have definite geometry of arrangements.
• When amorphous solids are cut with a sharp edge tool they form pieces with irregular surfaces.
• Amorphous solids do not have definite heat of fusion due to its irregular arrangement of the particles.
• Amorphous solids are isotropic in nature which means the value of any physical property would be same along any direction because of the irregular arrangement of particles.

Amorphous Solids Uses

Amorphous silicons which is one of the best photovoltaic material converts sunlight into electricity.

Types of Unit Cells