Classification of solids: molecular, ionic, covalent and metallic solids

Introduction

Solid is a physical state in which all the  constituent particles are held together by strong forces of attraction, and rearrangement of solid particles closely packed symmetrically or asymmetrically so we can say a solid has a definite  shape, boundary, fixed volume. Some solids are strong, while others can break when pressed. There are two types of solid: crystalline and amorphous solids. A crystalline solid is a substance in which the constituent particles are arranged in a regular, orderly pattern. An amorphous solid is one in which the constituent particles are not arranged in a regular, orderly pattern.

On the basis of the binding forces between their constituent particles, solids are divided into four main categories:

1. Ionic Solids 

The constituent particles of an ionic solid are cations and anions. These ions are arranged in three-dimensional space in a systematic manner. The binding force in an ionic solid is the strong electrostatic attraction between cations and anions. In the crystal, the ions are alternately arranged as A+B–.

The number of ions that surround a specific ion of opposite charge is referred to as the ion’s coordination number. Ionic solids are hard and brittle, with high melting points and a high fusion enthalpy. In the solid state, ionic solids are insulators. They conduct electricity in the molten state and in aqueous solutions. Sodium chloride (NaCl), potassium chloride (KCl), and lead bromide (PbBr2) are examples of ionic solids.

1. Metallic Solids :

At room temperature, metals such as iron, copper, gold, silver, sodium, and potassium exist as solids. Metallic bonding is a strong force of attraction that holds the atoms together in a metallic crystal. The metal occupies fixed positions in metallic crystals, but the valence electrons move. Some characteristics of metallic solids :

• They are hard, but ductile and malleable.
• They are good heat and electricity conductors.
• They have a lustrous appearance.
• The melting and boiling points of these substances are extremely high.

Fusion heats are moderate.

1. Covalent Solids:

Covalent solids are solid substances in which covalent bonds bind the atoms throughout the crystal. Networks of atoms or molecules are held together by covalent bonds to form covalent solids. A single giant molecule is a perfect single crystal of a covalent solid. The covalent network in diamond, for example, is made up entirely of carbon atoms. They’re extremely hard. The melting points of these substances are extremely high.

Both in terms of heat and electricity, they’re poor conductors (except graphite which is an atomic solid and is a good conductor of electricity).

Their fusion heat is extremely high.

1. Molecular Solids:

These are solid substances in which the constituent particles are molecules. London dispersion forces, hydrogen bonds,Dipole–dipole interactions, or any combination of these hold atoms or molecules together in molecular solids. Types of molecular solids :

• NON-POLAR: Nonpolar molecules are constituent particles in these types of solids. Van der Waals’ force of attraction, also known as the London dispersion force, is responsible for holding these molecules together. This molecular attraction force is weak. They are soft. Their melting point is low. They have a low fusion enthalpy. They’re bad conductors. They are flammable as well as electrical insulators. Some nonpolar molecules solidify and emit their distinctive odour. 
• Examples are dry ice, iodine, naphthalene, wax, etc.
• POLAR: Polar molecules with permanent dipole moments are constituent particles in such solids. The force that holds polar molecules together is called dipole-dipole interaction, and it is much stronger than dispersion forces. Example: Solid ammonia (NH3) and solid sulphur dioxide (SO2).

1. These solids are a bit flimsy.
2. They are poor heat and electricity conductors.
3. They melt at a higher temperature than nonpolar molecules.
4. Their melting points are lower than those of ionic solids.
5. Some molecular solids sublimate and emit a distinct odour.

• HYDROGEN BONDED: This type of bonding is possible in all solids that have hydrogen atoms bonded to highly electronegative atoms(like F, O, N) in their molecules. 

1. They are hard solids.
2. They are inefficient heat and electricity conductors.
3. They have a low melting point.

Conclusion

Solids are classified as ionic, molecular, covalent, or metallic. The strength of the bonding is reflected in the lattice energy of ionic solids, which are made up of positively and negatively charged ions held together by electrostatic forces. Ionic solids are relatively hard and have a high melting point. Dipole–dipole interactions, hydrogen bonds, and London dispersion forces are all relatively weak forces that hold molecular solids together. As a result, they’re soft and have low melting points, which are determined by their molecular structure. Covalent solids are made up of two- or three-dimensional networks of atoms held together by covalent bonds and have high melting points. Metallic solids have unusual properties, such as high thermal and electrical conductivity, malleability, and ductility, as well as lustre, or a gleaming, light-reflecting surface.