Solid State

In molecular solids, molecules are formed due to covalent bonds between the atoms. The bonding between the molecules depends on whether the molecules are polar or nonpolar. If the center of negative charge in a molecule coincides with the center of the positive charge, the molecule is called nonpolar (e.g., hydrogen, oxygen, chlorine). Otherwise, the molecule is called a polar molecule (e.g., water). The bond between polar molecules is called a dipole-dipole bond, and the bond between nonpolar molecules is called a van der Waals bond. Molecular solids are usually soft, have low melting points, and are poor conductors of electricity.

In an ionic solid, the lattice points are occupied by positive and negative ions. The electrostatic attraction between these ions binds the solid. These attraction forces are quite strong, so the material is usually hard and has a fairly high melting point. They are poor conductors of electricity.

In a covalent solid, atoms are arranged in the crystalline form. The neighboring atoms are bound by shared electrons. Such covalent bonds extend in space so as to form a large solid structure (e.g., diamond, silicon). Each carbon atom is bonded to four neighboring carbon atoms in a diamond structure. They are quite hard, have high melting points, and are poor conductors of electricity.

In a metallic solid, positive ions are situated at the lattice points. These ions are formed by detaching one or more electrons from the constituent atoms. These electrons are highly mobile and move throughout the solid, just like a gas. They are very good conductors of electricity.

There are several solids which do not exhibit a long range ordering. However, they still show a local ordering so that some molecules (say 4–5) are bonded together to form a structure. Such independent units are randomly arranged to form the extended solid. In this respect, the amorphous solid is similar to a liquid which also lacks any long range ordering. However, the intermolecular forces in amorphous solids are much stronger than those in liquids. This prevents the amorphous solid to flow like a fluid. A typical example is glass made of silicon and oxygen together with some other elements like calcium and sodium. The structure contains strong Si–O–Si bonds, but the structure does not extend too far in space. The amorphous solids do not have a well-defined melting point. Different bonds have different strengths and as the material is heated the weaker bonds break earlier starting the melting process. The stronger bonds break at higher temperatures to complete the melting process.