Most solid substances are crystalline in nature. Based on the intermolecular forces acting between them, the Crystalline Solids can be further classified
Most molecular solids are insulators, relatively soft, have low densities. Examples of molecular solids are sugar, solid halogens, sulphur and ice (made of water). Molecular solids themselves are further classified into different types.
Non – Polar Molecular Solids:
Such solids have an asymmetrical distribution of electrons and therefore there is no abundance of charge on either side. Opposite charges cancel each other out. Examples are methane, chlorine, hydrogen, oxygen. At room temperature and pressure, they are liquids or gases. The force binding or holding the molecules together in these solids, are Vander Waals forces (weak dispersion or London forces). These forces are weaker than ionic or covalent bonds.
Polar Molecular Solids:
In these solids, the geometry is such that one side has a negative charge, and the other has a positive charge. The force holding them together is a dipole – dipole force of attraction. Their melting and boiling points are higher than non-polar molecular solids but still relatively low. Examples of polar molecules are ethanol and ammonia.
Hydrogen-Bonded Molecular Solids:
In these kinds of solids, the intermolecular forces are strong hydrogen bonds. Their boiling and melting points are higher compared to polar and non – polar molecular solids. They exist as volatile liquids or soft solids at room temperature and pressure. Water is an example of hydrogen-bonded molecular liquid.
Constituent particles in ionic solids of the Crystalline Solids are anions (negatively charged) and cations (positively charged). An ion is surrounded by a typical number of opposite charges. For example, in NaCl, the Na+ ion is surrounded by 6 Cl- ions. Ions in these solids are held together by strong electrostatic forces. They have high melting and boiling points and are soluble in polar solvents but not in non-polar solvents.
Covalent Solids or Network Solids
What are covalent solids? In these solids, atoms of molecules are held to each other by covalent bonds. A network of interconnecting covalent bonds throughout the crystal thus leads to the formation of a giant molecule. They have high melting and boiling points and are generally bad conductors with the exception of graphite which is a good conductor (due to the fourth electron being free).
What are metallic solids? In these solids, the constituent particles in metallic solids are metal atoms which have valence electrons to be given/lost, therefore ending up being positively charged. The sea of electrons available is spread all through the crystal and can easily move about. The attractive force between positively charged ions and sea of electrons is the cause for metallic bond formation. This is the force holding the metal ions together. Metallic solids have a regular structure with high melting and boiling points. Because of the sea of electrons they have high thermal and metallic conductivity. All metals and alloys are metallic solids.
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