Three most common physical states of matter are solid, liquid and gases. These three states of matter are different from each other in the arrangement of particles. Solids have a regular and fixed arrangement of particles which are held together with strong intermolecular forces of attractions. The liquid state is characterized with the intermediate force of attraction between constituent particles. The gaseous particles have maximum kinetic energy and minimum intermolecular force of attraction between them. They move randomly in system and acquire the shape and volume of container. Solids can be classified as crystalline solids and amorphous solids. Crystalline solids are composed of atoms, ions, or molecules which are arranged in a regular, well-defined arrangement. Unit cell is the smallest repeating pattern of crystalline solids. Non crystalline solids or amorphous solids do not have much ordered arrangement of constituent particles such as glass and plastics.
On the basis of constituent particles; crystalline solids can be classified as ionic, molecular, atomic or covalent-network and metallic solids. Ionic solids are composed of oppositely charged particles which are held together with electrostatic force of attraction. Since ions are bonded with strong force of attraction, therefore such solids are non-conductor or insulator of heat and electricity in solid state. In the solution of ionic solids, the constituent ions become free to move and can conduct heat and electricity easily. Sodium chloride and potassium chloride are most common examples of ionic solids. Ionic solids have very high melting points. They are brittle and poor conductors in the solid state. Molecular solids are composed of atoms or molecules. The constituent particles are held together with some of the weak interactions like London dispersion forces, sulphide linkage, dipole-dipole forces, or hydrogen bonds. They have low melting points, flexibility and poor conductors. Sucrose and other carbohydrates are good examples of molecular solids. Covalent-network solids are composed of atoms which are bonded with covalent bonds. Due to strong covalent bonds, such solids are with high melting points. But they are poor conductor are electricity due to lack of ions. Diamond, graphite, and the fullerenes are good examples of covalent solids which are composed of carbon atoms. All these three solids have different arrangement of carbon atoms which affects their physical and chemical properties.
The solids, in which the constituent particles are metal atoms, are called as metallic solids. In such solids, the particles are packed with metallic bond such as Fe, Mg etc. They have variable hardness and melting point which depend on the strength of metallic bond. They are good conductor of heat and electricity due to presence of free electrons or loosely held electrons and metal ions.
All the metals tend to form metallic solids except the alkali and few alkaline earth metals since they are very soft due to weak metallic bonds. Transition metals are most common metallic solids as they can show variable oxidation state and have incomplete d-orbitals. Some common examples of Cu, Al, Fe and alloys like brass and bronze.
Since metallic solids contain metal atoms therefore in such solids, metals atoms are held together with metallic bonds. Do you know what metallic bonds are? Like ionic and covalent bonds, metallic bonds are a type of chemical bond in which attraction force held between metal ions and free valance electrons. Here valence electrons are considered to be delocalized as they are not bound to any particular metal atoms. These delocalized electrons are free to move in solid and make the metallic solid good conductor of heat and electricity. Hence we can say that free electrons move in the sea of metal ions. This is known as Electron Sea model of solids. Due to these free electrons, metallic solids have high thermal and electrical conductivity. They are hard, shiny and ductile. Metal atoms can’t share electrons with other atoms as they don't have enough electrons to fill their valence shells. Therefore the valence electrons are delocalized close to metal ions. These free electrons can conduct heat electricity. Metallic solids are opaque, lustrous solids, malleable and ductile. Malleability stands for softness and ability to change the shaped and pressed into thin sheets. Ductility stands the ability to pull the wires. Hence we can say that metallic bond is the bonding of valence electrons with metal ions without donation or sharing of electrons.
The strength of metallic bonds depends on the number of valance electrons present in the metal atom. For example, alkali metals and alkaline earth metals have less number of valence electrons therefore they form weak metallic bonds. Due to weak metallic bonds, the melting points of these metals are low and they are soft enough. On the contrary transition metals have more number of valance electrons therefore they have high melting points. Some of the metallic solids with their melting points are listed below.
Ionic solids, molecular solids and network solids have certain arrangement of constituent particles and have ionic or covalent bonding between particles. Unlike these solids, metallic solids do not have ionic or covalent bonding which form either by sharing or complete transfer of electrons between particles. They have metallic bonding between metal atoms which is an electrostatic force of attraction between metal ions and their free valence electrons. The presence of free valance electrons makes the metallic solids conductor and ductile in nature.