Solid materials are made up of large collection of atoms where the individual atoms are bonded together. The type of bonding which exists between atoms is different for all material.
An atom with a completely filled outer electron shell has a high degree of stability. The atoms of other elements with incomplete outer shells attempts, by combination with other atoms, to satisfy this condition for stability. The principal types of inter atomic bonds which may be formed are the ionic, covalent, metallic, Van Der Wall’s or molecular bonds.
The solids are also classified as per the bond structure found in them. They are Ionic, Metallic, Covalent and Molecular.
Metallic Bond. In all the metal atoms, valence electron are loosely held by there nuclei. So valence electron requires very less energy to detach themselves from their nuclei. The metal atom after leaving their outer most electrons becomes positive charged ions. These atoms are surrounded by a large number of free electrons. So the electrostatic force of attraction between the electron cloud and positive ion, form a bond which is known as metallic bond. In such bond, electrons are shared by all most all the atoms. There is no way of determining to which nucleus any particular valence electrons belong.
When aluminium atoms are grouped together in a block of metal, the outer electrons leave individual atoms to become part of common “electron cloud.” In this arrangement, the valence electrons have considerable mobility and are able to conduct heat and electricity easily. Also, the delocalized nature of the bonds, makes it possible for the atoms to slide past each other when the metal is deformed instead of fracturing like glass or other brittle material
Since the aluminium atoms lose three electrons, they end up having a positive charge and are designated Al3+ ions (cations). These ions repel each other but are held together in the block because the negative electrons are attracted to the positively charged ions. As a result of sharing of electrons, the cations arrange themselves in a regular pattern. This regular pattern of atoms is the crystalline structure of metals. In the crystal lattice, atoms are packed closely together to maximize the strength of the bonds. An actual piece of metal consists of many tiny crystals called grains that touch at grain boundaries.
Metallic Solids. Metallic solids are those in which positive ions occupy the lattice sites. The atoms of a metal assume nearly fixed position relative to each other. A solid metal usually is composed of a multitude of crystals. Within any one crystal, the atomic arrangement is repeated by adjacent atoms.
Properties of Metallic Solid. As these solids have unlimited number of free electrons, the electrical conductivity of metallic solids are very high. Due to the presence of free electrons which serves as carriers of heat, these solids are also having high thermal conductivity. These solids have high optical reflection and absorption co-efficient. It is having high tensile strength and ductility. Examples of metallic solids are Al, Mg, Si, etc.