METALLIC BOND:
When positive metallic ions are surrounded by a sea of negative electrons, the solid being held together by electrostatic forces then it is called metallic bond. Metallic bonds are usually formed between solids composed of electro positive metallic atoms, such as those with three or less valance electrons. These atoms achieve stable electron configuration by 'donating' electrons to a cloud of electrons shared by atoms within the metallic compound. As a result, each atom in the compound becomes a positively charged ion core (consisting of its nucleus and core elements). The atoms are held together by attractive forces between the positive ion cores and negatively charged sea of 'delocalized' electrons. Let us first consider the example of sodium (Na). Na has one valance electron, which is in the 3rd orbital. When two Na atoms bond to form a gaseous Na2 molecule, the two valance electrons (one from each Na atom) are found primarily between the two Na nuclei. To make crystalline solid, many atoms are packed together in a regular form. In metal solids the atoms adopt a "closest packed" configuration, in which the atoms are equally spaced and space between the atoms is minimized. Electrons in diffuse orbital are not tightly constrained to a small space, and hence the interactions between the one Na atom and an adjacent Na atom or "nearest atom" are weak. Simultaneously, each atom in a metallic solid has several (up to 12) "nearest neighbour" each atom interacts with various other atoms. Therefore, the individual interactions between atoms are weak, there are various interactions, and the aggregate result is a well bonded metallic solid. The low electromagnetic metal atoms "give up". Their valance electrons allowing them to found throughout to found the "mixed" orbital of the valance band. Hence, the band of orbitals is filled to certain energy according to the number valance electron provided by all of the Na atoms in the solid. Each atom shares electrons with all its many neighbours in all directions, so these bonds are weak covalent bonds. Because the orbitals are big and diffuse, the bonds they form do not have important directional preferences. Another term frequently used to describe this type of bonding is "metallic bonding", because solids with type of bonding exhibits metallic and are therefore categorized as metals.