Metallic bond accounts for several properties in metals. The properties arise due to the presence of delocalized electrons in the entire crystal lattice.
Metallic strength depends on the electrostatic force of attraction between the free electrons and the positively charged metal ion. The smaller the size of the metal and higher is the nuclear charge, the greater will be the attraction, and more will be the strength.
Most of the metals have a shiny appearance. The free electrons reflect the light off the metal surface to give a shiny appearance.
â€¢ Electrical conductivity
For a substance to conduct electricity, it must contain mobile carriers. Metals contain free electrons that carry a charge and are also free to move throughout the crystal lattice. Silver conducts electricity readily followed by copper, gold, and aluminum.
â€¢ Thermal conductivity
Metals are good conductors of heat. The thermal conductivity is due to the vibrations in the free electrons which spread throughout the crystal. The free electrons help in conducting heat from the source in the form of a wave.
â€¢ Malleability and ductility
Malleability refers to the property of metal being molded into shape on the application of pressure and ductility is the property of metal by which it can be drawn into wires. This is because the bonds between the metal atoms can be broken easily and reformed. The force binding the metals is non-directional and the ions in its crystal structure are displaced concerning one another, hence they can be made into any shape. A metal gets deformed not fractured.
The metallic bond also explains other properties like thermionic effect, high density, high melting and boiling point, and low volatility with a few exceptions like zinc, cadmium, and mercury being volatile and mercury being a liquid.
In simple metals like the alkali or alkaline earth, metals possess s and p orbitals in their outermost energy shell. These orbitals overlap to form sp hybridized orbital. The valence electrons lie in these orbitals. The metallic bond is formed from the electrons in sp orbitals. At least one valence electron is free to move from one atom to another. These are called conduction electrons and are responsible for the electrical conductivity in the metal. The metals with electrons in sp-orbitals have low cohesive energy i.e., the energy required to separate the atoms from each other, and thus forms a weak bond. To increase the strength of these metals like aluminum, they are alloyed or made polycrystalline.
The transition metals have partially filled d-orbitals. The electrons in d-orbitals are tightly bound to the metal ion and do not move away. These orbitals are involved in covalent bonding with the neighboring d-orbital. The cohesive energy is quite large, and the bonds are strong. Titanium, chromium, iron, and tungsten have high strength. Where there is a possibility for both the types of bonding, the d-orbital bond dominates over the sp-orbital bond.
Band Model in Metallic Bonding
Bonding in metals is described with the help of the band theory. The atomic orbitals overlap to form molecular orbitals which may be bonding, antibonding, and non-bonding. The bonding molecular orbital is of lowest energy, the antibonding molecular orbital has the highest energy, and those molecular orbitals with intermediate energy will correspond to the non-bonding molecular orbitals. To overlap, the atomic orbitals should have the same symmetry and the same energy. The continuous set of the allowed energy levels form an energy band and the difference between the highest and lowest energy level is the bandwidth.
The number of energy levels corresponds to the number of molecular orbitals which depend on the number of interacting atomic orbitals. Consider a metal with s-orbital in the valence shell and contains n energy levels. An s-orbital has a maximum capacity to accommodate 2 electrons. So, the energy band can accommodate 2n electrons. However, for metals that contain 1 electron in its s-orbital, n electrons will fill the band and they will occupy the lower half of the energy band which corresponds to bonding molecular orbital and will result in a strong bond.