The metallic bonds are non-directional and allow the ionic centers to go past one another therefore metals can bend without fracture. Metals are ductile, malleable and possess metallic luster.
Alloys are physical mixtures of different metals but of similar atomic size. Whereas covalent compound or ionic compounds are chemical compounds. Chemical compostion cannot change but stochiometric coefficient of different metals in an alloy can change.
Hydrogen of Group I is an exception. In gaseous, liquid or solid state at normal atmospheric pressure, hydrogen remains non-conducting. This is because Hydrogen is always in H 2 molecular state hence the two electrons are tightly held to the molecular state of Hydrogen.
Under unusually high pressure in Jupiter and Saturn, hydrogen molecules are brought so close together that electrons probability waves overlap and spread to the whole lattice. Under these conditions liquid Hydrogen becomes metallic and conductive. The electric currents set up in the metallic core of Jupiter and Saturn due to the planet’s spin create magnetic fields 20 times stronger than the terrestrial magnetic fields.
Metallic Hydrogen if achieved at normal pressure could be used as super conductors, rocket propellants and in fuel cells. Solid Hydrogen Propellants develop 5 times as much thrust per kg as the present day rocket fuels. This is the reason why ISRO is in process of developing cryogenic engine based on liquid-hydrogen propellant rocket system. Solid Hydrogen in form of Deutrium and Tritium could become very efficient fuel for fusion reactors.
1.10.3.4. VAN DER WAAL’s BOND- INDUCED BONDING .
It has been found over a century that Nobel Gases which are chemically inert and monatomic exhibit inter-atomic attractive forces. It is because of these weak attractive forces that Argon, Helium and Neon liquefy at low temperatures. These forces were first proposed by Dutch Physicist Johannes van de Waals[Appendix XXXXVI] to explain the deviation from the ideal gas laws when applied to real gases. The physical basis of these forces were only recently understood.
In absence of covalent bond, ionic bond and metallic bond, it is Vanm der Waals weak inter-atomic force which provides the cohesive force for liquefaction and solidification particularly in inert elements. The macroscopic behavior of solids such as surface tension, friction, viscosity, adhesion and cohesion have their basis in this force only. This force varies inversely as the seventh power of inter-atomic distance i.e. as 1/r 7 hence they act only when molecules are very close together.
Van der Waals force act between polar molecules as well as in non-polar molecules. In polar molecules it is simply dipole-dipole electrostatic attraction. In non-polar it is by induction.
The induced bonding is shown in Figure 1.51.
Figure 1.51. The mechanism of generating induced bonding in non-polar molecules.
As shown in Figure 1.51, on a time average basis there may be charge symmetry and generation of dipole but on instantaneous basis the material does not have charge symmetry hence it has no dipole If the dipole generation is properly synchronized then a net force can develop. This is referred to as Van der Waals induced bonding.