P3.THE METALLIC BOND.
In this post, we shall discuss the last kind of primary bond – The metallic bond.
We see many objects in our everyday life which are made of pure metals like gold bars/coins, copper wires, etc. In all these objects the atoms of the pure element are held together by some force.
How do metal atoms bond with each other? The metallic bond can be thought of as, the force which binds several metal atoms together.
Certain observations were made, which demanded that a new type of bond be described, to explain the bonding in metals. The observations indicated that the bond between two metal atoms is neither ionic nor covalent. These observations were as follows-
OBSERVATION # 1 – Values of Electrical conductivity.
The value of electrical conductivity is expressed in Siemens per meter(S/m).
Value for silver =6.1 × 107 = 61000000 S/m
Value for Germanium =2000 S/m.
Such a huge difference!
The value of electrical conductivity of metals was observed to be very high as compared to other substances. As seen in the above table, the metals (red color) have very high values of electrical conductivity. Thus, metals can conduct electricity very efficiently. Conduction of electricity is related to the free electrons in the structure(it is the movement of electrons that produces electricity). However, in ionic and covalent compounds the valence electrons are tightly bound and so these compounds are not good electrical conductors. So, there had to be a different kind of bond in metals that could explain this observation.
Note – Graphite is the ONLY NON METAL to have such high electrical conductivity. This is due to its structure.
OBSERVATION # 2 – Metals are malleable and ductile.
As we know, metals can be beaten into sheets and drawn into wires. These unique properties are shown exclusively by metals and there was no bonding theory that could explain this behavior.
Electron Gas Model
German physicist, Paul Drude, introduced the concept of metallic bonding. He proposed that the metal atoms consists of –
i) Atomic Core ⇒ This included the nucleus of the atom and all its inner shell electrons.
ii)Valence electrons ⇒ The outer electrons of the atom.
The inner electrons remain bound to the nucleus and the valence electrons are delocalized(not on any single atom). This model considers the lattice as being made up of metal ions surrounded by a sea of delocalized electrons. The metal ions can vibrate but remain in a fixed position, whereas the delocalized electrons are free to move throughout the lattice.
Normally electrons move in a random direction, but when a potential difference is applied, they move from higher potential to lower potential. This produces an electric current. Thus, metals are good conductors of electricity.
The metals are strong and are not brittle, as when hit ( when stress is applied), the layers of metal ions are able to slide over each other. Thus, metals are malleable and ductile.
Though this “electron-gas” model gave an adequate qualitative rationale of metallic properties, it could not correctly predict the heat capacity and the temperature dependence of electrical conductivity of metals.
The band theory, which was developed later, provided a better explanation of bonding in metals. We will study this theory in the next post. Till then,
Be a perpetual student of life and keep learning ..
References and Further reading –
Image Source –