ರಸಯೋಗಿ- ಎಂ. ಆರ್. ಎನ್.
METALS AND ALLOYS
It may
appear strange but it is a true fact. The craze for gold, a metal, created
alchemists and thus began chemistry, a new branch of science. A fantasy, an
unscientific and impossible dream led people to study about metals which have
unique property such as lustre and longevity relative to gold. People also had
a belief that all metals are gold, but due to some impurity had prevented them
from becoming gold. In
Alchemists
were unsuccessful in producing Sparshmani, a magical substance which they
believed would convert baser metals to gold. But the temptation of creating a
Sparshmani led to the formulation of a wide variety of apparatus and other
devices. They went on accumulating the nature and properties of matter which
included the properties of elements, compounds and mixtures.
By‐products of alchemistry
however turned out to be valuable. This led to the conventional chemistry which
started with Robert Boyle giving distinct definition of element. Many
contemporaries of Boyle including Sir Isaac Newton secretly practised Alchemy!
Elements,
as we know today, are classified in several ways. But the earliest
classification was just two‐ metals and non‐metals. Non‐metallic elements produced acids on oxidation and dissolution in
water. The acids turned blue litmus to red. Whereas metals on oxidation and
dissolution produced bases which turned red litmus to blue.
It is
interesting to know that the study and use of metals existed much earlier to
the definition of elements and classification of elements.
It has been
recorded that the metals known in ancient times were not many. Just 5 metals
were known to our ancients. Not surprisingly, ancient sculptures talk of
Panchaloha, or five metals. These metals are gold, silver, tin, zinc and
copper. Why were only these metals known to our ancients? The answer is their
chemistry. Gold and silver are native metals that occur mostly in pure form and
their purification is easy. Nowdays, platinum too is considered a valuable
metal like gold, but its occurrence is rare. The remaining three metals occur as
sulphide ores which are light, distinct and can be easily separated from mud.
Moreover, their extraction is also easy. It is only later that lead, iron and
other metals were included. It is surprising to note that in the beginning the
metals were identified with planets and sun. Gold was identified with sun,
silver with venus, lead with earth, mars with iron and Saturn with copper. The
last of the naturally occurring metals discovered are Uranium, Neptunium and
Plutonium. Neptunium and Plutonium are present in trace quantities with
Uranium. The three outer most planets then were Uranus, Neptune and Pluto!
Metals have
common physical properties but diverse stability. They can’t be converted into
Gold through any chemical change. This failure was in a way good because
success would have destroyed the diversity of metals that we have today.
Although we may expect half of all elements to be metals and the other half non‐metals, in reality 80% of the known 118 elements are metals. In the
twentieth century many elements were synthetically produced. Rather these
elements do not occur in nature in any form and are formed during interactions
between atoms of heavier elements for a fleeting second. As far as these
synthetic elements are concerned, they are all metals since their huge atomic
size prevents them to attract any more electrons. The current definition of a
metal is that substance which in a chemical reaction donates electrons rather
than attracting them.
Metals
donate electrons and form positive ion and hence they are also called
electropositive elements.
Ofcourse,
it is foolish to convert every metal into gold since we need metals for various
requirements. From household utensils to aircrafts and space probes that travel
beyond solar system, metals are put to use. It is believed that metals came
into use immediately after the Neolithic age when primitive man used stone
implements to suit his hunter‐gatherer lifestyle. Later he began modified his stone‐tools with methods
which must have led to the discovery of metals. Metal usage was not dependent
upon only the availability of the ore. There were other factors also that must
have contributed to the use of a particular metal. They are:
1. Identification of ores
2. Isolation from mud and extraction
3. Easy metallurgy
4. Technology available then to accomplish all the above.
The
earliest period of chemistry evolution had only thermal reduction procedures
for extraction. Hence only reactive ores such as sulphide ores could be
converted to metals. Later on the carbonate and oxide ores were used in
metallurgical processes and other metals became available.
Discovery
of electricity and electrochemical reactions wherein metal was an inevitable
component as a conductor led to the discovery of reactive metals like sodium,
potassium , magnesium and others. Metals are named ending with ium. Humphry
Davy initiated electrometallurgy that led to the discovery of many more metals.
And also, it led to the realisation that chemical bond must be electrical in
nature since electricity could tear apart salts into their respective atoms!
Isolating a
group of metals occupying the same place in the periodic table or the elements
which had indistiguashable property except their atomic weights was
challenging. Both physical and chemical methods of separation had to be applied
to isolate them . Thus were identified the lanthanide and actinide metals.
Separation techniques got a boost because of this challenge.
Now‐a‐days the line of
demarcation between metals and non‐metals is disappearing, thanks to some clever chemistry. Graphene
produced from a non‐metallic carbon is nothing but a metallo‐carbon. Graphite from which graphene is produced
has some properties such as electrical conductivity similar to metals. It has
lustre. It has a high melting point and thermal resistance which are usually
found among metals. Isolation of graphene is nothing but separation of a layer
from the multilayered structure of graphite. Isolation of a single layer also
eliminates the weak bonds that is responsible for the smooth nature of
graphite. Elimation of weaker bonds obviously makes the material stronger! This
is a landmark development in nanotechnology and thus we could produce a metallo‐carbon from a non‐metal.
Alloys are
solid solutions of metals. Mere mixing of molten metals in any proportion will
not produce alloys. Some alloys have to be produce d by mixing constituent
metals in specific proportions. Such alloys are called stoichiometric alloys.
There are also non‐stoichiometric alloys wherein metals are mixed in all proportions.
Alloys after solidification will have both physical and chemical properties
different from their constituent metals. Thus stainless steel shines better
than iron, its major component, and it also does not rust. Brass is yellowish
whereas its constituents copper and zinc are red and gray respectively. Alloys
can be prepared to suit the requirement both by changing the constituents and
composition. In fact, alloys are more useful than pure metals. Combination with
mercury, the only metal which is liquid at room temperature, is called an
amalgam.
Metals can
be useful in many other ways also. Smearing a molten metal on the surface of
another metal is called galvanization. Metals can be deposited on another metal
using electrolysis. This is called electroplating. Depositing a conducting
metal on a non‐conducting plastic or polymer is called electroless plating.
Non‐reactive metals which
come under the category of transition metals are lustrous, long lasting and can
get fine polish. Hence they were used extensively in the production of mirrors,
idols and ornaments. The name Rupee for the Indian currency comes from rupiyaa
which means silver, an indication that early currency were of silver. Later to
reduce the cost, it became inevitable to use alloys of cheaper metals like
nickel and copper in the place of silver and gold. The Sanskrit word ‘suvarna
varaha’ refers to golden currency.
Metals and
alloys gained importance particularly after the electricity was invented.
Electricity is generated with the help of metal electrodes and transmitted
through metallic wires, connected through fuses. Even the present day
integrated circuits and chips have circuits etched with metals. Perhaps, one of
the reasons for the depletion of copper ores is the conducting properties of
the metal. Batteries which use metals are small and efficient!
It is
surprising but true metals and their salts misled the early electrochemists too
to believe that electricity conduction in a metal is a chemical change.
Metallic conduction, however, is a physical change where as ionic conduction of
metallic salts in a solution is a chemical change. Michael Faraday called it the
chemical effect of electric current and this was rectified later.
There are
many applications of metals and alloys. Cooking is impossible without metallic
containers which efficiently conduct the heat. Music is made more interesting
by vibrating metal wires. Moulding of machines and spare parts is possible with
the use of metals which provide the required hardness.
The modern
world owes much to industrialisation. The industrial renaissance has its roots
in excessive use of metals in general and iron in particular. The dream of
going beyond the earth could materialise because of titanium alloys. These
alloys can withstand very high temperatures generated due to the friction
caused between the fast moving rocket and the atmospheric air. The titanium
alloys bear that heat and do not undergo any physical or chemical change at
that high temperature and that too in the presence of highly reactive oxygen!
Any other substance would have burnt to ashes.
There are
some metals which have properties of both metals as well as non‐metals. They are known
as metalloids. In that sense, zinc is not a metal but a metalloid. Their oxides
in aqueous solution react with acids as well as bases. Metals which occur in
nature in the form of compounds have a tendency to revert back to their original
state. This is called corrosion. Mechanical friction will erode the metals and
alloys. Enhancing the longevity of metals has its own merits. This is
significant since the resources of metallic ores are finite. The study of
corrosion and inhibition of corrosion by several techniques has helped mankind
to optimally utilize the metals.
The chemistry of metals and alloys also includes their compounds besides their direct application. Discovery of polymers have provided an alternative to metals, but yet one cannot imagine a world without metals and alloys. Their reach is very vast ‐‐ from containers to cosmos.
-Prof. M. R. Nagaraju
# F‐3, SFS Flats, 7th B Cross, Yelahanka Upanagara
BANGALORE‐560 106(Phone:94800938750)
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