CHEM1011 Lecture 12: Bridging Course Topic 12
2 Jul 2018
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TOPIC 12.
THE ELEMENTS - the Periodic Table.
For millennia, humans have been discovering and extracting elements from nature and
using them either in their elemental state or in combination with others as compounds.
This process has accelerated over the past few centuries and in today’s highly technical
environment we are dependent on a continued supply of many elements, the names of
some of which were rarely mentioned or even recognised by chemists just a few
decades ago. Without an assured supply, much of the technology that is now widely
taken for granted would no longer be viable and potential future developments will be
hampered or rendered impossible. As an example, note how dependent current
technology has become on the supply of the so-called rare earth elements which
underpin many of the advances made in computing, communications and the many
applications in which powerful rare earth magnets are the basis. Other less exotic
elements are even more important for today’s living standards - consider how
dependent agriculture is on a continued supply of phosphorous in the form of
phosphate fertilisers, a supply which may be fated to be fully depleted in the future. In
these notes, a selection of elements which are among those that are essential
components of technology today are discussed in the context of the Periodic Table.
This system of classification of the elements is not only a convenient summary of the
chemistry associated with various families of elements in which the Table’s origin lies,
but in its modern form the Periodic Table provides the scaffold underlying the
electronic structures of the atoms and upon which their various properties and
reactions depend.
The following are a few illustrative examples to ponder concerning the elements which
are discussed in this Topic.
Nitrogen molecules in the atmosphere contain one of the most stable bonds yet about
half the nitrogen atoms in our bodies were extracted artificially from the air.
Salt is destroying vast areas of agricultural land in Australia and has undesirable
consequences if consumed in excess in our diet yet is a valuable source of essential
products.
The air surrounding us contains a large proportion of a highly corrosive gas that
originated as the most polluting ever and which reacts with almost all other elements
yet is indispensable to all animal life - oxygen.
The rechargeable batteries which power mobile phones and computers are based on
the extremely small size of the lithium ion.
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Many nuclear diagnostic procedures rely on using artificially produced atoms that
originate from nuclear reactors such as that at Lucas Heights in Sydney.
An irreplaceable gas with vital applications in magnetic resonance imaging machines
is used and lost - filling party balloons!
An object made from some metals retains a memory of its initial shape and if distorted,
it will return to the original shape when heated.
Origin of the elements.
As discussed in Topic 1, there are 90 naturally occurring elements. In addition, there
are about 28 other elements which have been produced synthetically but some of these
exist only as very short-lived radioactive species which have been produced in
extremely small quantities using high energy particle accelerators. How did the 90
naturally occurring elements originate? The most widely accepted version of the
origin of all matter in the universe is the BIG BANG THEORY which proposes that
in an instant, all of space, energy and matter which had been confined to a volume the
size of a grain of sand at an infinitely high temperature underwent an explosive
expansion. Within 10 minutes of the big bang, nuclei of mostly hydrogen and helium
were formed from more basic particles and over the next million years these nuclei
cooled enough to capture electrons and form atoms. After about a billion years, the
gravitational attraction between atoms which were still mostly hydrogen and helium,
lead to clumps of matter which by gravitational attraction gradually increased in size.
With increasing size, the temperature of these clumps also increased and in some
regions of space the larger clumps became hot enough to initiate fusion reactions
between nuclei, forming stars which are in effect giant nuclear fusion reactors. Our
own sun, like all stars, converts hydrogen to helium with the concurrent release of
extremely large amounts of energy known as the BINDING ENERGY associated with
the strong nuclear force which was discussed briefly in Topic 2. Within a few billion
years vast numbers of new stars were formed and these in turn, through gravitational
attraction, clustered to create galaxies, each of which contains enormous numbers of
stars. By that point in time, the universe would have looked much as it does today. As
the hydrogen fuelling the fusion reactions in a star is consumed, other fusion reactions
can occur in which heavier elements form. Fusion reactions leading to new atoms of
elements as heavy as iron all release energy and can continue to fuel a star. Ultimately,
when a star has consumed most of its available fuel, it may simply cool and dim or in
some cases it may initially implode and then undergo an enormous explosion which
flings much of its constituent material and energy out into space in what is called a
SUPERNOVA EVENT. It is only during supernovae that elements heavier than iron
are formed. It is estimated that the present elemental composition of the universe is
92.7% hydrogen atoms, 7.2% helium atoms and just 0.1% atoms of all the other
elements. The shock waves of energy and material sent into space from supernovae
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may interact with existing clouds of gas, ice and dust to eventually form new stars and
planets such as our solar system. Our sun was not one of the original stars in the
universe but is probably a second or third generation star, formed in part from the
energy and residues released by previous supernovae. Although already 5 billion
years old, it still contains 71% hydrogen and 27% helium, so the sun will burn for
several billion more years.
Thus on the basis of this theory, the elements which constitute all matter on earth apart
from hydrogen were originally formed from stars that existed before our sun and which
had consumed all their available hydrogen, converted it to helium and progressed to
other nuclear fusion reactions that created heavier elements and finally underwent a
supernova explosion in which more of the heavier elements were produced.
Discovery and isolation of the elements.
Most of the non-gaseous elements on earth are chemically combined with other
elements as compounds. Few non-gaseous elements are found in the free state. For
thousands of years gold, silver, copper, sulfur and carbon had been known because
they do occur in the free form, although they were not necessarily recognised as
elements - indeed the concept of an element as we know it today was not firmly
established until the 18th century through the visionary work of Lavoisier. While
some metals such as gold and silver are so unreactive that they can be found as free
elements, most elements occur as compounds in MINERALS. Extraction of
metals such as copper and tin from their ores by the process of SMELTING was
probably accidentally discovered when minerals were used as fireplaces. The
extraction process relied upon the use of bellows made from animal hides to
increase the heat obtained from a fire to the point where decomposition could occur
of rocks containing for example copper (malachite, a copper carbonate compound)
and tin (cassiterite, an oxide of tin). Charcoal in the fire reduced the copper and tin
compounds to the free elements. Later it was discovered that mixing about 9 parts
of copper and 1 part of tin together and melting them produced an alloy called
BRONZE which is much harder than either of its constituent elements, a feature
exploited in the bronze age from about 5000 years ago. Later, iron was isolated
from its ores by similar means - probably more than 3000 years ago. Prior to 1600,
the elements gold, silver, carbon, sulfur, copper, tin, lead, iron and mercury had
been discovered by persons unknown. However arsenic, isolated by Albertus
Magnus in 1250, is the first recorded instance of an attributed method of isolation
of an element.
The prehistoric technical advance of using crude furnaces to smelt ores set the
pattern for future discoveries of elements which resulted from newly devised
processes. The rate of isolation of the elements was a process that tended to occur in
steps where development of a new method or technology gave impetus to a flurry of
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Document Summary
For millennia, humans have been discovering and extracting elements from nature and using them either in their elemental state or in combination with others as compounds. Without an assured supply, much of the technology that is now widely taken for granted would no longer be viable and potential future developments will be hampered or rendered impossible. In these notes, a selection of elements which are among those that are essential components of technology today are discussed in the context of the periodic table. The following are a few illustrative examples to ponder concerning the elements which are discussed in this topic. Nitrogen molecules in the atmosphere contain one of the most stable bonds yet about half the nitrogen atoms in our bodies were extracted artificially from the air. Salt is destroying vast areas of agricultural land in australia and has undesirable consequences if consumed in excess in our diet yet is a valuable source of essential products.
