The Age of Discovery of Elements "The nineteenth century was the golden age for the discovery of
elements. Scientists began to look for patterns of behaviour between
elements."*1 Johann DÃ¶bereiner, a German chemist, was the first to
attempt to categorise the elements. He used their atomic weights,
which we now know as atomic masses.
In 1863, John Newlands, produced something that he called the 'Law of
Octaves'. He used this to produce his own version of the periodic
table, but Newlands came across some problems. "After about 20
elements his table became ragged,"*2 Newlands had left no gaps for
undiscovered elements and even had to put two elements in one space.
It was Dimitri Mendeleev, a Russian chemistry professor, in 1869 that
produced a much improved table. He amended many of the atomic weight
values and left gaps for undiscovered elements.
"Mendeleev was so confident of the basis upon which he had drawn up
his table that he made predictions about elements which had yet to be
discovered."*3 Since Mendeleev's table all the gaps he left have been
filled, three of the five elements whose properties he predicted were
found within fifteen years, and a whole new group has been introduced
- the Noble Gases.
Atomic spectroscopy is one way which has been used to increase our
knowledge about chemical elements. It excites atoms which then emit
light; this light can be split by a prism which will show the
'emission spectrum'. It is used widely to find the composition of a
sample, such as blood, or to estimate the content of a substance.
By placing the sample in the flame and through a prism you can
identify the atoms in an element/sample. *4
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"The sequence of lines in an atomic spectrum is characteristic of the
atoms in the element."*5
This technique was only discovered in the 1860's, so was a very new
method when Paul Emile Lecoq de Boisbaudran, 12 years later, in 1875,
discovered a "faint violet line at 416 nm" on the spectrum of zinc
sulphide ore. "Every element has it own characteristic set of lines. A
new line (or lines) in a spectrum meant a new element."*6
He called it Gallium, this new element had very similar properties to
Mendeleev's predicted element 'eka-aluminium'; this therefore
supported his idea of the Periodic table hugely.
Table 1, comparing Mendeleev's prediction with the properties of
element 31, gallium. *7
Gallium has some very unusual properties, some which suggest that it
is a metal and some that imply it is a non-metal.
Its physical properties, a melting point of 29.78Â°C (which is almost
UK room temperature) making it a liquid metal along with Mercury and
Caesium. It also has an extremely...