Chemistry Reference
In-Depth Information
9
The Periodic Table
Development of the Periodic Table
Electronic structure and the Periodic Table
Group 0 - the noble gases
Transition elements
Group I - the alkali metals
Group II - the alkaline earth metals
The position of hydrogen
Checklist
Group VII - the halogens
Displacement reactions
Additional questions
●
Development of the
Periodic Table
The Periodic Table is a vital tool used by chemists
to predict the way in which elements react during
chemical reactions. It is a method of categorising
elements according to their properties. Scientists
started to look for a way in which to categorise the
known elements around 200 years ago.
The Periodic Table was devised in 1869 by the
Russian Dmitri Mendeleev, who was the Professor of
Chemistry at St Petersburg University (Figure 9.1).
His periodic table was based on the chemical and
physical properties of the 63 elements that had been
discovered at that time.
However, other scientists had also attempted to
categorise the known elements. In 1817, Johann
Döbereiner noticed that the atomic weight (now
called atomic mass) of strontium fell midway between
the weights of calcium and barium. These were
elements which possessed similar chemical properties.
They formed a
triad
of elements. Other triads were
also discovered, composed of:
chlorine, bromine, iodine
lithium, sodium, potassium
He called this the '
Law of Triads
'. This encouraged
other scientists to search for patterns.
In 1865, John Newlands, an English chemist,
arranged the 56 known elements in order of
increasing atomic weight. He realised when he did
this that every eighth element in the series was similar.
H
Li
Be B C N O F
Na
Mg Al Si P S Cl
K
He likened this to music and called it the '
Law of
Octaves
'. It fell down, however, because some of the
weights were inaccurate and there were elements that
had not been discovered then.
Mendeleev's classification proved to be the most
successful. Mendeleev arranged all the 63 known
elements in order of increasing atomic weight but in
such a way that elements with similar properties were
in the same vertical column. He called the vertical
columns
groups
and the horizontal rows
periods
(Figure 9.2). If necessary he left gaps in the table.
As a scientific idea, Mendeleev's periodic table
was tested by making predictions about elements
that were unknown at that time but could possibly
fill the gaps. Three of these gaps are shown by the
symbols * and † in Figure 9.2. As new elements were
discovered, they were found to fit easily into the
classification. For example, Mendeleev predicted the
properties of the missing element 'eka-silicon' (†).
He predicted the colour, density and melting point as
well as its atomic weight.
Figure 9.1
Dmitri Mendeleev (1834-1907).
