By now, including synthetic elements, there are more than 115
elements. As the number of elements increased, it became difficult to
keep in memory the chemistries of individual elements and their
compounds.
In previous classes we had learnt the classifications of elements in
metals and non-metals. Examples Sodium, Pottasium, etc. are metals
Sulphur, Clorien are non-metals. But this classification had so many
limitations Aluminium has some metallic properties and some non-metallic
properties. Elements of this type are considered as metallides or semi
metals. So, there was a need to classify them in other ways. Hence, chemists
started to frame ways to group these elements and compounds on the basis
of their physical and chemical properties.
In the beginning of the 18th century Joseph Louis Proust stated that
hydrogen atom is the building material and atoms of all other elements
are simply due to the combination of number of hydrogen atoms. (It is to
be noted that at his time the atomic weights of all elements were given as
whole numbers and the atomic weight of hydrogen was taken as one.)
Dobereiner’s law of Triads
A German chemist Johann Wolfgang Döbereiner (1829) noted that
there were groups of elements with three elements known as triads in
each group with similar chemical properties. He tried to give a relationship
between the properties of elments and their atomic weights.
Döbereiner stated that when elements with similar properties are taken
three at a time and arranged in the ascending order of their atomic weights,
the atomic weight of the middle element is the average of the atomic
weights of the first and third elements. This statement is called the
Dobereiner’slaw of triads.
