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PERIODIC CLASSIFICATION OF ELEMENTS
Dobereiner’s Triads:-
In the year 1817 a German scientist Dobereiner suggested that properties of elements
are related to their atomic masses.
He made groups of three elements each, having similar chemical properties and called them
triads.
“When the three elements in a triad are arranged in an increasing order of atomic mass then
the atomic mass of the middle element was approximately equal to the mean of the atomic
masses of the other two elements.”
Newlands’ Law of Octaves :-
1. In the year 1866 Newlands arranged the 56 elements known at that time in an increasing
order of their atomic masses.
2. It started with the lightest element hydrogen and ended up with thorium.
3. He found that every eighth element had properties similar to those of the first.
4. For example, sodium is the eighth element from lithium and both have similar properties.
5. Newlands compared this similarity with the octaves in music.
6. He called the similarity observed in the eighth and the first element as the Law of octaves.
“When the elements are arranged in increasing order of their atomic mass number then every
eighth element had properties similar to those of the first.”
Amjad Sir
, Demerits of Newlands’ Law of Octaves:-
➢ This law was found to be applicable only upto calcium.
➢ He placed some elements with different properties under the same note in the octave.
For example:- Newlands placed the metals Co and Ni under the note ‘Do’ along with
halogens, while Fe having similarity with Co and Ni, away from them along with the
nonmetals O and S under the note ‘Ti’.
➢ Newlands’ octaves did not have provision to accommodate the newly discovered
elements.
Mendeleev’s Periodic table:-
Mendeleev’s periodic table is the most important step in the classification of elements.
Mendeleev considered the fundamental property of elements, namely, the atomic mass.
At the time of Mendeleev 63 elements were known.
Then he transformed this into the periodic table of elements in accordance with the physical and
chemical properties of these elements.
“Properties of elements are periodic function of their atomic masses.”
Merits of Mendeleev’s periodic table:-
1. Atomic masses of some elements were revised so as to give them proper place in the Periodic
table in accordance with their properties.
For example, the previously determined atomic mass of beryllium, 14.09, was changed to the
correct value 9.4, and beryllium was placed before boron.
2. Mendeleev kept vacant places in the periodic table for elements not discovered till
then. Three of these unknown elements were given the names eka-boron, eka-aluminium
and eka-silicon.
Later on these elements were discovered and named as scandium (Sc), gallium (Ga) and
germanium (Ge) respectively.
The properties of these elements matched well with those predicted by Mendeleev.
3. There was no place reserved for noble gases in Mendeleev’s original periodic table.
Mendeleev created the ‘ zero’ group without disturbing the original periodic table in which the
noble gases were fitted very well.
Demerits of Mendeleev’s periodic table
1. The whole number atomic mass of the elements cobalt (Co) and nickel (Ni) is the same.
Therefore there was an ambiguity regarding their sequence in Mendeleev’s periodic
table.
Amjad Sir
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