Answer:
Mendeleev is presented the periodic
table containing 63 known elements. This table was arranged on the basis of atomic
mass of the elements. The original table consisted eight vertical columns
called groups. This table consisted a number of discrepancies such as anomalous
pairs; position of isotopes and position of lanthanides and actinides etc.
Some of these discrepancies are
described below:
(i) Anomalous pairs: The
original periodic law based on atomic masses is violated in case of four pairs
of elements in order to give them positions on the basis of properties. The
elements having higher atomic masses have been assigned position before the
elements having lower atomic masses at four places as shown below:
The discrepancy disappears, if the
elements are arranged in order of increasing atomic numbers.
(ii) Position of isotopes: Isotopes
are atoms of the same element having different atomic masses but same atomic number.
The isotopes of the same element will be given different positions, if atomic
mass is taken as a basis. This will disturb the symmetry of the table. In modem
table, one position is<fixed for one atomic number and since all the
isotopes of an element have the same atomic number, these are assigned only one
position.
(a) Ar k
(b) Co Ni
(c) Te I
(d) Th Pa
At. mass
40 39
59. 58.6
127.5 127
232 2 31
At. no.
18 19
27 28
52 53
90 91
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