Now let's see what are the merits of
 
Mendeleev periodic table. Mendeleev had
 
already predicted that there will be
 
certain elements which will be
 
discovered after some years. And the
 
elements was really discovered. The eka-
 
boron that Mendeleev had named is now
 
known as scandium, eka-aluminium is known
 
as Gallium and eka-silicon is known
 
as Germanium. Mendeleev had also
 
predicted certain properties of these
 
elements which found to be very much
 
correct. For example, atomic mass of eka-
 
aluminium was predicted to be 68 and
 
when Gallium was actually found out its
 
atomic masses was found out to be 69.7, which
 
is approximately close to the prediction.
 
He also predicted that eka-aluminium
 
formula with oxygen will be E2O3 and
 
gallium formula with oxygen is actually
 
Ga2O3.
 
He also found out that the formula of
 
chlorine with eka-aluminium will be ECl3
 
and actually formula of gallium with
 
chlorine is GaCl3. The noble gases that we
 
found out later could easily be
 
accommodated in the stable. Because
 
noble gases do not react with either oxygen or hydrogen
 
So they could be easily accommodated in a separate column.
 
This table was established in the year
 
1869.  And the  mendeleev periodic law
 
states that "the properties of the
 
elements are the periodic function of
 
their atomic masses. That means if you
 
arrange the element according to their
 
atomic masses we will find that elements
 
having similar chemical properties can
 
be found after a certain interval. After
 
the discovery of such a brilliant table.
 
There was certain flaws in the table.
 
Let us see what are the limitations of this table.
 
This table was arranged according to
 
their increasing order of the atomic
 
masses of the elements, but there was no
 
information about the isotopes of the
 
elements present.
 
For example, chlorine has two isotopes:
 
one of the isotope has mass 35, whereas
 
the other isotope has mass 37 but
 
chlorine 35 was placed in the table
 
and chlorine 37 was nowhere. So, no
 
information about the isotope was given
 
in the table. So, the limitation is the
 
position of the isotopes cannot be
 
explained.
 
Now according to the second limitation
 
the position of hydrogen in the table was
 
not clear. Hydrogen was placed in the
 
first column in the column of the
 
metals. But hydrogen is not a metal.
 
It also behaves like a halogen. So it has
 
to be placed with halogens. So, this was
 
the confusion which was created in the
 
periodic table regarding hydrogen. Other
 
limitation was a wrong order of atomic
 
masses of some elements could not be
 
explained. When you check the table you
 
will find that cobalt which has mass 58.93
 
was placed before nickel which
 
has mass 58.71. This is the wrong
 
order if we are considering the
 
increasing order of their atomic masses.
