DEFECTS IN MENDELEEFS TABLE

Defects in the Mendeleef’s Periodic Table
(i) Position of hydrogen Hydrogen has been placed in group IA (alkali metals). but it also
resembles with halogens of group VIlA. Thus. its position in the Mendeleef‟s Periodic Table is
controversial.
(ii) Position of isotopes As Mendeleef‟s classification is based on atomic weight, Isotopes
would have to be placed in different positions due to therr different atomic weights,
e.g.,
1
H1
2
H1
3
H1 would occupy different positions.
(iii) Anomalous positions of some elements Without any proper justification. in some cases
the element with higher atomic mass precedes the element with lower atomic mass. For
example, AI (atomic weight = 39.9) precedes K (atomic weight = 39.1) and similarly Co
(atomic weight. = 58.9) has been placed ahead of Ni (atomic weight = 58.7).
(iv) Position of Lanthanoids and actinoids Lanthanoids and actinoids were not placed in the
main Periodic Table.

Anomalous behaviour of the first element of a group. The first element of a group differs
considerably from its congeners (i.e., the rest of the elements of its group).
This is due to (i) small size (ii) high electronegativity and (iii) non availability of d·orbitals for
bonding. Anomalous behaviour is observed among the second row elements (i.e., Li to F)

Limitations of Long Form of Periodic Table
In the long form of the Periodic Table :
1. The position of hydrogen still remains uncertain.
2. The inner-transition elements do not find a place in the main body of the table. They are
placed separately.


Predicting the Position of an Element in the Periodic Table:
First of all write the complete electronic configuration. The principle quantum number of the
valence shell represents the period of the element.
Group of the element is predicted from the electrons present in the outermost (n) or penultimate
(n -1) shell as follows:

For s-block elements;
group number = number of ns-electrons
For p-block elements;
group number = 10 + number of ns and np electrons
For d-block elements;
group number = the sum of the number of (n -1) d
and ns electrons.
For f-block elements; group number is 3.