Certain elements of highest atomic
weights precede those with lower atomic weights.
For example, tellurium (atomic weight 127.6) precedes iodine (atomic
weight 126.9).
2. Dissimilar elements placed together :
elements with dissimilar
properties were placed in same group as sub-group A and sub-group B.
For example, alkali metal like Li, Na, K etc., of IA group have little
resemblance with coinage metals like Cu, Ag, Au of IB group.Cl is of
VII A group and ‘Mn’ is of VII B, but chlorine is a non metal, where as
manganese is a metal
Modern Periodic Table
H.J. Moseley (1913) found that each element emits a
characteristic pattern of X-rays when subjected to
bombardment by high energy electrons. By analyzing the X-ray
patterns, Moseley was able to calculate the number of positive
charges in the atoms of respective elements. The number of
positive charges (protons) in the atom of an element is called
the atomic number of the element. With this analysis Moseley
realised that the atomic number(Z) is more fundamental
characteristic of an element than its atomic weight.
After knowing the atomic numbers of elements, it was
recognized that a better way of arranging the elements in the
periodic table is according to the increasing atomic number.
This arrangement eliminated the problem of anomalous series. For
example, though tellurium (Te) has more atomic weight than iodine (I), it
H.J. Moseley (1913) found that each element emits a
characteristic pattern of X-rays when subjected to
bombardment by high energy electrons. By analyzing the X-ray
patterns, Moseley was able to calculate the number of positive
charges in the atoms of respective elements. The number of
positive charges (protons) in the atom of an element is called
the atomic number of the element. With this analysis Moseley
realised that the atomic number(Z) is more fundamental
characteristic of an element than its atomic weight.