Mendeleev and Modern Periodic Law:
Mendeleev Periodic Law:
The physical and chemical properties of elements are a periodic function of their atomic weights.
- Eight vertical columns called groups. These are numbered as I, II, III, IV, V, VI, VII, VIII.
- Each group is further divided into two sub-groups ‘A‘ and ‘B‘. ‘A‘ group is on the left-hand side and ‘B’ group is on the right-hand side.
- Six horizontal rows are called as series or periods. These are numbers as one to six.
- Gallium and Germanium were not known at that time when Mendeleev’s proposed his periodic table. He named these elements has Eka aluminium and Eka silicon.
Important Drawbacks or Defects:
- Anomalous position of hydrogen- It was placed in group I-A but its properties also resemble with group VII-A i.e halogens. Hence the position of hydrogen is controversial.
- Anomalous pairs of elements- Some elements with higher atomic weights were placed earlier with lower atomic weights. Example- Argon (atomic weight 39.9) proceeds Potassium (atomic weight 39.1), Cobalt (atomic weight 58.9) proceeds Nickel (atomic weight 58.7).
- Position of Isotopes- Isotopes always possess different atomic weights. Thus their position should be different in periodic table but Mendeleev arranged Isotopes at the same place. Example- 1H1, 1H2, 1H3 are isotopes of hydrogen but are placed in group I-A.
- Some dissimilar elements were group together and some similar elements are placed in different groups. Example- Lithium, Sodium, Potassium is group I-A are group together with Copper, Silver, Gold group I-B.
- Copper group I-B and mercury group II-B are placed in different groups.
- No proper place was allotted for group VIII.
Modern Periodic Law:
It states that physical and chemical properties of the elements are a periodic function of their atomic numbers i.e. if the elements are arranged in order of their increasing atomic numbers, the elements with similar properties are repeated after certain regular intervals.
Cause of Periodicity– The cause of periodicity in properties is the repetition of similar outer electronic configuration at certain regular intervals.