Mendeleyv’s left many vacant spaces for the still unknown elements. For example, next to Calcium (40)
should be Titanium (48) but it resembled silicon (28) instead of Aluminium (27). He left vacant space for
element with atomic mass 44.
Discovery of New Element
Mendeleyv’s discovered new elements and also guessed their atomic mass and properties.
Atomic Mass Correction
Mendeleyv’s corrected the atomic masses of certain elements on basis of their properties and provided
proper place to them in the periodic table.
Defects in Mendeleyv’s Periodic Table
The Mendeleyv’s Period Table has following defects:
Irregular Position of Some Elements
According to Mendeleyv’s Periodic Law Potassium (39) should be placed before Argon (40) but he placed
Argon (40) before Potassium (39) which goes against his law.
Position of Isotopes
Mendeleyv’s periodic table gives no indication about the position of isotopes.
Structure of Atom
Mendeleyv’s Periodic table gives no idea about structure of atoms.
Position of Lanthanides and Actinides
Lanthanides and Actinides have not been given proper place in Periodic Table.
Coinage and Alkali Metals
Alkali metals and coinage metals with different properties are placed in the same group. This defect has
been replaced by placing them into two sub groups.
Modern Periodic Law and Modern Periodic Table
Modern Periodic Law
Physical and chemical properties of the elements are periodic function of their atomic number. Mosely
(1913) says that atomic mas is not fundamental property. Due to some defects present in Mendeleyv’s
periodic law, Mosely introduced the concept of anomic number for the elements.
When isotopes were discovered, it was thought advisable to arrange the elements on basis of their atomic number instead o increasing atomic mases. Isotopes were needed different position in the
Mendeleyv’s periodic table. Hence Mendeleyv’s periodic law was modified.
Modern Periodic Table
When Mendeleyv’s periodic law was modified and new elements were discovered. This forcd the
scientists to change Mendeleyv’s periodic law.
The electronic configuration of atoms also played an important role in he arrangement of the modern
periodic law. This form of periodic table is called “Long form of Periodic Table” because it contains
eighteen groups instead of eight but seven periods instead of twelve.
Group I – The Alkali Metals
The elements of group I