Detailed and minute observations revealed that atomic masses cannot be considered as the sole basis to classify elements into groups and periods as given by Mendeleev.

This led to development of Modern periodic law and modern periodic table.

The positive aspects of the modern periodic table relative to Mendeleev's periodic table are:

1. The wrong position of some elements like argon and potassium, cobalt and nickel due to atomic weights, has been solved by arranging the elements in the order of their increasing atomic number without changing their own places.
2. The isotopes of the same element have the same atomic numbers. Therefore, they find the same position in the modern periodic table.
3. It separates metals from non-metals as well as representative elements from transition elements.
4. The elements such as iron, cobalt and nickel of groups VIII of Mendeleev's periodic table are placed at 8th, 9th and 10th groups respectively.