Compare and contrast the arrangement of elements in Mendeléev’s Periodic Table and the Modern Periodic Table.

Mendeleev’s periodic table

1. Elements are arranged in the increasing order of their atomic masses.

2. There are a total of 7 groups (columns) and 6 periods (rows).

3. Elements having similar properties were placed directly under one another.

4. The position of hydrogen could not be explained.

Modern periodic table

1. Elements are arranged in the increasing order of their atomic numbers.

2. There are a total of 18 groups (columns) and 7 periods (rows).

3. Elements having the same valence shell are present in the same period while elements having the same number of valence electrons are present in the same group.

4. Hydrogen is placed above alkali metals.