Difference between Mendeleev's Periodic table and Modern Periodic table

Before moving to the difference between Mendeleev's Periodic table and modern Periodic table, let's see the basics of both these periodic tables. And why Mendeleev's Periodic table was modified to the modern Periodic table.

Mendeleev classified the elements on the basis of similarity in physical properties and similarity in the formulae of their hydrides and oxides.

Mendeleev's Periodic Law states that the physical and chemical properties of the elements are a periodic function of their atomic masses.

In Mendeleev's Periodic Table, elements are arranged in order of their increasing atomic masses in such a way that elements with similar properties are placed in the same vertical column called the group. To be sure that elements with similar properties fell in the same group, Mendeleev had to place an element with slightly higher atomic mass (i.e ., Co) before an element of slightly lower atomic mass (i.e ., Ni). Similarly, tellurium was placed ahead of iodine.

Mendeleev even left some gaps in the periodic table for those elements which were yet to be discovered. For example, scandium, gallium and germanium.

Mendeleev's Periodic Table consists of seven horizontal rows called periods and eight vertical columns called groups. The periods are numbered from 1 to 7 while the groups are designated as I, II, III, IV, V, VI, VII and VIII. Except group VIII, each group is divided into two subgroups designated as A and B.

Limitations of Mendeleev's Periodic Table are :

(i) anomalous position of hydrogen

(ii) anomalous position of isotopes

(iii) anomalous position of some element

(iv) no resemblance of elements within sub-groups

(v) different groups for similar elements

(vi) uncertainty in prediction of new elements.

Modern Periodic table

Henry Moseley in 1913 showed that atomic number was a more fundamental property of an element than its atomic mass. Therefore, atomic number or proton number was adopted as the basis of classification of elements and Mendeleev's periodic law was modified to Modern periodic law which states that the properties of elements are periodic functions of their atomic numbers.

When the elements are arranged in increasing order of their atomic number, the anomalies of Mendeleev's Periodic table are removed. However, the position of hydrogen still remains anomalous. It can be placed either along with alkali metal or group 1 or along with halogens of group 17 of the modern periodic table.

In the Modern or Long form of the Periodic Table, elements are arranged in increasing order of their atomic numbers.

The Modern Periodic Table is based upon electronic configuration of elements.

The periodicity in properties of elements is due to periodicity in their outer electron configurations.

The numbers 2, 8, 8, 18, 18 and 32 after which the properties of elements get repeated are called magic numbers.

The Modern Periodic Table consists of 18 vertical columns called groups and 7 horizontal rows called periods.

Each period starts with the filling of electrons in a new electronic shell and the elements in a period have consecutive atomic numbers.

All the elements in a group have identical outer shell electronic configuration. However, the number of inner filled energy shells increases as we go down the group. Further, the elements in a group do not have consecutive atomic numbers.

The valency of elements in a group is fixed but in a period first it increases from 1 to 4 and then decreases from four to zero.

The atomic size decreases across a period from left to right but increases down a group.

Across a period, the metallic character decreases while the non metallic character increases. Conversely, on moving down a group, the metallic character increases while non metallic character decreases.

The oxides of metal are basic while those of nonmetals are acidic in nature.

The periodicity is the property of elements such as valency, atomic size and metallic/non metallic character can be explained on the basis of outer shell electronic configuration of the elements.

Now let's see the difference between Mendeleev's Periodic table and modern Periodic table

Mendeleev's Periodic Table	Modern Periodic Table
1. Elements are arranged in increasing order of their atomic masses.	1.  Elements are arranged in increasing order of their atomic numbers or proton numbers.
2. There is no provision for separate positions for isotopes of an element since their atomic masses are different.	2. Separate positions for isotopes of an element are not required because they have the same atomic number.
3. Some elements with higher atomic masses have been placed before elements of lower atomic masses. For example, Co (58.9 u) is placed before Ni (58.7 u) and Te (127.6 u) is placed before 1 (126.9 u).	3. No such discrepancy arises because elements are arranged in serial order of their increasing atomic numbers irrespective of their atomic masses. Thus, Co with atomic number 27 comes before Ni with atomic number 28. Similarly, Te with atomic number 52 comes before 1 with atomic number 53.
4. Except group VIII, all other groups have been divided into two sub-groups, namely A and B. Group VIII, however, contains three sets of three elements each.	4. Each group has one vertical column of elements and there is no sub-division of groups.
5. Some dissimilar elements are grouped together while some similar elements are placed in different groups. For example, alkali metals (Li, Na, K, etc.) of group IA are grouped together with coinage metals (Cu, Ag, Au) of group IB though their properties are quite different. At the same time, certain chemically similar elements like Cu (group IB) and Hg (group II B) have been placed in different groups.	5. There is no such discrepancy. All similar elements have been grouped together. For example, alkali metals are put together in group 1 while coinage metals are placed in group 11.
6. Many metals and non-metals are grouped together. For example, non-metals such as O, S and Se are placed in group VIA while metals like Cr, Mo and W are placed in group VIB.	6. Metals and non-metals are widely separated from one another.
7. Electronic configuration of an element cannot be predicted from its position in the periodic table.	7. Electronic configuration of an element can be easily predicted from its position in the periodic table.