Achievements and Limitations of Mendeleev Periodic Table

Mendeleev’s Periodic Table

(Published in a German journal in 1872): In the formula of oxides and hydrides at the top of the columns, the letter ‘R’ is used to represent any of the elements in the group.

Features of Mendeleev’s Periodic Table:

The important features of this table are:

This table contains 8 vertical columns, called groups and 6 horizontal rows, called periods.

In his table, Mendeleev’s left gaps for the elements not discovered at that time. He named such elements by prefixing a Sanskrit numeral Eka (one), divi (two), tri(three) etc., to the name of the preceding similar (analogous) element in the same group. e.g. Eka-boron, Eka-aluminium, Eka-silicon, which after their discovery were named as scandium, gallium and germanium respectively.

He also predicted the atomic masses and properties of several elements that were not known at that time. Elements like scandium, gallium and germanium have properties similar to those predicted by Mendeleev. The properties of Eka-aluminium predicted by Mendeleev and those of element, gallium which was discovered later and replaced Eka-aluminium.

In order to group the elements having similar properties together, at some places, Mendeleev had to place an element with a slightly greater atomic mass before an element with a slightly lower atomic mass

Noble gases like helium (He), neon (Ne), argon (Ar) etc., were discovered very late because they are very inert and present in extremely low concentrations in our atmosphere.

One of the strengths of Mendeleev’s periodic table was that, when these gases were discovered, they could be placed in a new group without disturbing the existing order.

Limitation of Mendeleev’s Periodic Table:

Although this table was greatly helpful for the study elements but a few anomalies could not be explained on the basis of this table. These anomalies are:

Position of Hydrogen: The electronic configuration of hydrogen resembles with alkali metals. Moreover, it combines with halogen, oxygen and sulphur to give similar type of compounds as given by alkali metals, e.g. (HCl, NaCl), (H2O, Na2O (H2S, Na2S). But just like halogen it exists in diatomic form and combines with metals and non-metals to form covalent compounds. Thus, its position was not fixed in the Mendeleev’s periodic table but it was kept alkali metals.

Position of Isotopes: isotopes of the elements have similar chemical properties but different atomic masses. In Mendeleev’s periodic table, no place was given to these elements, e.g. the element chlorine has two isotopes, Cl-35 and Cl-37, having atomic, masses of 35 and 37 respectively. The placing of these two Isotopes of chlorine (having different atomic masses) in the same group of periodic table could not be explained by Mendeleev periodic law.

Uncertainty in Atomic Masses: Another problem was that the atomic masses do not increase in a regular manner in going from one element to the next. So, it was not possible to predict how many elements could be discovered between two elements especially when we consider the heavier elements.

Placing of Heavier Element before the Lighter One: Few elements, those possess higher atomic mass were placed before elements having a lower atomic mass. e.g. argon (39.9) was placed before potassium (39.1), cobalt (58.9) before nickel (58.6), tellurium (127.60) before iodine (126.9)

Sample Questions

(More Questions for each concept available in Login)

Question : 1

The MendeLeev’s Periodic Table contains _____________________ vertical columns, called groups and 6 horizontal rows, called periods.
A. 7	B. 8	C. 10	D. 14
Right Option : B
View Explanation