Chemistry

Mendeleev’s Periodic Table

Discovery

In 1871, Russian Chemist, Dmitri Mendeleev, gave a more useful and comprehensive scheme for classification of elements. He presented the first regular periodic table in which elements of similar chemical properties were arranged in eight vertical columns called Groups. The horizontal rows of the table were called Periods and there are 12 Periods in Mendeleev’s periodic table.

Mendeleev’s Periodic Law

“If the elements are arranged in ascending order of their atomic masses, their chemical properties repeat in a periodic manner.”

Significance of Mendeleev’s Periodic Table

Some important contributions of Mendeleev’s Periodic Table are:

  • Systematic Study of Elements
  • Prediction of New Elements
  • Correction of Atomic Masses
  • Valency of Elements

Systematic Study of Elements:

He made the study of 105 elements quite convenient. Knowing the properties of one element in the group, the properties of other elements in the group can be guessed. Thus it becomes very useful in studying and remembering the properties of a large number of elements.

Prediction of New Elements:

While arranging elements, he left certain vacant  places. These gaps represented the undiscovered elements. Mendeleev predicted the properties of these undiscovered elements on the basis of their positions. For example, he predicted the properties of scandium, gallian and germanium which were discovered later. The observed properties of these elements were found to be similar to those predicted by Mendeleev.

Correction of Atomic Masses:

The Mendeleev’s periodic table helped in correcting the atomic masses of some elements based on their positions in the table. For example, atomic mass of beryllium was corrected from 13.5 to 9. With the help of this table, atomic masses of indium, gold, platinum etc., were corrected.

Valency of Elements:

Mendeleev’s classification helped in understanding the valency of elements. The valency of the elements s given by the group number.

Defects in Mendeleev’s Periodic Table

In spite of its useful role in the study  of chemistry, Mendeleev’s Table possessed many drawbacks. Some of these drawbacks are:

  • Position of hydrogen
  • Position of isotopes
  • Position of lanthanides and actinides
  • Dissimilar elements placed together
  • Similar elements separated
  • Anomalous pairs

Position of hydrogen: Hydrogen is placed in Group IA. However, it actually resembles the elements of Group-IA (alkali metals) as well as the elements of Group VII-A (halogens). Thus, the position of hydrogen in the periodic table is not clear.

Position of isotopes: On the basis of atomic weight, various isotopes of the same elements should be assigned different places in the periodic table. Mendeleev could not provide separate places for isotopes.

Position of lanthanides and actinides:  Fourteen elements following lanthanum (known as lanthanides or rare earths) and the fourteen elements following Actinium (known as actinides or transuranic elements) have not been provided separate and proper places in the Mendeleev’s table, rather they have been placed in two rows at the bottom of the table.

Dissimilar elements placed together:  Nobel metals like Cu, Ag and Au  are placed along with chemically dissimilar alkali metals in Group I. Similarly, Mn possessing very few similarities with halogens have been placed in VII group.

Similar elements separated:  In Mendeleev’s periodic table, certain chemically similar elements such as copper and mercury; gold and platinum have been placed in different groups.

Anomalous pairs:  In the Mendeleev’s Table based on atomic weight the positions of certain pairs, e.g. Argon (at. wt=39.94) and potassium (at. wt=39.1) : cobalt(at wt=58.93) and nickel ( Atomic weight=58.71); Tellurium (atomic weight=127.60) and iodine (atomic weight=126.90) would be reversed. In other words, certain pairs of elements are misfit in the periodic table, if atomic weight is the basis of classification.

Improvements in Mendeleev’s Periodic Table

There are following Improvements in Mendeleev’s Periodic Table are that:

i) Atomic Number:

The elements were arranged in ascending order of their atomic numbers instead of their atomic masses.

ii) Extra Group:

For the arrangement of Nobel gases, one extra group (VIII-A) was added at the extreme right of the periodic table. These gases had not been discovered in Mendeleev’s time.

iii) Sub-group:

In Mendeleev’s Periodic table, the elements like Be, Mg, Ca, Sr, Ba, and Zn, Cd, Hg were placed in a single vertical group although they possess different properties.

This confusion was removed by dividing the elements in two types of vertical group A and B.

iv) Position of Isotopes:

If the elements are arranged in the order of their increasing atomic masses, it is not possible to accommodate a large number of isotopes. This problem was solved by arranging elements in ascending order of their atomic numbers.

v) Regular Arrangement of Pairs:

Anomalous pairing can be rectified by arranging them in increasing order of their atomic numbers. e.g. 18Ar should be placed before 19K. Similarly 27Co should be placed before 28Ni and 52Te should be placed before 53i.

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