Mendeleev’s Periodic Table

Aug 26, 2022, 16:45 IST

Mendeleev’s Periodic Table:

Mendeleev arranged 63 - 65 elements known at that point of time in the increasing order of their atomic weights in the form of a table, known as Mendeleev’s periodic table and published in 1905. It is noted that at this time noble gases were not discovered.

Mendeleev’s original table consists of eight vertical column (called groups) I-VIII & six horizontal rows (called periods). The group number were indicated by Roman numerals.

He placed elements with similar nature in same group with respect to atomic weight. However

i) At some places the order of atomic weight was changed in order to justify the chemical and physical nature. g. Cobalt (at. Wt. = 58.9) is placed before Ni (At. Wt = 58.7)

ii) Some places were left vacant for new elements which were not discovered at that point of time

e.g., scandium, gallium and germanium etc., were not known at that time. He left vacant space for them and named them as eka-boron, eka-aluminium and eka-silicon believing them to be similar to boron, aluminium and silicon respectively.

iii) The first half of elements were placed in the upper left corner and second half in the lower right corner of each box.

e.g., In 4^th period of group I, K is written in the top left corner, while Cu is written in the lower right corner.

Merits of Mendeleev’s Periodic Table:

Mendeleev’s classification was considered superior to the others proposed earlier because of the following reasons.

i) Systematic study of the elements: Mendeleev’s classification condensed the study of about 90 elements (only 63 – 65 elements were known at that time but he left a provision for many more) to the study of only 8 groups of elements.

ii) Prediction of new elements and their properties: He left some gaps in his periodic table for the undiscovered elements and could even predict the properties of undiscovered elements.

e.g., eka – boron = scandium

eka – aluminium = gallium

eka - silicon = germanium

iii) Correction of certain atomic masses: By placing elements strictly according to the similarity in their properties, he was also able to correct certain atomic masses.

e.g., He corrected the atomic masses of beryllium (Be), gold (Au) and platinum (Pt).

‘Be’ had been assigned atomic mass is 13.5.

Atomic mass = equivalent mass ´ valency factor = 4.5 ´ 3 = 13.5

It should be placed between carbon and nitrogen but its properties resembles with magnesium and calcium with valence of 2 and the corrected mass of Be came out to be 9. i.e., 4.5 ´ 2 = 9.

Defects in the Mendeleev’s periodic table:

It was a brilliant attempt for the classification of elements but it had certain defects in it. These defects are described below:

i. Position of hydrogen: In Mandeleev’s periodic table, position of hydrogen was not made clear. i.e., It can be placed either in group – I or VII.

ii. Separation of chemically similar elements: Certain elements which appear to be chemically similar like Cu and Hg, Au and Pt etc., have been placed in separate groups.

iii. Grouping of chemically dissimilar elements: Certain chemically dissimilar elements have been grouped together in the Mendeleev’s periodic table. e.g., Cu, Ag and Au have no resemblance with alkali metals (Li, Na, K etc),but these have been grouped together in group I.

Inversion in periodic table: Certain pairs of elements had to be placed in the reverse order of atomic masses in order to confirm the periodic law. e.g., Co (58.9) has been placed before Ni (58.7) and Ar (39.9) has been placed before K (39.1) etc.

Position of Isotopes: No separate places were given to isotopes.

Position of lanthanides and actinides: Lanthanide & actinide were not given places in periodic table. From these anomalies, atomic weight does not appear to be a good basis for the periodic classification of elements.

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