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based on their atomic mass which helped scientists in coming years to study

the properties of these elements in great depth. Mendeleev was born in a
Siberian family in Tobolsk, Russia. Though his early childhood was difficult
his mother sent him to a good school for his academic progress. He was a

brilliant student and could progress vertically in Russian academic circles.

In celebration of the 150th anniversary of this epochal moment in science,
the UN declared 2019 the International year of the Periodic Table.

UNESCO announced in their press release, “The Periodic Table of
Chemical Elements is more than just a guide or catalogue of the entire
known atoms in the universe; it is essentially a window on the universe,
helping to expand our understanding of the world around us.” Mendeleev
emphasized more on the properties of elements rather than the atomic mass

of the elements. His periodic table consists of vertical columns called as
‘groups’ and horizontal rows called as ‘periods’. It consists of eight groups and
seven periods. The speciality of this table was that he left few empty spaces

for undiscovered elements names as eka aluminum, eka boron and eka
silicon, as scandium gallium and germanium respectively, (Sanskrit ‘eka’
means one). These elements were discovered later and found to be correct
in terms of position, physical and chemical properties. It showed the
immense visionary power of the chemist who could foresee such discoveries

in the future and brilliantly incorporated them in the periodic table.

Mendeleev suggested that elements that could not find a proper place in the
periodic table were elements whose atomic masses were not weighed

properly. It helped in correcting the atomic masses of elements. Example-
Beryllium atomic mass 13.9 was corrected to 9 and atomic masses of indium,
gold and platinum were corrected. Separate column (the group was allotted

for halogen in the extreme right of the periodic table. Inert gases (noble
gases) discovered by William Ramsay in 1904 were placed in a column called
as zero group elements.

However, there were some basic limitations in this periodic table. Elements
with higher atomic mass were placed in front of elements with lower atomic
mass. Isotopes discovered could not find a place in the periodic table.
Hydrogen could not find a specific place in the periodic table as it shows
similarities with both alkali metals and halogens.

Using atomic number instead of atomic mass as the organizing principle was
first proposed by British chemist Henry Moseley in 1913, and it solved
anomalies like this one. For example, the metals cobalt and nickel had been

assigned the atomic numbers 27 and 28, respectively, based on their known
chemical and physical properties, even though they have nearly the same

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