Mendeleev claimed the famous periodic law that “Element properties are a periodic function of their atomic weight.” Mendeleev placed elements in the order of their atomic weights in the form of a table known as the Periodic Table of Mendeleev.
The Mendeleev periodic table is based on atomic mass, which is one of the key distinctions between itself and the modern periodic table. The current periodic table is based on the number of atoms. Noble gases were not included in Mendeleev’s periodic table because they were not known at the time. Noble gases are classified as group-18 in the modern periodic table.
Merits of mendeleev’s periodic table
Mendeleev’s periodic law and table is the first systematic attempt to classify elements on the fundamental property of the element/atom.
i) Give more weight to similarities in properties than to atomic mass in the arrangement.
Iodine, though of smaller weight (127) than Tellurium (128) is placed later as it resembles more to chlorine and bromine than to oxygen and Sulphur. Similarly cobalt and nickel do not follow increasing atomic mass.
ii) Correction of atomic mass
From the periodic properties, elements of beryllium, indium, gold and platinum were predicted to be having incorrect mass and lead to the re-estimation of their real mass.
iii) Prediction of new elements
It could predict the existence and properties of three elements, of properties similar to boron, aluminum and silicon. These elements are named Eka-boron, Eka-aluminum and Eka-silicon. They were isolated and named as scandium, gallium and germanium respectively.
Experimentally determined atomic weight, physical properties and their chemical formula for their oxides and halides exactly matched with that predicted by Mendeleev. The incomplete periods indicated the existence of yet to be discovered elements.
iv) Noble gases were discovered much later.
Since they show inert behaviour and zero valences, they could be easily attached to the Mendeleev’s table.