CHEMISTRY CH12 Periodic Classification of Elements — Science STD 9 — Question

1. It has eight vertical columns called ‘groups’ and seven horizontal rows called ‘period’.2. Each group has two subgroups ‘A’ and ‘B’. All the elements appearing in a group were found to have similar properties.3. For the first time,-elements were comprehensively classified in such a way that elements of similar properties were placed in the same group.4. It was noticed that certain elements could not be placed in their proper groups in this manner. The reason for this was wrongly determined atomic masses, and consequently those wrong atomic masses were corrected. Eg: The atomic mass of beryllium was known to be 14. Mendeleev reassessed it as 9 and assigned beryllium a proper place;5. Columns were left vacant for elements which were not known at that time and their properties also were predicted. This gave me the motivation to experiment in Chemistry. Eg: Mendeleev gave the names Eka Aluminium and Eka Silicon to those elements which were to be placed below aluminium and silicon respectively in the periodic table and predicted their properties. The discovery of Germanium later on, during his lifetime, proved him correct.

Property	Mendeleev’s prediction (1871)	Actual property (1886)
Atomic Mass	About 72	72.59
Specific Gravity	5.5	5.47
Colour	Dark grey	Dark grey
Formula of oxide	EsO2	GeO2
Nature of chloride	ESCl4	GeCl4