Question

How many elements were known at the time of Mendeleev? Name one element which was not known at the time of Mendeleev, but when discovered later found to have the same properties as predicted by Mendeleev.

Answer 1

No. of Elements That were unknown at his time

Mendeleev created a table that listed all $63$elements found up to his time, in increasing order relative atomic masses. Mendeleev's Periodic Table is the name given to it. There were $8$ columns and $7$ rows in the table.

Name of one of those elements

• Geranium was undiscovered elements at the time of Mendeleev's classification. He left spaces for the undiscovered elements and correctly guessed the characteristics of some of them.
• Mendeleev anticipated the atomic masses as well as other chemical properties of gallium, germanium, and scandium, which were all unknown in $1871$. Within $15$ years, the "unknown" elements were found, and they were found to match Mendeleev's basic characteristics.