Which of the following is a Dobereiner triad?

Describe Mendeleev's periodic table.

What were the important contributions of Mendeleev's periodic table?

Write the drawbacks in Mendeleev's periodic table that led to its modification.

Which of the following information was not known at the time when Mendeleev's periodic table was constructed?

State whether the following statements are true or false:
$(i)$ The chart of elements presented by $Courtois$ could predict the stoichiometry of oxides of some metals.
$(ii)$ $Li, Na, Rb$ are known as $Döbereiner's$ triads.
$(iii)$ In $1863$,$Newlands$ classified the $56$ discovered elements into $18$ groups.
$(iv)$ $Mendeleev's$ periodic table is based on the atomic number of elements.