Which of the following is a Dobereiner's triad?

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.

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

Which elements did Mendeleev leave gaps for in his periodic table and what names did he give them?

The telluric helix was given by

The modern periodic table is based on the atomic number of the elements. The experiment which proved the significance of the atomic number was: