It was assumed by Newlands that only :

Write true or false for the statement: Electronic configuration of hydrogen resembles that of alkaline earth metals.

When Mendeleev started his work,the number of known elements were:

$(i)$ State Modern Periodic Law.
$(ii)$ How many :
$(a)$ groups and
$(b)$ periods are present in the modern periodic table?
$(iii)$ State how the problem of placing:
$(a)$ hydrogen,
$(b)$ isotopes of an element has been solved in this periodic table.

Calcium is an element with atomic number $20$.
$(i)$ Will it be a metal or a non-metal?
$(ii)$ What will be its valency?
$(iii)$ What would be the formula of its chloride?
$(iv)$ Will it be larger or smaller than $K$ (Potassium)?

How many vertical columns are there in the modern periodic table and what are they called?