Why are lithium (atomic number $3$) and potassium (atomic number $19$) placed in group $1$? What will be the atomic numbers of the first two elements in group $2$?

Name the element which has:
$(a)$ the electronic configuration $2, 8, 1$.
$(b)$ a total of two shells,with $4$ electrons in the valence shell.
$(c)$ a total of three shells,with $3$ electrons in the valence shell.
$(d)$ one shell which is completely filled with electrons.
$(e)$ twice as many electrons in the second shell as in the first shell.

$Lithium$,$sodium$,and $potassium$ are all metals that react with water to liberate hydrogen gas. Is there any similarity in the atoms of these elements?

Why do you think the noble gases are placed in a separate group?

$(i)$ 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.

The element with atomic number $18$ belongs to which group?