Arrange the following metals in decreasing order of their atomic size: $Ca, Mg, Ba, Be$.

Arrange the following elements in the order of their increasing nonmetallic character:
$Li, O, C, Be, F$

$K$ and $Ca$ have atomic numbers $19$ and $20$ respectively,and they belong to the same period.
$(a)$ Which among them would have a smaller atomic size?
$(b)$ Which one would be more electropositive?
$(c)$ To which group would each of them belong?

The elements $A$,$B$,$C$,$D$,and $E$ have atomic numbers $9$,$11$,$17$,$12$,and $13$ respectively. Which pair of elements belongs to the same group?

$(a)$ State the main characteristics of elements on which the modern periodic table is based.
$(b)$ No fixed position can be assigned to hydrogen in the periodic table. Why?

If an element $X$ is placed in group $14$,what will be the formula and the nature of bonding of its chloride?