State the Modern Periodic Law.

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

State the reason for the following:
$(a)$ The elements of the same group have similar chemical properties.
$(b)$ The elements of the same period have different properties.

An element $X$ (atomic number $17$) reacts with an element $Y$ (atomic number $20$) to form a divalent halide.
$(a)$ Where in the periodic table are elements $X$ and $Y$ placed?
$(b)$ Classify $X$ and $Y$ as metal$(s)$,non-metal$(s)$,or metalloid$(s)$.

$(i)$ 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.

An element which is an essential constituent of all organic compounds belongs to