Define Newlands' law of Octaves.

Complete the following crossword puzzle (Figure):
**Across:**
$(1)$ An element with atomic number $12$.
$(3)$ Metal used in making cans and member of Group $14$.
$(4)$ $A$ lustrous non-metal which has $7$ electrons in its outermost shell.
**Down:**
$(2)$ Highly reactive and soft metal which imparts yellow colour when subjected to flame and is kept in kerosene.
$(5)$ The first element of second Period.
$(6)$ An element which is used in making fluorescent bulbs and is second member of Group $18$ in the Modern Periodic Table.
$(7)$ $A$ radioactive element which is the last member of halogen family.
$(8)$ Metal which is an important constituent of steel and forms rust when exposed to moist air.
$(9)$ The first metalloid in Modern Periodic Table whose fibres are used in making bullet-proof vests.

The atomic numbers of a few elements are given below: $10, 20, 7, 14$.
$(a)$ Identify the elements.
$(b)$ Identify the group number of these elements in the Periodic Table.
$(c)$ Identify the periods of these elements in the Periodic Table.

Atomic radii of the elements of the second period are given below :

Period $II$ elements	$B$	$Be$	$O$	$N$	$Li$	$C$
Atomic radius $(pm)$	$88$	$111$	$66$	$74$	$152$	$77$

$(i)$ Arrange them in decreasing order of their atomic radii.
$(ii)$ Are the elements now arranged in the pattern of a period in the Periodic Table?
$(iii)$ Which elements have the largest and the smallest atoms?
$(iv)$ How does the atomic radius change as you go from left to right in a period?

$(a)$ Predict which of the following will form anions and which will form cations:
$(i)$ $Na$ $(ii)$ $Al$ $(iii)$ $Cl$ $(iv)$ $O$
$(b)$ Name two elements that are inert.

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