Which of the following gives the correct increasing order of the atomic radii of $O$,$F$ and $N$?

Why does atomic size decrease as we move from left to right along a period in the periodic table?

Identify and name the metals from the following elements whose electronic configurations are given below:
$(a)$ $2, 8, 2$
$(b)$ $2, 8, 1$
$(c)$ $2, 8, 7$
$(d)$ $2, 1$

''Elements in Periodic Table show periodicity of properties.'' List any four such properties.

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 order of increasing atomic size is: