Study the variation in the atomic radii of Group $I$ elements given below and arrange them in an increasing order.
$(i)$ Name the elements which have the smallest and the largest atoms.

Group $I$ elements

$Li: 52\, pm, Na: 61\, pm, K: 231\, pm, Rb: 244\, pm, Cs: 262\, pm$

$(ii)$ How does the atomic size vary as you go down a group?

Explain the basic character of oxides of elements down the group and across the period.

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.

Elements have been arranged in the following sequence on the basis of their increasing atomic masses.
$F, Na, Mg, Al, Si, P, S, Cl, Ar, K$
$(a)$ Pick two sets of elements which have similar properties.
$(b)$ The given sequence represents which law of classification of elements?

Two elements $X$ and $Y$ have atomic numbers $11$ and $16$ respectively.
$(a)$ Write the electronic configuration of both.
$(b)$ Which type of bond will they form?
$(c)$ Write the formula of the compound formed by their combination (in terms of $X$ and $Y$).

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 $2$?