Which of the following has the maximum number of atoms?

One mole of $H_{2}O$ means:
$(i)$ $18 \text{ g}$ of $H_{2}O$
$(ii)$ $6.022 \times 10^{23}$ molecules of $H_{2}O$
$(iii)$ $180 \text{ g}$ of $H_{2}O$
$(iv)$ $1.8 \text{ g}$ of $H_{2}O$

$(a)$ What do the following formulae stand for:
$(i)$ $2O$
$(ii)$ $O_{2}$
$(iii)$ $O_{3}$
$(iv)$ $H_{2}O$
$(b)$ Give the chemical formulae of the following compounds:
$(i)$ Potassium carbonate
$(ii)$ Calcium chloride
$(c)$ Calculate the formula unit mass of $Al_{2}(SO_{4})_{3}$.
(Given atomic mass of $Al = 27 \, u, S = 32 \, u, O = 16 \, u$)

$(a)$ Define polyatomic ions. Write an example.
$(b)$ Calculate the formula unit mass of $CaCO_{3}$. (Atomic mass of $C = 12 \ u, Ca = 40 \ u, O = 16 \ u$)
$(c)$ Calculate the molecular mass of the following:
$(i)$ $HNO_{3}$
$(ii)$ $CH_{3}COOH$
(Atomic mass of $H = 1 \ u, N = 14 \ u, O = 16 \ u, C = 12 \ u$)

Complete the following crossword puzzle by using the names of the chemical elements based on the data provided in the table.

Across	Down
$2$. The element used by Rutherford during his $\alpha$-scattering experiment.	$1$. $A$ white lustrous metal used for making ornaments and which tends to get tarnished black in the presence of moist air.
$3$. An element which forms rust on exposure to moist air.	$4$. Both brass and bronze are alloys of this element.
$5$. $A$ very reactive non-metal stored under water.	$6$. The metal which exists in the liquid state at room temperature.
$7$. Zinc metal when treated with dilute hydrochloric acid produces a gas of this element which when tested with a burning splinter produces a pop sound.	$8$. An element with the symbol $Pb$.

$(i)$ $(a)$ What do you understand by $1 \text{ amu}$ or $1 \text{ u}$?
$(b)$ How is it linked with relative atomic mass?
$(ii)$ How do we know the presence of atoms if they do not exist independently for most of the elements?