What formed the basis for John Dalton to formulate Dalton's atomic theory?

$(a)$ What are molecules? Using examples, differentiate between the molecules of an element and a compound.
$(b)$ Derive the chemical formulae of the molecules formed between the following ions:
$(i)$ $Al^{3+}$ and $SO_{4}^{2-}$
$(ii)$ $Ba^{2+}$ and $NO_{3}^{-}$

Which of the following represents $1 \, u$?

Calculate the number of atoms in $0.02$ mole of carbon.

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 $Pb$.

Write the molecular formulae of all the compounds that can be formed by the combination of the following ions: $Cu^{2+}, Na^{+}, Fe^{3+}, Cl^{-}, SO_{4}^{2-}, PO_{4}^{3-}$.