Why is it not possible to see an atom with naked eyes?

In a reaction,$5.3 \, g$ of sodium carbonate reacted with $6 \, g$ of acetic acid. The products were $2.2 \, g$ of carbon dioxide,$0.9 \, g$ of water,and $8.2 \, g$ of sodium acetate. Show that these observations are in agreement with the law of conservation of mass.
$\text{Sodium carbonate} + \text{Acetic acid} \to \text{Sodium acetate} + \text{Carbon dioxide} + \text{Water}$

Write the chemical formulae of the following:
$(i)$ Magnesium chloride
$(ii)$ Calcium oxide
$(iii)$ Copper nitrate
$(iv)$ Aluminium chloride
$(v)$ Calcium carbonate.

Calculate the number of molecules of sulphur $(S_8)$ present in $16 \, g$ of solid sulphur.

Which has more number of atoms,$100\,g$ of sodium or $100\,g$ of iron (given,atomic mass of $Na = 23\,u, Fe = 56\,u$)?

Which postulate of Dalton's atomic theory can explain the law of definite proportions?