Mention the name given to the number of $6.022 \times 10^{23}$ particles.

$(i)$ Name the body which approves the nomenclature of elements and compounds.
$(ii)$ The symbol of sodium is written as $Na$ and not as $S$. Give reason.
$(iii)$ Name one element which forms diatomic and one which forms tetraatomic molecules.

Mass of one atom of oxygen is

Why is the atomic mass of chlorine taken as $35.5\, u$ and not a whole number like $35\, u$ or $36\, u$? Explain.

Compute the difference in masses of one mole each of aluminium atoms and one mole of its ions. (Mass of an electron is $9.1 \times 10^{-28} \,g$). Which one is heavier (in $,g$)?

Calculate the number of moles in $5.75 \, g$ of sodium (atomic mass of sodium $= 23$).