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

$(i)$ 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.

State whether the following statement is true or false:
The relative atomic masses of all elements have been found with respect to an atom of oxygen-$16$.

Express each of the following in kilograms:
$(a)$ $5.84 \times 10^{-3} \,mg$
$(b)$ $58.34 \,g$
$(c)$ $0.584 \,g$
$(d)$ $5.873 \times 10^{-21} \,g$

$A$ sample of vitamin $C$ is known to contain $2.58 \times 10^{24}$ oxygen atoms. How many moles of oxygen atoms are present in the sample?

The chemical symbol for sodium is