Give one relevant reason why scientists chose $1/16$ of the mass of an atom of naturally occurring oxygen as the atomic mass unit.

The chemical symbol for sodium is

The chemical symbol for nitrogen gas is

$(a)$ 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 $NO_{3}^{-}$

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

Which of the following statements is not true about an atom?