What is atomicity? Give two examples for monoatomic and diatomic elements.

Represent a molecule of the compound water using chemical symbols / formulae:

$\mathrm{A}:{\mathrm{H}}_2\mathrm{O}, \mathrm{B}:{\mathrm{CO}}_2, \mathrm{C}:{\mathrm{NH}}_3$ (Enter your correct answer as $\mathrm{A}, \mathrm{B} \mathrm{or} \mathrm{C}$)

Represent four molecules of the compound carbon dioxide using chemical symbols / formulae. 

$$\begin{gathered} \mathrm{A}) 4{\mathrm{H}}_2\mathrm{O} \\ \mathrm{B}) 4{\mathrm{CO}}_2 \\ \mathrm{C}) 2{\mathrm{CO}}_2 \end{gathered}$$

(Enter your correct answer as $\mathrm{A}, \mathrm{B} \mathrm{or} \mathrm{C}$)

Write the chemical formula of the following compounds:

Sulphuric acid, Nitric acid and Hydrochloric acid.

Name the elements in the following compounds:

${\mathrm{CO}}_2, \mathrm{NaCl}, {\mathrm{CH}}_{4,} \mathrm{MgO}$

Explain the following observations:

Iron objects turn reddish-brown when exposed to air.