How many atoms are present in a $(i)$ $H_2S$ molecule and $(ii)$ $PO_4^{3-}$ ion?

What are ionic and molecular compounds? Give examples.

Classify each of the following on the basis of their atomicity:
$(a) F_2$
$(b) NO_2$
$(c) N_2O$
$(d) C_2H_6$
$(e) P_4$
$(f) H_2O_2$
$(g) P_4O_{10}$
$(h) O_3$
$(i) HCl$
$(j) CH_4$
$(k) He$
$(l) Ag$

Name two biologically important compounds that contain both oxygen and nitrogen.

State the number of atoms present in each of the following chemical species:
$(a)$ $CO_{3}^{2-}$
$(b)$ $PO_{4}^{3-}$
$(c)$ $P_{2}O_{5}$
$(d)$ $CO$

Give reasons for the following:
$(a)$ Ions are more stable than atoms.
$(b)$ Noble gases show low reactivity.