What is atomicity?

Select the correct options from the following statements:

I. ${}_6{}^{12}\mathrm{C}$ and ${}_6{}^{14}\mathrm{C}$ are isobars of each other.

II. ${}_6{}^{12}\mathrm{C}$ reacts with ${}_8{}^{16}\mathrm{O}$ to form a product which contains ionic bonds.

III. ${}_{20}{}^{40}\mathrm{Ca}$ and ${}_{18}{}^{40}\mathrm{Ar}$ are isobars of each other.

IV. ${}_{20}{}^{40}\mathrm{Ca}$ reacts with ${}_8{}^{16}\mathrm{O}$ to form a compound whose aqueous solution is known as lime water.

 In the preparation of ozone we require _____ atoms of oxygen.

Molecules of which element is made up of only one atom of that element?