The diagram shows three containers, each filled with a different gas - Leaving Cert Chemistry - Question (d) - 2021

According to Avogadro's law, equal volumes of gases at the same temperature and pressure contain an equal number of molecules. In this case, all three containers have equal volumes.

Now, looking at the chemical compositions:

• Helium (He) is a monoatomic gas, so it has 1 atom per molecule.
• Chlorine (Cl₂) is diatomic, consisting of 2 atoms per molecule.
• Carbon dioxide (CO₂) is triatomic, consisting of 3 atoms per molecule (1 carbon atom and 2 oxygen atoms).

Since all containers have the same number of molecules, CO₂, with its triatomic structure, contains the most atoms compared to He and Cl₂.