The atomicity of molecules

The program holds eight molecules, each as a dictionary of its atoms. You will make it find the atomicity of each, name it monatomic, diatomic, triatomic or polyatomic, and count all the atoms in a mixture of molecules.

The idea. Atomicity is the number of atoms in one molecule. A number in front of a formula multiplies all of it.

Your tasks.

1. In atomicity, add up the numbers of atoms in the dictionary.
2. In kind, return "monatomic", "diatomic", "triatomic" or "polyatomic".
3. Find total, the atoms in $3\,\text{H}_2\text{O} + 2\,\text{CO}_2 + 5\,\text{He}$.