Atoms and bonds in a molecule

Every covalent bond uses one unit of valency from each of its two atoms. You will make the program add up the valencies of a molecule, turn them into a number of bonds, and spot a formula that cannot exist.

The idea. Bonds $=$ (sum of the valencies) $\div 2$, counting a double bond as two. If the sum is odd, the units cannot pair up, and no such molecule exists.

Your tasks.

1. In units, return the sum of the valencies of all the atoms.
2. In bonds, return the number of bonds, or None if the sum is odd.
3. Find propanol_bonds, the bonds in propanol, $\text{C}_3\text{H}_7\text{OH}$.