Picking the best structure

You will write a formal charge function, add up the sizes of the charges, and use them to choose the best Lewis structure of the phosphate ion.

The idea. Formal charge $= V - L - B$. The best structure has the smallest sum of the sizes of its formal charges.

Your tasks.

1. Complete formal_charge(valence, lone_electrons, bonds).
2. Complete size_sum(charges), the sum of the sizes of a list of charges.
3. The program draws $\text{PO}_4^{3-}$ with $0$, $1$ or $2$ $\text{P=O}$ bonds. Set best_k to the number of double bonds with the smallest sum.