Where is the mass of the atom?

Each atom in the list is given by its numbers of protons, neutrons and electrons. You will make the program add up the mass of each atom, find the share of the mass that is in the nucleus, and find the atom whose electrons carry the biggest share.

The idea. A proton and a neutron each have a mass of about $1\ \text{u}$, and an electron only $\dfrac{1}{1836}\ \text{u}$. So the nucleus holds almost all the mass.

Your tasks.

1. In mass, return the mass of an atom in u.
2. In nucleus_share, return the percentage of the mass that is in the nucleus.
3. Find most_electron, the name of the atom whose electrons carry the biggest share of its mass.