The mass spectrum of chlorine gas

A $\text{Cl}_2$ molecule is made of two chlorine atoms, each $35$ or $37$. You will work out the share of each kind of molecule, the average molar mass of the gas, and the tallest peak. Then the program draws the spectrum.

The idea. With shares $p_{35}$ and $p_{37}$: mass $70$ has $p_{35}^2$, mass $72$ has $2p_{35}p_{37}$, and mass $74$ has $p_{37}^2$.

Your tasks.

1. In peaks, return a dictionary of the share of each mass.
2. Find average, the average molar mass of $\text{Cl}_2$, in g/mol.
3. Find tallest, the mass with the largest share.