def atomic_mass(element):
    """Return the mass of the most common isotope of the selected element."""

    # Set mass for each atom
    mass = {'H' :  1.007825,
            'He':  4.002603,
            'Li':  7.016004,
            'Be':  9.012182,
            'B' : 11.009305,
            'C' : 12.000000,
            'N' : 14.003074,
            'O' : 15.994914,
            'F' : 18.998403,
            'Ne': 19.992440,
            'Na': 22.989769,
            'Mg': 23.985041,
            'Al': 26.981538,
            'Si': 27.976926,
            'P' : 30.973761,
            'S' : 31.972070,
            'Cl': 34.968852,
            'Ar': 39.962383,
            'K' : 38.963706,
            'Ca': 39.962591,
            'Sc': 44.955910,
            'Ti': 47.947947,
            'V' : 50.943963,
            'Cr': 51.849511,
            'Mn': 54.938049,
            'Fe': 55.934941,
            'Co': 58.933199,
            'Ni': 57.935347,
            'Cu': 62.929600,
            'Zi': 63.929146,
            'Ga': 68.925581,
            'Ge': 73.921178,
            'As': 74.921596,
            'Se': 79.916522,
            'Br': 78.918337,
            'Kr': 83.911508,
            'Rb': 84.911792,
            'Sr': 87.905616,
            'Y' : 88.905848,
            'Zr': 89.904702,
            'Nb': 92.906376,
            'Mo': 97.905406,
            'Tc': 98.000000,
            'Ru': 101.904348,
            'Rh': 102.905504,
            'Pd': 105.903484,
            'Ag': 106.905093,
            'Cd': 113.903358,
            'In': 114.903879,
            'Sn': 119.902198,
            'Sb': 120.903822,
            'Te': 129.906222,
            'I' : 126.904468,
            'Xe': 131.904154,
            'Cs': 132.905447,
            'Ba': 137.905242,
            'La': 138.906349,
            'Ce': 139.905435,
            'Pr': 140.907648,
            'Nd': 141.907719,
            'Pm': 145.000000,
            'Sm': 151.919729,
            'Eu': 152.921227,
            'Gd': 157.924101,
            'Tb': 158.925343,
            'Dy': 163.929171,
            'Ho': 164.930319,
            'Er': 165.930290,
            'Tm': 168.934211,
            'Yb': 173.938858,
            'Lu': 174.940768,
            'Hf': 179.946548,
            'Ta': 180.947996,
            'W' : 183.950932,
            'Re': 186.955750,
            'Os': 191.961479,
            'Ir': 192.962923,
            'Pt': 194.964774,
            'Au': 196.966551,
            'Hg': 201.970625,
            'Tl': 204.974412,
            'Pb': 207.976636,
            'Bi': 208.980384,
            'Po': 209.000000,
            'At': 210.000000,
            'Rn': 222.000000,
            'Fr': 223.000000,
            'Ra': 226.000000,
            'Ac': 227.000000,
            'Th': 232.038049,
            'Pa': 231.035878,
            'U' : 238.050783
           }

    if element is None:
        return None
    else:
        return mass[element]