def atomic_mass(atom):
    """
atomic_mass
    description:
        Returns a monoisotopic mass with high precision (6 decimal places) for a specified atom. Reference data taken 
        from https://www.sisweb.com/referenc/source/exactmas.htm
    parameters:
        atom (str) -- atom id 
    returns:
        (float) -- high precision atomic mass 
"""
    # reference dictionary for converting atoms to monoisotopic masses with high precision
    a_to_m = {
        'H': 1.007825,
        'C': 12.,
        'N': 14.003074,
        'O': 15.994915,
        'F': 18.998403,
        'Na': 22.989770,
        'P': 30.973763,
        'S': 31.972072,
        'Cl': 34.968853,
        'K': 38.963708
    }
    if atom not in a_to_m:
        raise ValueError('atomic_mass: atom id {} not available in a_to_m'.format(atom))
    return a_to_m[atom]