def parse_line(line):
    """parse composition line by deleting whitespace
       and separating the isotope and atomic density

    Parameters
    ----------
    line: str
        line of isotope and composition

    Returns
    -------
    tuple : (str, float)
        (isotope, atomic density)
    """
    
    # remove whitespace in front
    line = line.lstrip()
    isotope, atom_density = line.split("  ")
    return (isotope, float(atom_density))