def abundance(isotope):
    """
    Natural abundance.

    :Parameters:
        *isotope* : Isotope

    :Returns:
        *abundance* : float | %

    Reference:
        *Coursey. J. S., Schwab. D. J, and Dragoset. R. A., NIST Atomic
        Weights and Isotopic Composition Database.*
    """
    return isotope._abundance
