def density(iso_el):
    """
    Element density for natural abundance. For isotopes, return
    the equivalent density assuming identical inter-atomic spacing as the
    naturally occuring material.

    :Parameters:
        *iso_el* : isotope or element
            Name of the element or isotope.

    :Returns:
        *density* : float | |g/cm^3|

    Reference:
        *ILL Neutron Data Booklet, original values from CRC Handbook of Chemistry and Physics,
        80th ed. (1999).*

    """
    if hasattr(iso_el, 'element'):
        return iso_el.element._density * (iso_el.mass / iso_el.element.mass)
    return iso_el._density
