def METHOD_NAME(ion_elements):
    """Reorder elements to be consistently ordered.
    Per https://en.wikipedia.org/wiki/Chemical_formula#Hill_system
    """
    if ion_elements['C'] != 0:
        return ['C', 'H', *sorted(key for key in ion_elements.keys() if key not in ('C', 'H'))]
    return sorted(key for key in ion_elements.keys())