def _atom_to_str(atom):
    """ Get the string representation of an atom in a monomeric form in an
    alphabet

    Args:
        atom (:obj:`core.Atom`): atom

    Returns:
        :obj:`str`: string representation
    """
    if atom.charge:
        return '{0}{1}{2:+d}'.format(atom.element, atom.position, atom.charge)
    else:
        return '{0}{1}'.format(atom.element, atom.position)
