def eta1_Bouledroua(TK):
    """Viscosity of the monomers

    Parameters
    ----------
    TK : float
        K, temperature

    Returns
    -------
    η₁ : float
        Pa s, viscosity of a monomer gas

    References
    ----------
    Bouledroua, M, A Dalgarno, and R Côté.
    “Viscosity and Thermal Conductivity of Li, Na, and K Gases.”
    Physica Scripta 71, no. 5 (January 1, 2005): 519–22.
    https://doi.org/10.1238/Physica.Regular.071a00519.

    Notes
    -----
    "The numerical values in the Tables
    can be reproduced by the simple formula

    .. math::
        \eta = A T^{\alpha}, \quad A = 0.234, \quad \alpha = 0.903

    "
    """
    A = 0.234
    alpha = 0.903
    eta1 = 1e-7 * A * TK**alpha
    return eta1