def lambda1_Vargaftik_1991_Eq_5(TK):
    """Thermal conductivity of a monomer gas

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

    Returns
    -------
    λ₁ : float
        W / m K, thermal conductivity

    References
    ----------
    Equation (5) of
    Vargaftik, N. B., Yu. K. Vinogradov, V. I. Dolgov, V. G. Dzis,
    I. F. Stepanenko, Yu. K. Yakimovich, and V. S. Yargin.
    "Viscosity and Thermal Conductivity of Alkali Metal Vapors at
    Temperatures up to 2000 K." International Journal of
    Thermophysics 12, no. 1 (January 1991): 85–103.
    https://doi.org/10.1007/BF00506124.
    """
    lambda1 = (541.0 + 0.485 * (TK - 1000)) * 1e-4
    return lambda1