In the transition elements, the ionization energy of the metals tends to increase slowly across rows. The density, electronegativity, electrical and thermal conductivities increase from the left of 3D series to the right corner 5D transition elements. The enthalpies of hydration in the metal cations, however, decrease in magnitude. All this leads to the transition metals becoming y less reactive and more “noble” in character. Hence they are referred to as noble metals. Their high ionization energies and increasing electronegativity, and the decreasing enthalpies of hydration make the d block elements highly unreactive. They are thus justifying the term Noble metals. Check out Vedantu’s website or download the app for a detailed and comprehensive explanation.