On descending a group, the I.P value generally decreases because of the pronounced increase in the size of the atoms. For this reason, the 1st I.P of Ag is less than that of Cu. But due to the effect of lanthanide contraction, there is practically no increase in the size of Au, whereas its nuclear charge is higher. Hence, due to the higher nuclear charge of Au as compared to Ag, the I.P of Au becomes higher than that of Ag i.e. Cu ˃ Ag ˂ Au.