Question

compare the ionization energy between Au, Ag and Cu

Answer 1

However, the actual trend is Cu > Au > Ag.

The ionisation energy of gold is greater than silver due to the relativistic effect. Due to the weak shielding effect of the d and f orbitals the effective nuclear attraction on the 6s orbital electron increases greatly. This leads to a relativistic contraction of the s orbital, and consequently the 6s electron is held more tightly by the nucleus. As a result, the ionisation energy of gold is greater than that of silver.

Hence, the above observed trend is observed.

I hope this helps. :)