Introduction:
Ionization energy is the energy required to remove an electron from an atom or ion in its gaseous state. It is dependent on the atomic structure of the element, including the number of electrons and their arrangement in the electron shell.

Explanation:
Copper has a higher ionization energy than nickel despite having only one electron in the s-orbital due to the following reasons:

1. Effective Nuclear Charge: The effective nuclear charge is the net positive charge experienced by an electron in an atom. In copper, the effective nuclear charge is higher than nickel due to the presence of a full d-subshell. The d-subshell electrons shield the s-electron from the nucleus, reducing the positive charge felt by the s-electron. This results in a higher ionization energy for copper.

2. Electron Configuration: Copper has a unique electron configuration with a half-filled d-subshell. This configuration is energetically favorable and leads to a more stable atom. The removal of an s-electron from copper would result in a less stable configuration, requiring more energy than the removal of an electron from nickel.

3. Screening Effect: The screening effect is the ability of electrons in the inner shells to shield the outer electrons from the positive charge of the nucleus. In copper, the d-electrons in the inner shells provide better screening and reduce the effective nuclear charge felt by the s-electron. This results in a higher ionization energy for copper.

Conclusion:
In conclusion, although copper has only one electron in the s-orbital, its unique electron configuration, higher effective nuclear charge, and better screening effect result in a higher ionization energy compared to nickel.

Copper has attained such configuration to get stability...hence it's not so easy to remove an electron from such a stable atom.....