Which of the following correctly identifies which has the higher first-ionization energy, \(\ce{Cl}\) or \(\ce{Ar}\), and supplies the best justification?

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Although chlorine does have a higher electronegativity and electron affinity, these are simply properties and do not correspond to other properties like ionization energy.

Although argon does have a filled valence shell, this property does not justify another property.

The ionization energy is the energy required to remove an electron. The energy keeping an electron attached to the nucleus can be approximated by Coulomb's law. Every electron in chlorine is effectively attracted to the nucleus, which has a charge of +17 from its protons. In argon, there are 18 protons, so the force of attraction is slightly greater. It would therefore take more energy to remove any given electron.