Which of the following correctly identifies which has the higher first-ionization energy, Cl or Ar, — Atomic Structure Chemistry Question

STEPS:

1. Define first-ionization energy: First-ionization energy is the minimum energy required to remove the most loosely bound valence electron from a gaseous atom in its ground state.
2. Identify the positions of Cl and Ar on the periodic table: Both Chlorine ($\text{Cl}$, atomic number 17) and Argon ($\text{Ar}$, atomic number 18) are located in Period 3 of the periodic table. This means their valence electrons reside in the same principal energy level ($n = 3$) and experience a similar amount of shielding from the inner-shell (core) electrons.
3. Compare their effective nuclear charge ($Z_{\text{eff}}$): Effective nuclear charge is the net positive charge from the nucleus that pulls on the valence electrons. Because argon has 18 protons in its nucleus compared to chlorine's 17 protons, and both experience similar shielding, the valence electrons in argon experience a greater attractive force from the nucleus. Thus, argon has a higher $Z_{\text{eff}}$.
4. Relate $Z_{\text{eff}}$ to ionization energy: A higher effective nuclear charge holds the valence electrons tighter and closer to the nucleus. Because these electrons are held more tightly, it requires significantly more energy to overcome the electrostatic attraction and remove one.
5. Formulate the final conclusion: Since argon's valence electrons are held more tightly due to its higher effective nuclear charge, argon ($\text{Ar}$) has a higher first-ionization energy than chlorine ($\text{Cl}$). This makes Option D the correct answer.

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WHY_OTHERS_WRONG:

• A is incorrect: Chlorine has a *lower* first-ionization energy than argon. Additionally, electronegativity refers to an atom's ability to attract shared electrons within a covalent bond, which is not the correct physical concept to justify the energy needed to remove an electron from an isolated atom.
• B is incorrect: Chlorine has a *lower* first-ionization energy than argon. Furthermore, electron affinity is the energy change associated with *adding* an electron to a gaseous atom, which cannot be used to explain the energy required to *remove* an electron.
• C is incorrect: While argon does have a completely filled valence shell (a stable octet), a "filled shell" is a descriptive rule of thumb rather than the fundamental physical cause of the trend. On the AP Chemistry exam, citing a stable octet without addressing Coulombic forces (nuclear pull and shielding) is considered an incomplete and incorrect justification. The actual physical reason the electrons are hard to remove is the strong nuclear pull.